Combined deletions of pfhrp2 and pfhrp3 genes result in Plasmodium falciparum malaria false-negative rapid diagnostic test

High Frequency of Deletions in the pfhrp2 and pfhrp3 Genes of Plasmodium falciparum in the Middle Rio Negro Region of the Brazilian Amazon

Several countries are reporting natural populations of P. falciparum with deletions in the pfhrp2/3 genes that can lead to false-negative results in rapid diagnostic tests. To investigate the prevalence of deletion in the pfhrp2/3 genes in the Rio Negro basin in the Brazilian Amazon and identify whether there is clinical differentiation between individuals infected by these parasites, clinical samples collected from 2003 to 2016 were analyzed from symptomatic and asymptomatic P. falciparum-infected individuals. The molecular deletion of pfhrp2 and pfhrp3 genes was evaluated using the protocols recommended by the WHO. From 82 samples used, 28 (34.2%) had a single deletion in pfhrp2, 19 (23.2%) had a single deletion in pfhrp3, 15 (18.3%) had a double deletion (pfhrp2/3), and 20 (24.4%) did not have a deletion in either gene. In total, 29.3% of individuals had an asymptomatic plasmodial infection and were 3.64 times more likely to have parasites with a double deletion (pfhrp2/3) than patients with clinical malaria (p = 0.02). The high prevalence of parasites with pfhrp2/3 deletions shows the need to implement a surveillance program in this area. Deletions in parasites may be associated with the clinical pattern of the disease in this area. More studies must be carried out to elucidate these findings.

Diagnostic value of rapid test for malaria among febrile neonates in a tertiary hospital in North-East Nigeria: a prospective cross-sectional study

OBJECTIVE

The WHO recommends testing using microscopy or rapid diagnostic test (RDT) before treatment for malaria. However, the use of RDT to diagnose neonatal malaria has not been widely validated with most studies limited to the first week of life. Thus, we conducted this study to determine the utility of RDT in the diagnosis of congenital and acquired malaria in febrile neonates in Nigeria.

DESIGN

This prospective cross-sectional descriptive study consecutively recruited 131 febrile neonates at the Special Care Baby Unit (SCBU) of the Federal Teaching Hospital Gombe, Nigeria. All study participants concurrently had RDT (HRP2, LDH) and malaria microscopy. The performance of both methods was then compared.

RESULT

Seventy-eight of 131 neonates tested for malaria by blood smear microscopy demonstrated malaria parasites; a prevalence of 59.5%. Parasite count ranged from 16 to 520 /μL and the median parasite count was 81.0 /μL with IQR (40.0-134.5). The majority of patients (93.5%) had low-density parasitaemia (≤2+). All species identified were Plasmodium falciparum. None of the 131 neonates tested positive on RDT. The sensitivity and positive predictive value of RDT for neonatal malaria was zero. Congenital malaria was the most common form of neonatal malaria, accounting for 75.6%, while acquired and transfusion-related malaria were estimated at 12.8% and 11.6%, respectively.

CONCLUSION

The RDT used in this study was not sensitive in the diagnosis of congenital or acquired neonatal malaria; therefore, microscopy remains the preferred method of diagnosis of neonatal malaria.

Global polymorphism of Plasmodium falciparum histidine rich proteins 2/3 and impact on malaria rapid diagnostic test detection: a systematic review and meta-analysis

BACKGROUND

This review presents an overview of field findings on sequence variation of Plasmodium falciparum histidine-rich proteins 2/3 (PfHRP2/3) for which reference types (1-24) have been identified, and its critical impact on PfHRP2-based rapid diagnostic test (RDT) detection.

RESEARCH DESIGN AND METHODS

This systematic review and meta-analysis was registered with PROSPERO, CRD42022316027, and conducted as per the PRISMA guidelines, and the methodological quality of studies was assessed.

RESULTS

Of the 2184 records identified, 34 studies were included mostly from Africa (47.1%) and Asia (35.3%). The reference PfHRP2 types 1, 2, 3, 6, and 7 are invariably found at proportions ≥ 80-100% in all areas with the exception of The Americas where their proportion is very low. The proteins exhibited high diversity of variants/unknown types, especially for types 1, 2, 4, and 7. Eleven major PfHRP2 epitopes were found at pooled proportion > 90%. The existing models to predict RDT detection are greatly limited by the impact of factors such as low (very low) parasitemia, RDT brand, and PfHRP3 cross-reactivity. PfHRP2 length and presence/number of a given reference repeat type/variant did not seem to impact RDT detection.

CONCLUSIONS

PfHRP2/3 are highly polymorphic and current findings are insufficient, conflicting and not convincing enough to conclude on the role of PfHRP2/3 sequence polymorphism in PfHRP2-based RDT detection.

CRISPR/Cas advancements for genome editing, diagnosis, therapeutics, and vaccine development for Plasmodium parasites, and genetic engineering of Anopheles mosquito vector

Malaria as vector-borne disease remains important health concern with over 200 million cases globally. Novel antimalarial medicines and more effective vaccines must be developed to eliminate and eradicate malaria. Appraisal of preceding genome editing approaches confirmed the CRISPR/Cas nuclease system as a novel proficient genome editing system and a tool for species-specific diagnosis, and drug resistance researches for Plasmodium species, and gene drive to control Anopheles population. CRISPR/Cas technology, as a handy tool for genome editing can be justified for the production of transgenic malaria parasites like Plasmodium transgenic lines expressing Cas9, chimeric Plasmodium transgenic lines, knockdown and knockout transgenic parasites, and transgenic parasites expressing alternative alleles, and also mutant strains of Anopheles such as only male mosquito populations, generation of wingless mosquitoes, and creation of knock-out/ knock-in mutants. Though, the incorporation of traditional methods and novel molecular techniques could noticeably enhance the quality of results. The striking development of a CRISPR/Cas-based diagnostic kit that can specifically diagnose the Plasmodium species or drug resistance markers is highly required in malaria settings with affordable cost and high-speed detection. Furthermore, the advancement of genome modifications by CRISPR/Cas technologies resolves contemporary restrictions to culturing, maintaining, and analyzing these parasites, and the aptitude to investigate parasite genome functions opens up new vistas in the better understanding of pathogenesis.

Gene Coverage Count and Classification (GC3): a locus sequence coverage assessment tool using short-read whole genome sequencing data, and its application to identify and classify histidine-rich protein 2 and 3 deletions in Plasmodium falciparum

BACKGROUND

The ability of malaria rapid diagnostic tests (RDTs) to effectively detect active infections is being compromised by the presence of malaria strains with genomic deletions at the hrp2 and hrp3 loci, encoding the antigens most commonly targeted in diagnostics for Plasmodium falciparum detection. The presence of such deletions can be determined in publically available P. falciparum whole genome sequencing (WGS) datasets. A computational approach was developed and validated, termed Gene Coverage Count and Classification (GC3), to analyse genome-wide sequence coverage data and provide informative outputs to assess presence and coverage profile of a target locus in WGS data. GC3 was applied to detect deletions at hrp2 and hrp3 (hrp2/3) and flanking genes in different geographic regions and across time points.

METHODS

GC3 uses Python and R scripts to extract locus read coverage metrics from mapped WGS data according to user-defined parameters and generates relevant tables and figures. GC3 was tested using WGS data for laboratory reference strains with known hrp2/3 genotypes, and its results compared to those of a hrp2/3-specific qPCR assay. Samples with at least 25% of coding region positions with zero coverage were classified as having a deletion. Publicly available sequence data was analysed and compared with published deletion frequency estimates.

RESULTS

GC3 results matched the expected coverage of known laboratory reference strains. Agreement between GC3 and a hrp2/3-specific qPCR assay reported for 19/19 (100%) hrp2 deletions and 18/19 (94.7%) hrp3 deletions. Among Cambodian (n = 127) and Brazilian (n = 20) WGS datasets, which had not been previously analysed for hrp2/3 deletions, GC3 identified hrp2 deletions in three and four samples, and hrp3 deletions in 10 and 15 samples, respectively. Plots of hrp2/3 coding regions, grouped by year of sample collection, showed a decrease in median standardized coverage among Malawian samples (n = 150) suggesting the importance of a careful, properly controlled follow up to determine if an increase in frequency of deletions has occurred between 2007-2008 and 2014-2015. Among Malian (n = 90) samples, median standardized coverage was lower in 2002 than 2010, indicating widespread deletions present at the gene locus in 2002.

CONCLUSIONS

The GC3 tool accurately classified hrp2/3 deletions and provided informative tables and figures to analyse targeted gene coverage. GC3 is an appropriate tool when performing preliminary and exploratory assessment of locus coverage data.

Geographical distribution and genetic characterization of pfhrp2 negative Plasmodium falciparum parasites in the Peruvian Amazon

Malaria rapid diagnostic tests (RDTs) have been evaluated in the Peruvian Amazon region and their performance has been variable. This region is known for being the first with documented evidence of wild Plasmodium falciparum parasites lacking pfhrp2 and pfhrp3 genes, leading to false-positive results with HRP2-based RDTs. In our attempt to further characterize the deletion pattern of these genes and their evolutionary relationship, 93 P. falciparum samples, collected in different communities from the Peruvian Amazon region between 2009 and 2010, were analyzed in this study. Genomic DNA was used to amplify 18S rRNA, pfmsp2 and pfglurp to confirm the diagnosis and DNA quality, respectively; pfhrp2, pfhrp3, and their flanking genes were amplified by PCR to assess the pattern of the gene deletions. In addition, microsatellite analysis were performed using seven neutral microsatellites (MS) and five microsatellite loci flanking pfhrp2. The data showed the absence of pfhrp3 gene in 53.76% (50/93) of the samples, reflecting a higher frequency than the proportion of pfhrp2 gene deletions (33.33%; 31/93). Among the flanking genes, the highest frequency of deletion was observed in the PF3D7_0831900 gene (78.49%; 73/93) for pfhrp2. MS marker analysis showed the presence of 8 P. falciparum lineages. The lineage Bv1 was the most prevalent among parasites lacking pfhrp2 and pfhrp3 genes. Additionally, using MS flanking pfhrp2 gene, the haplotypes α and δ were found to be the most abundant in this region. This study confirms the presence in this area of field isolates with deletions in either pfhrp2, pfhrp3, or both genes, along with their respective flanking regions. Our data suggest that some pfhrp2/pfhrp3 deletion haplotypes, in special the lineage Bv1, are widely dispersed within the Peruvian Amazon. The persistence of these haplotypes ensures a proportion of P.falciparum parasites lacking the pfhrp2/pfhrp3 genes in this area, which ultimately leads to false-negative results on PfHRP2-detecting malaria RDTs. However, additional studies are needed to not only confirm this hypothesis but also to further delineate the origin and genetic basis for the pfhrp2- and pfhrp3 gene deletions in wild P. falciparum parasites.

Active case detection and treatment of malaria in pregnancy using LAMP technology (LAMPREG): a pragmatic randomised diagnostic outcomes trial-study protocol

INTRODUCTION

Malaria is one of the major public health problems in sub-Saharan Africa. It contributes significantly to maternal and fetal morbidity and mortality in affected countries. This study aims to evaluate the impact of enhanced case detection using molecular testing called loop-mediated isothermal amplification (LAMP) on birth outcomes in a prospective study design.

METHODS AND ANALYSIS

A pragmatic randomised diagnostic outcomes trial will be conducted in several health institutes in different Ethiopian regions. Women (n=2583) in their first and second trimesters of pregnancy will be included in the study and individually randomised to the standard of care or enhanced case detection arms, and followed until delivery. Enrolment will encompass the malaria peak transmission seasons. In the standard of care arm, a venous blood sample will be collected for malaria diagnosis only in symptomatic patients. In contrast, in the intervention arm, mothers will be tested by a commercially available Conformité Européene (CE)-approved LAMP malaria test, microscopy and rapid diagnostic test for malaria regardless of their symptoms at each antenatal care visit. The primary outcome of the study is to measure birth weight.

ETHICS AND DISSEMINATION

The study was approved by the following ethical research boards: Armauer Hansen Research Institute/ALERT Ethics Review Committee (FORM AF-10-015.1, Protocol number PO/05/20), the Ethiopia Ministry of Science and Higher Education National Research Ethics Review Committee (approval SRA/11.7/7115/20), the Ethiopia Food and Drug Administration (approval 02/25/33/I), UCalgary Conjoint Health Research Ethics Board (REB21-0234). The study results will be shared with the institutions and stakeholders such as the Ethiopia Ministry of Health, the Foundation for Innovative Diagnostics, WHO's Multilateral initiative on Malaria - Tropical Diseases Research (TDR-MIM), Roll Back Malaria and the Malaria in Pregnancy Consortium. The study results will also be published in peer-reviewed journals and presented at international conferences.

TRIAL REGISTRATION NUMBER

NCT03754322.

Plasmodium falciparum histidine-rich protein 2 and 3 genes deletion in global settings (2010–2021): a systematic review and meta-analysis

Background

The usefulness of histidine-rich protein-2/3 (HRP2/3)-based rapid diagnostic tests of malaria due to Plasmodium falciparum has been threatened by the appearance of mutant PfHRP2/3 genes. This study was undertaken to determine the global pooled estimates of PfHRP2/3gene deletions.

Methods

Relevant publications were identified from electronic databases such as; PubMed, EMBASE, and MEDLINE online. Besides, all the relevant literatures were retrieved through Google and Google Scholar. STATA software was used for data analysis. The pooled estimates were calculated using random effect model. The summary estimates were presented using forest plots and tables.

Results

A total of 27 studies were included in the systematic review. However, only 24 and 17 studies were included for PfHRP2 and 3 gene deletion meta-analysis, respectively. The prevalence of PfHRP2 gene deletion across the individual studies ranged from the highest 100% to the lowest 0%. However, the meta-analysis result showed that the global pooled prevalence of PfHRP2 and PfHRP3 gene deletions were 21.30% and 34.50%, respectively. The pooled proportion of PfHRP2 gene deletion among false negative PfHRP2-based RDTs results was found to be 41.10%. The gene deletion status was higher in South America and followed by Africa. The pooled estimate of PfHRP2 gene deletion among studies, which did not follow the WHO PfHRP2/3 gene deletion analysis protocol was higher than their counter parts (21.3% vs 10.5%).

Conclusions

This review showed that there is a high pooled prevalence of PfHRP2/3 gene deletions in Plasmodium falciparum confirmed isolates and also a high proportion of their deletions among false-negative malaria cases using PfHRP2-based RDT results. Hence, malaria diagnosis based on PfHRP2-based rapid tests seems to be less sensitive and warrants further evaluation of PfHRP2/3 gene deletions.

Plasmodium falciparum histidine rich protein 2 (pfhrp2): an additional genetic marker suitable for anti-malarial drug efficacy trials

Background

Genotyping of the three Plasmodium falciparum polymorphic genes, msp1, msp2 and glurp, has been adopted as a standard strategy to distinguish recrudescence from new infection in drug efficacy clinical trials. However, the suitability of a particular gene is compromised in areas where its allelic variants distribution is significantly skewed, a phenomenon that might occur in isolated parasite populations or in areas of very low transmission. Moreover, observation of amplification bias has diminished the value of glurp as a marker.

Methods

The suitability of the polymorphic P. falciparum histidine-rich protein 2 (pfhrp2) gene was assessed to serve as an alternative marker using a PCR-sequencing or a PCR–RFLP protocol for genotyping of samples in drug efficacy clinical trials. The value of pfhrp2 was validated by side-by-side analyses of 5 admission-recrudescence sample pairs from Yemeni malaria patients.

Results

The outcome of the single pfhrp2 gene discrimination analysis has been found consistent with msp1, msp2 and glurp pool genotyping analysis for the differentiation of recrudescence from new infection.

Conclusion

The findings suggest that under the appropriate circumstances, pfhrp2 can serve as an additional molecular marker for monitoring anti-malarials efficacy. However, its use is restricted to endemic areas where only a minority of P. falciparum parasites lack the pfhrp2 gene.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-04014-4.

Modelling the epidemiology of malaria and spread of HRP2-negative Plasmodium falciparum following the replacement of HRP2-detecting rapid diagnostic tests

Malaria rapid diagnostic tests (RDTs) are dominated by products which use histidine-rich protein 2 (HRP2) to detect Plasmodium falciparum. The emergence of parasites lacking the pfhrp2 gene can lead to high rates of false-negative results amongst these RDTs. One solution to restore the ability to correctly diagnose falciparum malaria is to switch to an RDT which is not solely reliant on HRP2. This study used an agent-based stochastic simulation model to investigate the impact on prevalence and transmission caused by switching the type of RDT used once false-negative rates reached pre-defined thresholds within the treatment-seeking symptomatic population. The results show that low transmission settings were the first to reach the false-negative switch threshold, and that lower thresholds were typically associated with better long-term outcomes. Changing the diagnostic RDT away from a HRP2-only RDT is predicted to restore the ability to correctly diagnose symptomatic malaria infections, but often did not lead to the extinction of HRP2-negative parasites from the population which continued to circulate in low density infections, or return to the parasite prevalence and transmission levels seen prior to the introduction of the HRP2-negative parasite. In contrast, failure to move away from HRP2-only RDTs leads to near fixation of these parasites in the population, and the inability to correctly diagnose symptomatic cases. Overall, these results suggest pfhrp2-deleted parasites are likely to become a significant component of P. falciparum parasite populations, and that long-term strategies are needed for diagnosis and surveillance which do not rely solely on HRP2.

A Comprehensive Analysis of the Genetic Diversity of Plasmodium falciparum Histidine-Rich Protein 2 (PfHRP2) in the Brazilian Amazon

Early diagnosis and treatment are fundamental to the control and elimination of malaria. In many endemic areas, routine diagnosis is primarily performed microscopically, although rapid diagnostic tests (RDTs) provide a useful point-of-care tool. Most of the commercially available RDTs detect histidine-rich protein 2 (HRP2) of Plasmodium falciparum in the blood of infected individuals. Nonetheless, parasite isolates lacking the pfhrp2 gene are relatively frequent in some endemic regions, thereby hampering the diagnosis of malaria using HRP2-based RDTs. To track the efficacy of RDTs in areas of the Brazilian Amazon, we assessed pfhrp2 deletions in 132 P. falciparum samples collected from four malaria-endemic states in Brazil. Our findings show low to moderate levels of pfhrp2 deletion in different regions of the Brazilian Amazon. Overall, during the period covered by this study (2002-2020), we found that 10% of the P. falciparum isolates were characterized by a pfhrp2 deletion. Notably, however, the presence of pfhrp2-negative isolates has not been translated into a reduction in RDT efficacy, which in part may be explained by the presence of polyclonal infections. A further important finding was the discrepancy in the proportion of pfhrp2 deletions detected using two assessed protocols (conventional PCR versus nested PCR), which reinforces the need to perform a carefully planned laboratory workflow to assess gene deletion. This is the first study to perform a comprehensive analysis of PfHRP2 sequence diversity in Brazilian isolates of P. falciparum. We identified 10 PfHRP2 sequence patterns, which were found to be exclusive of each of the assessed regions. Despite the small number of PfHRP2 sequences available from South America, we found that the PfHRP2 sequences identified in Brazil and neighboring French Guiana show similar sequence patterns. Our findings highlight the importance of continuously monitoring the occurrence and spread of parasites with pfrhp2 deletions, while also taking into account the limitations of PCR-based testing methods associated with accuracy and the complexity of infections.

Rapid diagnostic test negative Plasmodium falciparum malaria in a traveller returning from Ethiopia

Background

Plasmodium falciparum strains with mutations/deletions of the genes encoding the histidine-rich proteins 2/3 (pfhrp2/3) have emerged during the last 10 years leading to false-negative results in HRP2-based rapid diagnostic tests (RDTs). This can lead to unrecognized infections in individuals and to setbacks in malaria control in endemic countries where RDTs are the backbone of malaria diagnostics and control.

Case description

Here the detection of a pfhrp2/3-negative P. falciparum infection acquired in Ethiopia by a 63-year old female traveller is presented. After onset of symptoms during travel, she was first tested negative for malaria, most probably by RDT, at a local hospital in Harar, Ethiopia. Falciparum malaria was finally diagnosed microscopically upon her return to Germany, over 4 weeks after infection. At a parasite density of approximately 5387 parasites/µl, two different high-quality RDTs: Palutop + 4 OPTIMA, NADAL^RMalaria PF/pan Ag 4 Species, did not respond at their respective P. falciparum test lines. pfhrp2/3 deletion was confirmed by multiplex-PCR. The patient recovered after a complete course of atovaquone and proguanil. According to the travel route, malaria was acquired most likely in the Awash region, Central Ethiopia. This is the first case of imported P. falciparum with confirmed pfhrp2/3 deletion from Ethiopia.

Conclusion

HRP2-negative P. falciparum strains may not be recognized by the presently available HRP2-based RDTs. When malaria is suspected, confirmation by microscopy and/or qPCR is necessary in order to detect falciparum malaria, which requires immediate treatment. This case of imported P. falciparum, non-reactive to HRP2-based RDT, possibly underlines the necessity for standardized, nationwide investigations in Ethiopia and should alert clinicians from non-endemic countries to the possibility of false-negative RDT results which may increase in returning travellers with potentially life-threatening infections.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03678-2.

Detection of high prevalence of Plasmodium falciparum histidine-rich protein 2/3 gene deletions in Assosa zone, Ethiopia: implication for malaria diagnosis

Background

Rapid diagnostic tests (RDTs) targeting histidine rich protein 2(HRP2) are widely used for diagnosis of Plasmodium falciparum infections. Besides PfHRP2, the PfHRP3 antigen contributes to the detection of P. falciparum infections in PfHRP2 RDTs. However, the performance HRP2-based RDT is affected by pfhrp2/3 gene deletions resulting in false-negative test results. The objective of this study was to determine the presence and prevalence of pfhrp2/3 gene deletions including the respective flanking regions among symptomatic patients in Assosa zone, Northwest Ethiopia.

Methods

A health-facility based cross-sectional study was conducted in febrile patients seeking a malaria diagnosis in 2018. Blood samples were collected by finger-prick for microscopic examination of blood smears, malaria RDT, and molecular analysis using dried blood spots (DBS) prepared on Whatman filter paper. A total of 218 P. falciparum positive samples confirmed by quantitative PCR were included for molecular assay of pfhrp2/3 target gene.

Results

Of 218 P. falciparum positive samples, exon 2 deletions were observed in 17.9% of pfhrp2 gene and in 9.2% of pfhrp3 gene. A high proportion of deletions in short segments of pfhrp2 exon1-2 (50%) was also detected while the deletions of the pfhrp3 exon1-2 gene were 4.1%. The deletions were extended to the downstream and upstream of the flanking regions in pfhrp2/3 gene (above 30%). Of eighty-six PfHRP2 RDT negative samples, thirty-six lacked pfhrp2 exon 2. Five PfHRP2 RDT negative samples had double deletions in pfhrp2 exon 2 and pfhrp3 exon2. Of these double deletions, only two of the samples with a parasite density above 2000 parasite/µl were positive by the microscopy. Three samples with intact pfhrp3 exon2 in the pfhrp2 exon2 deleted parasite isolates were found to be positive by PfHRP2 RDT and microscopy with a parasite density above 10,000/µl.

Conclusion

This study confirms the presence of deletions of pfhrp2/3 gene including the flanking regions. Pfhrp2/3 gene deletions results in false-negative results undoubtedly affect the current malaria control and elimination effort in the country. However, further countrywide investigations are required to determine the magnitude of pfhrp2/3 gene deletions and its consequences on routine malaria diagnosis.

Performance evaluation of the highly sensitive histidine-rich protein 2 rapid test for plasmodium falciparum malaria in North-West Tanzania

Background

Precise detection of Plasmodium infections in community surveys is essential for effective malaria control. Microscopy and rapid diagnostic tests (RDTs) are the major techniques used to identify malaria infections in the field-based surveys. Although microscopy is still considered as the gold standard, RDTs are increasingly becoming versatile due to their rapid and adequate performance characteristics.

Methods

A malaria prevalence cross-sectional survey was carried out in north-western Tanzania in 2016, aimed at appraising the performance of high sensitivity Plasmodium falciparum (HSPf) tests compared to SD Bioline Pf and microscopy in detecting P. falciparum infections. A total of 397 individuals aged five years and above were tested for P. falciparum infections. The sensitivity, specificity, positive, and negative predictive values (PPV and NPV) of microscopy, Pf RDT and HSPf RDT was determined using PCR as the gold standard method.

Results

The prevalence of P. falciparum infections determined by microscopy, SD Bioline Pf, HSPf and PCR was 21.9, 27.7, 33.3 and 43.2%, respectively. The new HSPf RDT had significantly higher sensitivity (98.2%) and specificity (91.6%) compared to the routinely used SD Bioline Pf RDT(P < 0.001). The positive predictive value (PPV) was 81.8% and the negative predictive value (NPV) was 99.2% for the routinely used SD Bioline Pf RDT. Moreover, HSPf RDT had sensitivity of 69% and specificity of 76.8% compared to microscopy. The PPV was 45.5% and the NPV was 89.8% for microscopy. Furthermore, the analytical sensitivity test indicated that the newly developed HSPf RDT had lower detection limits compared to routinely used SD Bioline RDT.

Conclusions

HSPf RDT had better performance when compared to both microscopy and the currently used malaria RDTs. The false negativity could be associated with the low parasite density of the samples. False positivity may be related to the limitations of the expertise of microscopists or persistent antigenicity from previous infections in the case of RDTs. Nevertheless, HS PfRDT performed better compared to routinely used Pf RDT, and microscopy in detecting malaria infections. Therefore, HS Pf RDT presents the best alternative to the existing commercial/regularly available RDTs due to its sensitivity and specificity, and reliability in diagnosing malaria infections.

Deletions of pfhrp2 and pfhrp3 genes were uncommon in rapid diagnostic test-negative Plasmodium falciparum isolates from Uganda

BACKGROUND

Rapid diagnostic tests (RDTs) play a key role in malaria case management. The most widely used RDT identifies Plasmodium falciparum based on immunochromatographic recognition of P. falciparum histidine-rich protein 2 (PfHRP2). Deletion of the paralogous pfhrp2 and pfhrp3 genes leads to false-negative PfHRP2-based RDTs, and has been reported in P. falciparum infections from South America and Africa. However, identification of pfhrp2/pfhrp3 deletions has usually been based only on failure to amplify these genes using PCR, without confirmation based on PfHRP2 protein expression, and understanding of the true prevalence of deletions is incomplete.

METHODS

Deletions of pfhrp2/pfhrp3 in blood samples were investigated from cross-sectional surveys in 2012-13 in three regions of varied malaria transmission intensity in Uganda. Samples with positive Giemsa-stained thick blood smears, but negative PfHRP2-based RDTs were evaluated by PCR amplification of conserved subunit ribosomal DNA for Plasmodium species, PCR amplification of pfhrp2 and pfhrp3 genes to identify deletions, and bead-based immunoassays for expression of PfHRP2.

RESULTS

Of 3516 samples collected in cross-sectional surveys, 1493 (42.5%) had positive blood smears, of which 96 (6.4%) were RDT-negative. Of these 96 RDT-negative samples, P. falciparum DNA was identified by PCR in 56 (58%) and only non-falciparum plasmodial DNA in 40 (42%). In all 56 P. falciparum-positive samples there was a failure to amplify pfhrp2 or pfhrp3: in 25 (45%) pfhrp2 was not amplified, in 39 (70%) pfhrp3 was not amplified, and in 19 (34%) neither gene was amplified. For the 39 P. falciparum-positive, RDT-negative samples available for analysis of protein expression, PfHRP2 was not identified by immunoassay in only four samples (10.3%); these four samples all had failure to amplify both pfhrp2 and pfhrp3 by PCR. Thus, only four of 96 (4.2%) smear-positive, RDT-negative samples had P. falciparum infections with deletion of pfhrp2 and pfhrp3 confirmed by failure to amplify the genes by PCR and lack of expression of PfHRP2 demonstrated by immunoassay.

CONCLUSION

False negative RDTs were uncommon. Deletions in pfhrp2 and pfhrp3 explained some of these false negatives, but most false negatives were not due to deletion of the pfhrp2 and pfhrp3 genes.

Disruption of Plasmodium falciparum histidine-rich protein 2 may affect haem metabolism in the blood stage

Background

Haem is a key metabolic factor in the life cycle of the malaria parasite. In the blood stage, the parasite acquires host haemoglobin to generate amino acids for protein synthesis and the by-product haem for metabolic use. The malaria parasite can also synthesize haem de novo on its own. Plasmodium falciparum-specific histidine-rich protein 2 (PfHRP2) has a haem-binding site to mediate the formation of haemozoin, a biocrystallized form of haem aggregates. Notably, the gene regulates the mechanism of haemoglobin-derived haem metabolism and the de novo haem biosynthetic pathway in the Pfhrp2-disrupted parasite line during the intraerythrocytic stages.

Methods

The CRISPR/Cas9 system was used to disrupt the gene locus of Pfhrp2. DNA was extracted from the transgenic parasite, and PCR, Southern blotting and Western blotting were used to confirm the establishment of transgenic parasites. RNA-sequencing and comparative transcriptome analysis were performed to identify differences in gene expression between 3D7 and Pfhrp2^--3D7 parasites.

Results

Pfhrp2^- transgenic parasites were successfully established by the CRISPR/Cas9 system. A total of 964, 1261, 3138, 1064, 2512 and 1778 differentially expressed genes (DEGs) were identified in the six comparison groups, respectively, with 373, 520, 1499, 353, 1253 and 742 of these DEGs upregulated and 591, 741, 1639, 711, 1259 and 1036 of them downregulated, respectively. Five DEGs related to haem metabolism and synthesis were identified in the comparison groups at six time points (0, 8, 16, 24, 32, and 40 h after merozoite invasion). The genes encoding delta-aminolevulinic acid synthetase and ferrochelatase, both related to haem biosynthesis, were found to be significantly upregulated in the comparison groups, and those encoding haem oxygenase, stromal-processing peptidase and porphobilinogen deaminase were found to be significantly downregulated. No GO terms were significantly enriched in haem-related processes (Q value = 1).

Conclusion

Our data revealed changes in the transcriptome expression profile of the Pfhrp2^--3D7 parasite during the intraerythrocytic stages. The findings provide insight at the gene transcript level that will facilitate further research on and development of anti-malaria drugs.

Impact of Plasmodium falciparum gene deletions on malaria rapid diagnostic test performance

Background

Malaria rapid diagnostic tests (RDTs) have greatly improved access to diagnosis in endemic countries. Most RDTs detect Plasmodium falciparum histidine-rich protein 2 (HRP2), but their sensitivity is seriously threatened by the emergence of pfhrp2-deleted parasites. RDTs detecting P. falciparum or pan-lactate dehydrogenase (Pf- or pan-LDH) provide alternatives. The objective of this study was to systematically assess the performance of malaria RDTs against well-characterized pfhrp2-deleted P. falciparum parasites.

Methods

Thirty-two RDTs were tested against 100 wild-type clinical isolates (200 parasites/µL), and 40 samples from 10 culture-adapted and clinical isolates of pfhrp2-deleted parasites. Wild-type and pfhrp2-deleted parasites had comparable Pf-LDH concentrations. Pf-LDH-detecting RDTs were also tested against 18 clinical isolates at higher density (2,000 parasites/µL) lacking both pfhrp2 and pfhrp3.

Results

RDT positivity against pfhrp2-deleted parasites was highest (> 94%) for the two pan-LDH-only RDTs. The positivity rate for the nine Pf-LDH-detecting RDTs varied widely, with similar median positivity between double-deleted (pfhrp2/3 negative; 63.9%) and single-deleted (pfhrp2-negative/pfhrp3-positive; 59.1%) parasites, both lower than against wild-type P. falciparum (93.8%). Median positivity for HRP2-detecting RDTs against 22 single-deleted parasites was 69.9 and 35.2% for HRP2-only and HRP2-combination RDTs, respectively, compared to 96.0 and 92.5% for wild-type parasites. Eight of nine Pf-LDH RDTs detected all clinical, double-deleted samples at 2,000 parasites/µL.

Conclusions

The pan-LDH-only RDTs evaluated performed well. Performance of Pf-LDH-detecting RDTs against wild-type P. falciparum does not necessarily predict performance against pfhrp2-deleted parasites. Furthermore, many, but not all HRP2-based RDTs, detect pfhrp2-negative/pfhrp3-positive samples, with implications for the HRP2-based RDT screening approach for detection and surveillance of HRP2-negative parasites.

Genetic variations in histidine-rich protein 2 and histidine-rich protein 3 of Myanmar Plasmodium falciparum isolates

BACKGROUND

Malaria rapid diagnostic tests (RDTs) are precious tools to diagnose malaria. Most RDTs used currently are based on the detection of Plasmodium falciparum histidine-rich protein 2 (PfHRP2) in a patient's blood. However, concern has been raised in recent years that deletion of pfhrp2 in the parasite could affect the accuracy of PfHRP2-based RDTs. In addition, genetic variation in pfhrp2 might influence the accuracy and sensitivity of RDTs. In this study, the genetic variation in pfhrp2 and pfhrp3 in Myanmar P. falciparum isolates was analysed.

METHODS

Blood samples were collected from malaria patients who were infected with P. falciparum in Mandalay, Naung Cho, Tha Beik Kyin, and Pyin Oo Lwin, Upper Myanmar between 2013 and 2015. The pfhrp2 and pfhrp3 were amplified by nested polymerase chain reaction (PCR), cloned and sequenced. Genetic variation in Myanmar pfhrp2 and pfhrp3 was analysed using the DNASTAR program. Comparative analysis of Myanmar and global pfhrp2 and pfhrp3 isolates was also performed.

RESULTS

One-hundred and two pfhrp2 and 89 pfhrp3 were amplified from 105 blood samples, of which 84 pfhrp2 and 56 pfhrp3 sequences were obtained successfully. Myanmar pfhrp2 and pfhrp3 showed high levels of genetic variation with different arrangements of distinct repeat types, which further classified Myanmar pfhrp2 and pfhrp3 into 76 and 47 haplotypes, respectively. Novel amino acid changes were also found in Myanmar pfhrp2 and pfhrp3, but their frequencies were very low. Similar structural organization was shared by Myanmar and global pfhrp2 and pfhrp3, and differences in frequencies of repeat types and lengths were also observed between and among global isolates.

CONCLUSION

Length polymorphisms and amino acid substitutions generated extensive genetic variation in Myanmar pfhrp2 and pfhrp3. Comparative analysis revealed that global pfhrp2 and pfhrp3 share similar structural features, as well as extensive length polymorphisms and distinct organizations of repeat types. These results provide a better understanding of the genetic structure of pfhrp2 and pfhrp3 in global P. falciparum populations and suggest useful information to develop RDTs with improved quality.

Plasmodium falciparum isolate with histidine-rich protein 2 gene deletion from Nyala City, Western Sudan

In remote areas of malaria-endemic countries, rapid diagnostic tests (RDTs) have dramatically improved parasitological confirmation of suspected malaria cases, especially when skilled microscopists are not available. This study was designed to determine the frequency of Plasmodium falciparum isolates with histidine-rich protein 2 (pfhrp2) gene deletion as one of the possible factors contributing to the failure of PfHRP2-based RDTs in detecting malaria. A total of 300 blood samples were collected from several health centres in Nyala City, Western Sudan. The performance of PfHRP2-based RDTs in relation to microscopy was examined and the PCR-confirmed samples were investigated for the presence of pfhrp2 gene. A total of 113 out of 300 patients were P. falciparum positive by microscopy. Among them, 93.81% (106 out of 113) were positives by the PfHRP2 RDTs. Seven isolates were identified as false negative on the basis of the RDTs results. Only one isolate (0.9%; 1/113) potentially has pfhrp2 gene deletion. The sensitivity and specificity of PfHRP2-based RDTs were 93.81% and 100%, respectively. The results provide insights into the pfhrp2 gene deletion amongst P. falciparum population from Sudan. However, further studies with a large and systematic collection from different geographical settings across the country are needed.

Prevalence of Plasmodium falciparum lacking histidine-rich proteins 2 and 3: a systematic review

Objective

To calculate prevalence estimates and evaluate the quality of studies reporting Plasmodium falciparum lacking histidine-rich proteins 2 and 3, to inform an international response plan.

Methods

We searched five online databases, without language restriction, for articles reporting original data on Plasmodium falciparum-infected patients with deletions of the pfhrp2 and/or pfhrp3 genes (pfhrp2/3). We calculated prevalence estimates of pfhrp2/3 deletions and mapped the data by country. The denominator was all P. falciparum-positive samples testing positive by microscopy and confirmed positive by species-specific polymerase chain reaction testing (PCR). If microscopy was not performed, we used the number of samples based on a different diagnostic method or PCR alone. We scored studies for risk of bias and the quality of laboratory methods using a standardized scoring system.

Findings

A total of 38 articles reporting 55 studies from 32 countries and one territory worldwide were included in the review. We found considerable heterogeneity in the populations studied, methods used and estimated prevalence of P. falciparum parasites with pfhrp2/3 deletions. The derived prevalence of pfhrp2 deletions ranged from 0% to 100%, including focal areas in South America and Africa. Only three studies (5%) fulfilled all seven criteria for study quality.

Conclusion

The lack of representative surveys or consistency in study design impairs evaluations of the risk of false-negative results in malaria diagnosis due to pfhrp2/3 deletions. Accurate mapping and strengthened monitoring of the prevalence of pfhrp2/3 deletions is needed, along with harmonized methods that facilitate comparisons across studies.

Genetic polymorphism of histidine rich protein 2 in Plasmodium falciparum isolates from different infection sources in Yunnan Province, China

Background

Failed diagnoses of some falciparum malaria cases by RDTs are constantly reported in recent years. Plasmodium falciparum histidine-rich protein 2 (pfhpr2) gene deficiency has been found to be the major reason of RDTs failure in many countries. This article analysed the deletion of pfhpr2 gene of falciparum malaria cases isolated in Yunnan Province, China.

Methods

Blood samples from falciparum malaria cases diagnosed in Yunnan Province were collected. Plasmodium genomic DNA was extracted and the pfhrp2 gene exon2 region was amplified via nested PCR. The haplotype of the DNA sequence, the nucleic acid diversity index (PI) and expected heterozygosity (He) were analyzed. Count PfHRP2 amino acid peptide sequence repeat and its times, and predict the properties of PfHRP2 peptide chain reaction to RDTs testing.

Results

A total of 306 blood samples were collected, 84.9% (259/306) from which pfhrp2 PCR amplification products (gene exon2) were obtained, while the remaining 47 samples were false amplification. The length of the 250 DNA sequences ranged from 345 - 927 bp, with 151 haplotypes, with PI and He values of 0.169 and 0.983, respectively. The length of the PfHRP2 peptide chain translated from 250 DNA sequences ranged from 115 to 309 aa. All peptide chains had more than an amino acid codon deletion. All 250 PfHRP2 strands ended with a type 12 amino acid repeat, 98.0% (245/250) started with a type 1 repetition and 2.0% (5/250) with a type 2 repetition. The detection rate for type 2 duplicates was 100% (250/250). Prediction of RDT sensitivity of PfHRP2 peptide chains based on type 2 and type 7 repeats showed that 9.60% (24/250), 50.0% (125/250), 13.20% (33/250) and 27.20.5% (68/250) of the 250 peptide chains were very sensitive, sensitive, borderline and non-sensitive, respectively.

Conclusion

The diversified polymorphism of the pfhrp2 gene deletion from different infection sources in the Yunnan province are extremely complex. The cause of the failure of pfhrp2 exon2 amplification is still to be investigated. The results of this study appeal to Yunnan Province for a timely evaluation of the effectiveness and applicability of RDTs in the diagnosis of malaria.

Molecular surveillance of pfhrp2 and pfhrp3 genes deletion in Plasmodium falciparum isolates and the implications for rapid diagnostic tests in Nigeria

Prompt diagnosis and appropriate treatment of malaria remain the hallmark for reducing malaria-related mortality in high transmission areas. Plasmodium falciparum histidine-rich protein2 (PfHRP2) based rapid diagnostic tests (RDT) play a vital role in prompt and accurate malaria diagnosis. However, pfhrp2 gene deletion threatens the RDT test sensitivity. This study reports the presence of pfhrp2 and pfhrp3 genes deletion among parasite isolates in Nigeria. Febrile children were screened using histidine-rich protein (HRP2) specific RDT (SD-Bioline RDT) and microscopy for P. falciparum infections. All RDT negative samples were re-evaluated by polymerase chain reaction (PCR). The presence of parasite in RDT false negative cases and randomly selected RDT positive cases were validated using PCRs targeting glutamate-rich protein (glurp) and merozoite surface proteins (msp-1 and msp-2). Thereafter, exon 2 of pfhrp2 and pfhrp3 were amplified, and Sanger sequenced. A total of 511 febrile children were enrolled out of which 309 (61%) were positive by RDT. The presence of pfhrp2 and pfhrp3 genes were analyzed in 66 PCR positive samples comprising of 31 RDT false negative and 35 RDT true positive randomly selected samples. The pfhrp2 and pfhrp3 genes failed to amplify in 17% (11/66) and 6% (4/66) samples, respectively. Seven of the eleven samples had only pfhrp2 deletion while four had both pfhrp2 and pfhrp3 deletions. The absence of the pfhrp2 gene may be responsible for the seven RDT false negative cases observed. Three RDT positive cases lacked pfhrp2 whereas pfhrp3 was absent in only four RDT false negative cases. The pfhrp2 and pfhrp3 amino acid repeat sequences were highly diverse. The P. falciparum isolates lacking pfhrp2 and pfhrp3 genes may be circulating and contributing to RDT false negativity in Nigeria. More studies in larger population and seasonally defined cases will be needed to determine the extent of pfhrp2/3 genes deletion in different geographical areas of Nigeria.

Plasmodium falciparum glutamate dehydrogenase is genetically conserved across eight malaria endemic states of India: Exploring new avenues of malaria elimination

Accurate and timely diagnosis is very critical for management, control and elimination of the malaria. Malaria rapid diagnostic tests (RDTs) have improved the diagnosis and management of malaria in remote areas, community and places where microscopy is not available for diagnosis. According to WHO report 2018, Plasmodium falciparum malaria constitutes more than 50% of malaria cases in India. Most of the RDTs used for diagnosis of falciparum malaria today employ HRP2 as a target antigen. However, low density parasitemia and deletion of hrp-2 gene in P. falciparum leads to false negative results and necessitates the development of alternative/ new or improved RDT for malaria diagnosis. We have analysed the genetic diversity and homology modelling of Pfgdh (glutamate dehydrogenase), ldh (lactate dehydrogenase) and aldolase genes in P. falciparum isolates from the eight endemic states of India to assess their potential as antigen for RDT development. We observed negligible sequence diversity in Pfgdh in comparison to the low level of diversity in ldh and aldolase gene. No structural or functional changes were observed in modelling studies and all three genes were under negative purifying selection pressure. The highly conserved nature of pfgdh gene suggests that GDH could be a potential target molecule for Pan/Pf diagnostic test for malaria.

Rapid antigen diversification through mitotic recombination in the human malaria parasite Plasmodium falciparum

Malaria parasites possess the remarkable ability to maintain chronic infections that fail to elicit a protective immune response, characteristics that have stymied vaccine development and cause people living in endemic regions to remain at risk of malaria despite previous exposure to the disease. These traits stem from the tremendous antigenic diversity displayed by parasites circulating in the field. For Plasmodium falciparum, the most virulent of the human malaria parasites, this diversity is exemplified by the variant gene family called var, which encodes the major surface antigen displayed on infected red blood cells (RBCs). This gene family exhibits virtually limitless diversity when var gene repertoires from different parasite isolates are compared. Previous studies indicated that this remarkable genome plasticity results from extensive ectopic recombination between var genes during mitotic replication; however, the molecular mechanisms that direct this process to antigen-encoding loci while the rest of the genome remains relatively stable were not determined. Using targeted DNA double-strand breaks (DSBs) and long-read whole-genome sequencing, we show that a single break within an antigen-encoding region of the genome can result in a cascade of recombination events leading to the generation of multiple chimeric var genes, a process that can greatly accelerate the generation of diversity within this family. We also found that recombinations did not occur randomly, but rather high-probability, specific recombination products were observed repeatedly. These results provide a molecular basis for previously described structured rearrangements that drive diversification of this highly polymorphic gene family.

USAGE OF A RAPID DIAGNOSTIC TEST FOR MALARIA IN CHILDREN

BACKGROUND

Malaria is still the primary cause of pediatric deaths. The efficient management of pediatric malaria requires its rapid and accurate diagnosis. To fulfill this requirement, rapid diagnostic tests have been developed, but their evaluation before commercialization is never exhaustive. The aim of this study was to evaluate the performance of a rapid diagnostic test (SD Bioline Malaria Antigen P.f/Pan) to diagnose malaria in children.

MATERIALS AND METHODS

Testing was conducted on children aged between 6 months and 15 years who were examined at the "Centre Mère Enfant (CME) of the "Chantal Biya" Foundation (FCB). as a result of fever. Enrollment took place from April to October 2014. All children presenting with fever were sampled (3ml of blood). These blood samples were tested for malaria using microscopy on a thick blood smear and by a rapid diagnostic test (RDT) SD Bioline Malariae Antigen P.f/Pan.

RESULTS

A total of 249 children were enrolled in this study. Malaria presence as determined by microscopy and by RDT was 30.9% and 58.2% respectively. The sensitivity, specificity, positive and negative predictive values compared to microscopy were: 75; 48.8; 39, and 81.6%. With these performances, the malaria SD Bioline rapid test presents lower values compared to WHO recommendations for rapid tests (sensitivity > 95%) in children.

CONCLUSION

SD Bioline Malaria Antigen P.f/Pan test should only be used in peripheral health structures that lack resources, and should be aided by clinical diagnosis.

High proportions of pfhrp2 gene deletion and performance of HRP2-based rapid diagnostic test in Plasmodium falciparum field isolates of Odisha

BACKGROUND

With the documentation of cases of falciparum malaria negative by rapid diagnostic tests (RDT), though at low frequency from natural isolates in a small pocket of Odisha, it became absolutely necessary to investigate the status of HRP-2 based RDT throughout the state and in different seasons of the year.

METHODS

Suspected individuals were screened for malaria infection by microscopy and RDT in 25/30 districts of Odisha, India. Discrepancies in results were confirmed by PCR. False negative RDT samples for Plasmodium falciparum mono-infection were evaluated for detection of HRP2 antigen in ELISA and genotyped for pfhrp2, pfhrp3 and their flanking genes. Multiplicity of infection was ascertained based on msp1 and msp2 genotyping and parasitaemia level was determined by microscopy.

RESULTS

Of the total 1058 patients suspected for malaria, 384 were microscopically confirmed for P. falciparum mono-infection and RDT failure was observed in 58 samples at varying proportion in different regions of the state. The failure in detection was due to undetectable level of HRP-2. Although most of these samples were screened during rainy season (45/345), significantly high proportion (9/17) of RDT negative samples were obtained during the summer compared to rainy season (P = 0.0002; OR = 7.5). PCR genotyping of pfhrp2 and pfhrp3 in RDT negative samples showed 38/58 (65.5) samples to be pfhrp2 negative and 24/58 (41.4) to be pfhrp3 negative including dual negative in 17/58 (29.3). Most of the RDT negative samples (39/58) were with single genotype infection and high proportions of pfhrp2 deletion (7/9) was observed in summer. No difference in parasitaemia level was observed between RDT positive and RDT negative patients.

CONCLUSION

High prevalence of parasites with pfhrp2 deletion including dual deletions (pfhrp2 and pfhrp3) is a serious cause of concern, as these patients could not be given a correct diagnosis and treatment. Therefore, HRP2-based RDT for diagnosing P. falciparum infection in Odisha is non-reliable and must be performed in addition to or replaced by other appropriate diagnostic tools for clinical management of the disease.

Genetic diversity and deletion of Plasmodium falciparum histidine-rich protein 2 and 3: a threat to diagnosis of P. falciparum malaria

BACKGROUND

PfHRP2-based rapid diagnostic tests (RDTs), based on the recognition of the Plasmodium falciparum histidine-rich protein 2, are currently the most used tests in malaria detection. Most of the antibodies used in RDTs also detect PfHRP3. However, false-negative results were reported. Significant variation in the pfhrp2 gene could lead to the expression of a modified protein that would no longer be recognized by the antibodies used in PfHRP2-based RDTs. Additionally, parasites lacking the PfHRP2 do not express the protein and are, therefore, not identifiable.

AIMS

This review aims to assess the pfhrp2 and pfhrp3 genetic variation or the prevalence of gene deletion in areas where malaria is endemic and describe its implications on RDT use.

SOURCES

Publications of interest were identified using PubMed, Google Scholar and Google.

CONTENT

More than 18 types of amino acid repeats were identified from the PfHRP2 sequences. Sequencing analysis revealed high-level genetic variation in the pfhrp2 and pfhrp3 genes (>90% of variation in Madagascar, Nigeria or Senegal) both within and between countries. However, genetic variation of PfHRP2 and PfHRP3 does not seem to be a major cause of false-negative results. The countries that showed the highest proportions of pfhrp2-negative parasites were Peru (20%-100%) and Guyana (41%) in South America, Ghana (36%) and Rwanda (23%) in Africa. High prevalence of pfhrp2 deletion causes a high rate of false-negatives results.

IMPLICATIONS

Presence of parasites lacking the pfhrp2 gene may pose a major threat to malaria control programmes because P. falciparum-infected individuals are not diagnosed and properly treated.

Seasonal variations in Plasmodium falciparum parasite prevalence assessed by varying diagnostic tests in asymptomatic children in southern Ghana

Plasmodium falciparum infections presenting either as symptomatic or asymptomatic may contain sexual stage parasites (gametocytes) that are crucial to malaria transmission. In this study, the prevalence of microscopic and submicroscopic asexual and gametocyte parasite stages were assessed in asymptomatic children from two communities in southern Ghana. Eighty children aged twelve years and below, none of whom exhibited signs of clinical malaria living in Obom and Cape Coast were sampled twice, one during the rainy (July 2015) and subsequently during the dry (January 2016) season. Venous blood was used to prepare thick and thin blood smears, spot a rapid malaria diagnostic test (PfHRP2 RDT) as well as prepare filter paper blood spots. Blood cell pellets were preserved in Trizol for RNA extraction. Polymerase chain reaction (PCR) and semi-quantitative real time reverse transcriptase PCR (qRT-PCR) were used to determine submicroscopic parasite prevalence. In both sites 87% (95% CI: 78-96) of the asymptomatic individuals surveyed were parasites positive during the 6 month study period. The prevalence of asexual and gametocyte stage parasites in the rainy season were both significantly higher in Obom than in Cape Coast (P < 0.001). Submicroscopic gametocyte prevalence was highest in the rainy season in Obom but in the dry season in Cape Coast. Parasite prevalence determined by PCR was similar to that determined by qRT-PCR in Obom but significantly lower than that determined by qRT-PCR in Cape Coast. Communities with varying parasite prevalence exhibit seasonal variations in the prevalence of gametocyte carriers. Submicroscopic asymptomatic parasite and gametocyte carriage is very high in southern Ghana, even during the dry season in communities with low microscopic parasite prevalence and likely to be missed during national surveillance exercises.

Global analysis of Plasmodium falciparum histidine-rich protein-2 (pfhrp2) and pfhrp3 gene deletions using whole-genome sequencing data and meta-analysis

Many rapid diagnostic tests (RDT) used on suspected malaria cases are based on the detection of the protein encoded by the Plasmodium falciparum histidine-rich protein-2 (pfhrp2) gene, which shares a high sequence homology with pfhrp3 in the 3D7 reference genome. Parasite isolates showing pfhrp2 and pfhrp3 gene deletions have been emerging over the years, but a comprehensive genetic analysis of these variants is still lacking. With this purpose, genomic data from experimental P. falciparum genetic crosses between different laboratory lines (3D7, HB3, DD2, 7G8 and GB4) were first analysed (n = 98). The frequency of pfhrp2 deletions was consistent with a Mendelian prediction in HB3 × DD2 (56.7%; 95%CI = (39.5%-72.9%)). Moreover, the pfhrp2 and pfhrp3 deletions segregated independently of each other in the same genetic cross. Analysis of 3D7 × HB3 and 7G8 × GB4 estimated the probability of spontaneously generating a pfhrp2 deletion during sexual recombination to be up to 6.2%. Next, whole genome sequence data from 1970 P. falciparum isolates collected globally were analysed. Nine samples displayed depth of coverage consistent with pfhrp2 deletions (0.5%), but the corresponding split-read analysis could not confirm deletions in seven of these samples. Twenty-eight isolates had evidence of pfhrp3 deletions (1.4%), which are widespread in Southeast Asia. Finally, a meta-analysis of published data revealed a positive mean association between the frequencies of pfhrp2 and pfhrp3 deletions in Africa and South America. This result suggested a shared selective pressure acting on these genetic variants. In conclusion, evidence of genetic selection on both pfhrp2 and pfhrp3 deletions was presented, but experimental crosses do not provide evidence of a fitness cost of these variants. Further work is urgently needed to accurately determine the prevalence and the degree of association between these genetic variants, and the respective impact on diagnostic accuracy of many in-use RDT.

Repetitive sequences in malaria parasite proteins

Five species of parasite cause malaria in humans with the most severe disease caused by Plasmodium falciparum. Many of the proteins encoded in the P. falciparum genome are unusually enriched in repetitive low-complexity sequences containing a limited repertoire of amino acids. These repetitive sequences expand and contract dynamically and are among the most rapidly changing sequences in the genome. The simplest repetitive sequences consist of single amino acid repeats such as poly-asparagine tracts that are found in approximately 25% of P. falciparum proteins. More complex repeats of two or more amino acids are also common in diverse parasite protein families. There is no universal explanation for the occurrence of repetitive sequences and it is possible that many confer no function to the encoded protein and no selective advantage or disadvantage to the parasite. However, there are increasing numbers of examples where repetitive sequences are important for parasite protein function. We discuss the diverse roles of low-complexity repetitive sequences throughout the parasite life cycle, from mediating protein-protein interactions to enabling the parasite to evade the host immune system.

Identification of Plasmodium falciparum isolates lacking histidine-rich protein 2 and 3 in Eritrea

The histidine-rich protein 2 of Plasmodium falciparum is the most common malaria antigen targeted by rapid diagnostic tests for the specific diagnosis of P. falciparum. Recently, pfhrp2 gene deletions have been documented in P. falciparum isolates from South America and some multiple endemic countries in Africa and Asia. Parasites with such gene deletions can produce false negative diagnostic results using HRP2-based rapid diagnostic kits. In the present work, the prevalence of P. falciparum parasites lacking pfhrp2, pfhrp3, which produces a second P. falciparum antigen that is recognized by PfHRP2 -based rapid diagnostic tests, and their flanking genes was evaluated in 135 P. falciparum isolates from Gash Barka region and in 9 isolates from Debub region, in Eritrea. In the analyzed samples, 56% (81/144) of isolates were pfhrp2/pfhrp3 positive, while 9.7% (14/144) showed deletion of exon 2 of pfhrp2 gene and 43% (62/144) of isolates lacked the pfhrp3 gene. These results suggest that the pfhrp2 and pfhrp3 deletion phenomenon is present in a considerable proportion in the study areas, thus making the HRP2/3 based rapid diagnostic tests not completely reliable for malaria diagnosis in Eritrea.

Histidine-rich protein 2 (pfhrp2) and pfhrp3 gene deletions in Plasmodium falciparum isolates from select sites in Brazil and Bolivia

More than 80% of available malaria rapid diagnostic tests (RDTs) are based on the detection of histidine-rich protein-2 (PfHRP2) for diagnosis of Plasmodium falciparum malaria. Recent studies have shown the genes that code for this protein and its paralog, histidine-rich protein-3 (PfHRP3), are absent in parasites from the Peruvian Amazon Basin. Lack of PfHRP2 protein through deletion of the pfhrp2 gene leads to false-negative RDT results for P. falciparum. We have evaluated the extent of pfhrp2 and pfhrp3 gene deletions in a convenience sample of 198 isolates from six sites in three states across the Brazilian Amazon Basin (Acre, Rondonia and Para) and 25 isolates from two sites in Bolivia collected at different times between 2010 and 2012. Pfhrp2 and pfhrp3 gene and their flanking genes on chromosomes 7 and 13, respectively, were amplified from 198 blood specimens collected in Brazil. In Brazil, the isolates collected in Acre state, located in the western part of the Brazilian Amazon, had the highest percentage of deletions for pfhrp2 25 (31.2%) of 79, while among those collected in Rondonia, the prevalence of pfhrp2 gene deletion was only 3.3% (2 out of 60 patients). In isolates from Para state, all parasites were pfhrp2-positive. In contrast, we detected high proportions of isolates from all 3 states that were pfhrp3-negative ranging from 18.3% (11 out of 60 samples) to 50.9% (30 out of 59 samples). In Bolivia, only one of 25 samples (4%) tested had deleted pfhrp2 gene, while 68% (17 out of 25 samples) were pfhrp3-negative. Among the isolates tested, P. falciparum pfhrp2 gene deletions were present mainly in those from Acre State in the Brazilian Amazon. These results indicate it is important to reconsider the use of PfHRP2-based RDTs in the western region of the Brazilian Amazon and to implement appropriate surveillance systems to monitor pfhrp2 gene deletions in this and other parts of the Amazon region.

A Field-Tailored Reverse Transcription Loop-Mediated Isothermal Assay for High Sensitivity Detection of Plasmodium falciparum Infections

Highly sensitive and field deployable molecular diagnostic tools are critically needed for detecting submicroscopic, yet transmissible levels of malaria parasites prevalent in malaria endemic countries worldwide. A reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed and evaluated in comparison with thick blood smear microscopy, an antigen-based rapid diagnostic test (RDT), and an in-house RT-PCR targeting the same RT-LAMP transcript. The optimized assay detected Plasmodium falciparum infections in as little as 0.25ng of total parasite RNA, and exhibited a detection limit of 0.08 parasites/ μL when tested directly on infected whole blood lysates, or ~0.0008 parasites/ μL when using RNA extracts. Assay positivity was observed as early as eight minutes from initiation of the RT-LAMP and in most cases the reaction was complete before twenty minutes. Clinical evaluation of the assay on 132 suspected malaria cases resulted in a positivity rate of 90% for RT-LAMP using extracted RNA, and 85% when using whole blood lysates. The positivity rates were 70% for P. falciparum-specific RDT, 83% for RT-PCR, and 74% for thick blood smear microscopy (Mean parasite density = 36,986 parasites/ μL). Concordance rates between the developed RT-LAMP and comparator tests were greater than 75%, the lowest being with light microscopy (78%, McNemar's test: P = 0.0002), and the highest was with RT-PCR (87%, McNemar's test: P = 0.0523). Compared to reference RT-PCR, assay sensitivity was 90% for RT-LAMP on whole blood, and 96% for RT-LAMP using corresponding RNA extracts. Electricity-free heaters were further developed and evaluated in comparison with a battery-operated isothermal amplification machine for use with the developed test in resource-limited settings. Taken together, the data highlight the benefits of targeting high abundant RNA transcripts in molecular diagnosis, as well as the potential usefulness of the developed RT-LAMP-assay in malaria diagnosis in low to high parasite density settings.

Genetic Characterisation of Plasmodium falciparum Isolates with Deletion of the pfhrp2 and/or pfhrp3 Genes in Colombia: The Amazon Region, a Challenge for Malaria Diagnosis and Control

Most Plasmodium falciparum-detecting rapid diagnostic tests (RDTs) target histidine-rich protein 2 (PfHRP2). However, P. falciparum isolates with deletion of the pfhrp2 gene and its homolog gene, pfhrp3, have been detected. We carried out an extensive investigation on 365 P. falciparum dried blood samples collected from seven P. falciparum endemic sites in Colombia between 2003 and 2012 to genetically characterise and geographically map pfhrp2- and/or pfhrp3-negative P. falciparum parasites in the country. We found a high proportion of pfhrp2-negative parasites only in Amazonas (15/39; 38.5%), and these parasites were also pfhrp3-negative. These parasites were collected between 2008 and 2009 in Amazonas, while pfhrp3-negative parasites (157/365, 43%) were found in all the sites and from each of the sample collection years evaluated (2003 to 2012). We also found that all pfhrp2- and/or pfhrp3-negative parasites were also negative for one or both flanking genes. Six sub-population clusters were established with 93.3% (14/15) of the pfhrp2-negative parasites grouped in the same cluster and sharing the same haplotype. This haplotype corresponded with the genetic lineage B_V1, a multidrug resistant strain that caused two outbreaks reported in Peru between 2010 and 2013. We found this B_V1 lineage in the Colombian Amazon as early as 2006. Two new clonal lineages were identified in these parasites from Colombia: the genetic lineages E_V1 and F. PfHRP2 sequence analysis revealed high genetic diversity at the amino acid level, with 17 unique sequences identified among 53 PfHRP2 sequences analysed. The use of PfHRP2-based RDTs is not recommended in Amazonas because of the high proportion of parasites with pfhrp2 deletion (38.5%), and implementation of new strategies for malaria diagnosis and control in Amazonas must be prioritised. Moreover, studies to monitor and genetically characterise pfhrp2-negative P. falciparum parasites in the Americas are warranted, given the extensive human migration occurring in the region.

Recombination and Diversification of the Variant Antigen Encoding Genes in the Malaria Parasite Plasmodium falciparum

The most severe form of human malaria is caused by the protozoan parasite Plasmodium falciparum. These parasites invade and replicate within the circulating red blood cells of infected individuals leading to numerous disease manifestations, including severe anemia, altered circulation, and tissue inflammation. Malaria parasites are also known for their ability to maintain a chronic infection through antigenic variation, the ability to systematically alter the antigens displayed on the surface of infected cells and thereby avoid clearance by the host's antibody response. The genome of P. falciparum includes several large, multicopy gene families that encode highly variable forms of the surface proteins that are the targets of host immunity. Alterations in expression of genes within these families are responsible for antigenic variation. This process requires the continuous generation of new antigenic variants within these gene families, and studies have shown that new variants arise through extensive recombination and gene conversion events between family members. Malaria parasites possess an unusual complement of DNA repair pathways, thus the study of recombination between variant antigen encoding genes provides a unique view into the evolution of mobile DNA in an organism distantly related to the more closely studied model eukaryotes.

Genetic diversity of Plasmodium falciparum histidine-rich protein 2 in the China-Myanmar border area

Deletion of the Plasmodium falciparum histidine-rich protein 2 (pfhrp2) gene may affect the performance of PfHRP2-based rapid diagnostic tests (RDTs). Here we investigated the genetic diversity of the pfhrp2 gene in clinical parasite isolates collected in recent years from the China-Myanmar border area. Deletion of pfhrp2 has been identified in 4 out of 97 parasite isolates. Sequencing of the pfhrp2 exon2 from 67 isolates revealed a high level of genetic diversity in pfhrp2, which is reflected in the presence of many repeat types and their variants, as well as variable copy numbers and different arrangements of these repeats in parasite isolates. In addition, we observed pfhrp3 deletion in three of the four parasites harboring pfhrp2 deletion, suggesting of double deletions of both genes in these three isolates. Analysis of two cases, which were P. falciparum-positive by microscopy and PCR but failed by two PfHRP2-based RDTs, did not find pfhrp2 deletion. Further correlational studies of pfhrp2 polymorphisms with detection sensitivity are needed to identify factors influencing the performance of RDTs in malaria-endemic areas.

Diagnostic performance of rapid diagnostic tests for the diagnosis of malaria at public health facilities in north-west Ethiopia

OBJECTIVE

To assess the performance of RDTs against nested polymerase chain reaction (nPCR) for the diagnosis of malaria in public health facilities in north-western Ethiopia.

METHODS

Cross-sectional study at public health facilities in North Gondar, Ethiopia, of 359 febrile patients with signs and symptoms consistent with malaria. Finger prick blood samples were collected for testing in a P. falciparum/pan-malaria RDTs and for molecular analysis. Sensitivity, specificity and predictive values were determined for the RDTs using nPCR as reference diagnostic method. Kappa value was determined to demonstrate the consistency of the results between the diagnostic tools.

RESULTS

By RDTs, 22.28% (80/359) of patients tested positive for malaria, and by nPCR, 27.02% (97/359) did. In nPCR, 1.67% (6/359) and 0.28% (1/359) samples were positive for P. ovale and P. malariae, which had almost all tested negative in the RDTs. The sensitivity, specificity, positive and negative predictive values of RDTs for the diagnosis of malaria were 62.9%, 92.7%, 76.3% and 87.1%, respectively, with 0.589 measurement agreement between RDTs and nPCR. The sensitivity and specificity of RDTs for P. falciparum identification only were 70.8% and 95.2%, and 65.2% and 93.1% for P. vivax.

CONCLUSION

Although RDTs are commonly used at health posts in resource-limited environments, their sensitivity and specificity for the detection and species identification of Plasmodium parasites were poor compared to nPCR, suggesting caution in interpreting RDTs results. Particularly, in the light of expanded efforts to eliminate malaria in the country, more sensitive diagnostic procedures will be needed.

Malaria Diagnosis Across the International Centers of Excellence for Malaria Research: Platforms, Performance, and Standardization

Diagnosis is “the act of identifying a disease, illness, or problem by examining someone or something.” When an individual with acute fever presents for clinical attention, accurate diagnosis leading to specific, prompt treatment often saves lives. As applied to malaria, not only individual patient diagnosis is important but also assessing population-level malaria prevalence using appropriate diagnostic methods is essential for public health purposes. Similarly, identifying (diagnosing) fake antimalarial medications prevents the use of counterfeit drugs that can have disastrous effects. Therefore, accurate diagnosis in broad areas related to malaria is fundamental to improving health-care delivery, informing funding agencies of current malaria situations, and aiding in the prioritization of regional and national control efforts. The International Centers of Excellence for Malaria Research (ICEMR), supported by the U.S. National Institute of Allergy and Infectious Diseases, has collaborated on global efforts to improve malaria diagnostics by working to harmonize and systematize procedures across different regions where endemicity and financial resources vary. In this article, the different diagnostic methods used across each ICEMR are reviewed and challenges are discussed.

Genetic variation of pfhrp2 in Plasmodium falciparum isolates from Yemen and the performance of HRP2-based malaria rapid diagnostic test

Background

The genetic variation in the Plasmodium falciparum histidine-rich protein 2 (pfhrp2) gene that may compromise the use of pfhrp2-based rapid diagnostic tests (RDTs) for the diagnosis of malaria was assessed in P. falciparum isolates from Yemen.

Methods

This study was conducted in Hodeidah and Al-Mahwit governorates, Yemen. A total of 622 individuals with fever were examined for malaria by CareStart™ malaria HRP2-RDT and Giemsa-stained thin and thick blood films. The Pfhrp2 gene was amplified and sequenced from 180 isolates, and subjected to amino acid repeat types analysis.

Results

A total of 188 (30.2 %) participants were found positive for P. falciparum by the RDT. Overall, 12 different amino acid repeat types were identified in Yemeni isolates. Six repeat types were detected in all the isolates (100 %) namely types 1, 2, 6, 7, 10 and 12 while types 9 and 11 were not detected in any of the isolates. Moreover, the sensitivity and specificity of the used PfHRP2-based RDTs were high (90.5 % and 96.1 %, respectively).

Conclusion

The present study provides data on the genetic variation within the pfhrp2 gene, and its potential impact on the PfHRP2-based RDTs commonly used in Yemen. CareStart™ Malaria HRP2-based RDT showed high sensitivity and specificity in endemic areas of Yemen.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1008-x) contains supplementary material, which is available to authorized users.

Deletion of Plasmodium falciparum Histidine-Rich Protein 2 (pfhrp2) and Histidine-Rich Protein 3 (pfhrp3) Genes in Colombian Parasites

A number of studies have analyzed the performance of malaria rapid diagnostic tests (RDTs) in Colombia with discrepancies in performance being attributed to a combination of factors such as parasite levels, interpretation of RDT results and/or the handling and storage of RDT kits. However, some of the inconsistencies observed with results from Plasmodium falciparum histidine-rich protein 2 (PfHRP2)-based RDTs could also be explained by the deletion of the gene that encodes the protein, pfhrp2, and its structural homolog, pfhrp3, in some parasite isolates. Given that pfhrp2- and pfhrp3-negative P. falciparum isolates have been detected in the neighboring Peruvian and Brazilian Amazon regions, we hypothesized that parasites with deletions of pfhrp2 and pfhrp3 may also be present in Colombia. In this study we tested 100 historical samples collected between 1999 and 2009 from six Departments in Colombia for the presence of pfhrp2, pfhrp3 and their flanking genes. Seven neutral microsatellites were also used to determine the genetic background of these parasites. In total 18 of 100 parasite isolates were found to have deleted pfhrp2, a majority of which (14 of 18) were collected from Amazonas Department, which borders Peru and Brazil. pfhrp3 deletions were found in 52 of the100 samples collected from all regions of the country. pfhrp2 flanking genes PF3D7_0831900 and PF3D7_0831700 were deleted in 22 of 100 and in 1 of 100 samples, respectively. pfhrp3 flanking genes PF3D7_1372100 and PF3D7_1372400 were missing in 55 of 100 and in 57 of 100 samples. Structure analysis of microsatellite data indicated that Colombian samples tested in this study belonged to four clusters and they segregated mostly based on their geographic region. Most of the pfhrp2-deleted parasites were assigned to a single cluster and originated from Amazonas Department although a few pfhrp2-negative parasites originated from the other three clusters. The presence of a high proportion of pfhrp2-negative isolates in the Colombian Amazon may have implications for the use of PfHRP2-based RDTs in the region and may explain inconsistencies observed when PfHRP2-based tests and assays are performed.

Addressing Barriers to the Development and Adoption of Rapid Diagnostic Tests in Global Health

Immunochromatographic rapid diagnostic tests (RDTs) have demonstrated significant potential for use as point-of-care diagnostic tests in resource-limited settings. Most notably, RDTs for malaria have reached an unparalleled level of technological maturity and market penetration, and are now considered an important complement to standard microscopic methods of malaria diagnosis. However, the technical development of RDTs for other infectious diseases, and their uptake within the global health community as a core diagnostic modality, has been hindered by a number of extant challenges. These range from technical and biological issues, such as the need for better affinity agents and biomarkers of disease, to social, infrastructural, regulatory and economic barriers, which have all served to slow their adoption and diminish their impact. In order for the immunochromatographic RDT format to be successfully adapted to other disease targets, to see widespread distribution, and to improve clinical outcomes for patients on a global scale, these challenges must be identified and addressed, and the global health community must be engaged in championing the broader use of RDTs.

Development of monoclonal antibodies against Plasmodium falciparum thioredoxin peroxidase 1 and its possible application for malaria diagnosis

Rapid diagnostic tests (RDTs) have been considered as an ideal alternative for light microscopy to detect malaria parasites especially in remote areas. The development and improvement of RDTs is an area of intensive research in the last decade. To date, few parasite proteins have been targeted in RDTs which are known to have certain deficiencies and made the researchers to look for other promising candidates to address this problem. Plasmodium falciparum thioredoxin peroxidase 1 (PfTPx-1) is abundantly expressed in the cytoplasm of the parasite and well conserved across Plasmodium species, making this antigen a promising target for malaria diagnosis. Several monoclonal antibodies (mAbs) were produced against PfTPx-1. The binding affinities of mAbs were measured. Several immunochromatographic tests (ICTs) were developed using different combination of mAbs. All mAbs showed promising affinities to be used for diagnosis. The sensitivities of ICTs were evaluated using recombinant PfTPx-1 whose results lead us to the preparation of 4 different ICTs. These tests showed positive reaction with P. falciparum in vitro culture supernatant indicating the release of PfTPx-1 during schizont rupture. Altogether, these findings suggest that PfTPx-1 is a promising biomarker to diagnose P. falciparum infection. However, the diagnostic performance of this antigen should be further validated using clinical samples.

Comparison of a PfHRP2-based rapid diagnostic test and PCR for malaria in a low prevalence setting in rural southern Zambia: implications for elimination

Background

Rapid diagnostic tests (RDTs) detecting histidine-rich protein 2 (PfHRP2) antigen are used to identify individuals with Plasmodium falciparum infection even in low transmission settings seeking to achieve elimination. However, these RDTs lack sensitivity to detect low-density infections, produce false negatives for P. falciparum strains lacking pfhrp2 gene and do not detect species other than P. falciparum.

Methods

Results of a PfHRP2-based RDT and Plasmodium nested PCR were compared in a region of declining malaria transmission in southern Zambia using samples from community-based, cross-sectional surveys from 2008 to 2012. Participants were tested with a PfHRP2-based RDT and a finger prick blood sample was spotted onto filter paper for PCR analysis and used to prepare blood smears for microscopy. Species-specific, real-time, quantitative PCR (q-PCR) was performed on samples that tested positive either by microscopy, RDT or nested PCR.

Results

Of 3,292 total participants enrolled, 12 (0.4%) tested positive by microscopy and 42 (1.3%) by RDT. Of 3,213 (98%) samples tested by nested PCR, 57 (1.8%) were positive, resulting in 87 participants positive by at least one of the three tests. Of these, 61 tested positive for P. falciparum by q-PCR with copy numbers ≤ 2 x 10³ copies/μL, 5 were positive for both P. falciparum and Plasmodium malariae and 2 were positive for P. malariae alone. RDT detected 32 (53%) of P. falciparum positives, failing to detect three of the dual infections with P. malariae. Among 2,975 participants enrolled during a low transmission period between 2009 and 2012, sensitivity of the PfHRP2-based RDT compared to nested PCR was only 17%, with specificity of >99%. The pfhrp gene was detected in 80% of P. falciparum positives; however, comparison of copy number between RDT negative and RDT positive samples suggested that RDT negatives resulted from low parasitaemia and not pfhrp2 gene deletion.

Conclusions

Low-density P. falciparum infections not identified by currently used PfHRP2-based RDTs and the inability to detect non-falciparum malaria will hinder progress to further reduce malaria in low transmission settings of Zambia. More sensitive and specific diagnostic tests will likely be necessary to identify parasite reservoirs and achieve malaria elimination.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0544-3) contains supplementary material, which is available to authorized users.

Plasmodium falciparum parasites lacking histidine-rich protein 2 and 3: a review and recommendations for accurate reporting

Malaria rapid diagnostic tests (RDTs) play a critical role in malaria case management, surveillance and case investigations. Test performance is largely determined by design and quality characteristics, such as detection sensitivity, specificity, and thermal stability. However, parasite characteristics such as variable or absent expression of antigens targeted by RDTs can also affect RDT performance. Plasmodium falciparum parasites lacking the PfHRP2 protein, the most common target antigen for detection of P. falciparum, have been reported in some regions. Therefore, accurately mapping the presence and prevalence of P. falciparum parasites lacking pfhrp2 would be an important step so that RDTs targeting alternative antigens, or microscopy, can be preferentially selected for use in such regions. Herein the available evidence and molecular basis for identifying malaria parasites lacking PfHRP2 is reviewed, and a set of recommended procedures to apply for future investigations for parasites lacking PfHRP2, is proposed.

Performance of Paracheck™-Pf, SD Bioline malaria Ag-Pf and SD Bioline malaria Ag-Pf/pan for diagnosis of falciparum malaria in the Central African Republic

Background

Rapid diagnostic tests (RDTs) are the current complement to microscopy for ensuring prompt malaria treatment. We determined the performance of three candidate RDTs (Paracheck™-Pf, SD Bioline malaria Ag-Pf and SD Bioline malaria Ag-Pf/pan) for rapid diagnosis of malaria in the Central African Republic.

Methods

Blood samples from consecutive febrile patients who attended for laboratory analysis of malaria at the three main health centres of Bangui were screened by microscopy and the RDTs. Two reference standards were used to assess the performance of the RDTs: microscopy and, a combination of microscopy plus nested PCR for slides reported as negative, on the assumption that negative results by microscopy were due to sub-patent parasitaemia.

Results

We analysed 436 samples. Using the combined reference standard of microscopy + PCR, the sensitivity of Paracheck™-Pf was 85.7% (95% CI, 80.8–89.8%), that of SD Bioline Ag-Pf was 85.4% (95% CI, 80.5–90.7%), and that of SD Bioline Ag-Pf/pan was 88.2% (95% CI, 83.2–92.0%). The tests performed less well in cases of low parasitaemia; however, the sensitivity was > 95% at > 500 parasites/μl.

Conclusions

Overall, SD Bioline malaria Ag-Pf and SD Bioline malaria Ag-Pf/pan performed slightly better than Paracheck™-Pf. Use of RDTs with reinforced microscopy practice and laboratory quality assurance should improve malaria treatment in the Central African Republic.