Development of a Plasmodium PCR for monitoring efficacy of antimalarial treatment

Oropouche virus cases identified in Ecuador using an optimised qRT-PCR informed by metagenomic sequencing

Oropouche virus (OROV) is responsible for outbreaks of Oropouche fever in parts of South America. We recently identified and isolated OROV from a febrile Ecuadorian patient, however, a previously published qRT-PCR assay did not detect OROV in the patient sample. A primer mismatch to the Ecuadorian OROV lineage was identified from metagenomic sequencing data. We report the optimisation of an qRT-PCR assay for the Ecuadorian OROV lineage, which subsequently identified a further five cases in a cohort of 196 febrile patients. We isolated OROV via cell culture and developed an algorithmically-designed primer set for whole-genome amplification of the virus. Metagenomic sequencing of the patient samples provided OROV genome coverage ranging from 68–99%. The additional cases formed a single phylogenetic cluster together with the initial case. OROV should be considered as a differential diagnosis for Ecuadorian patients with febrile illness to avoid mis-diagnosis with other circulating pathogens.

Oropouche virus (OROV) causes outbreaks of febrile illness in areas of South and Central America and we recently identified it in Ecuador for the first time, using metagenomic sequencing. The genome sequence data revealed that the Ecuadorian strain of the virus was not detected using a published qRT-PCR, as it differed genetically at the binding site of the reverse primer. To address this, we developed a modified qRT-PCR that showed increased sensitivity for the Ecuadorian strain. This test detected OROV infection in 6 out of 196 febrile patients from Esmeraldas, Ecuador in 2016. OROV was isolated from positive patient samples, viral genome sequences were compared to publicly available OROV sequences. This revealed that the Ecuadorian cases are genetically distinct, suggesting that local transmission of the virus should not be ruled out. This work highlights the need for a better understanding of OROV dynamics in Ecuador and surrounding areas, the importance of considering OROV as a cause of fever in Ecuadorian patients and the possibility of selectively using metagenomic sequencing in parallel to traditional molecular techniques in patient testing.

Mixed Infection with Plasmodium falciparum and Plasmodium ovale in a Returned Traveller: the First Case in Korea

Mixed-species malaria infections are often unrecognized or underestimated. We hereby report the first described case of mixed infection with Plasmodium falciparum and Plasmodium ovale malaria in a returned traveller in Korea. In August 2016, a 25-year-old returned traveller from Cameroon and Democratic Republic of Congo presented with fever. He was diagnosed as P. falciparum malaria and successfully treated with artesunate. And 5 weeks after the completion of treatment, he presented with fever and diagnosed as P. ovale infection. P. ovale infection is a rare cause of malaria and often shows delayed presentation due to its dormant liver stage as hypnozoites. At re-presentation, the immunochromatographic test and microscopic examinations of our patient did not reveal P. ovale, which was only detected via polymerase chain reaction (PCR) assay. This case highlights the importance of considering malaria infection even in persons who have previously received malaria treatment. It also shows the usefulness of PCR testing for diagnosing P. ovale infections, which often present with a low level of parasitaemia.

Molecular identification of Plasmodium species in symptomatic children of Democratic Republic of Congo

Background

Worldwide, the highest malaria mortality is due to Plasmodium falciparum infection. However, other species of Plasmodium (Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium knowlesi) can also cause malaria. Therefore, accurate identification of malaria species is crucial for patient management and epidemiological surveillance. This study aimed to determine the different Plasmodium species causing malaria in children under 5 years old in two provinces (Kinshasa and North Kivu) of the Democratic Republic of Congo (DRC).

Methods

From October to December 2015, a health-facility based cross-sectional study was conducted in General Reference Hospitals in Kinshasa and North Kivu. Four hundred and seven blood samples were collected from febrile children aged ≤ 5 years. Nested polymerase chain reaction assays were performed for Plasmodium species identification.

Results

Out of 407 children, 142 (34.9%) were infected with Plasmodium spp. and P. falciparum was the most prevalent species (99.2%). Among those infected children, 124 had a mono infection with P. falciparum and one with P. malariae. Mixed infections with P. falciparum/P. malariae and P. falciparum/P. vivax were observed in 6 (1.5%) and 8 (2.0%) children, respectively. The prevalence of infection was higher in females (64.8%) than in males (35.2%), p < 0.001. The age-specific distribution of infection showed that children of less than 2 years old were less infected (18.4%) compared to those aged above 2 years (81.6%), p < 0.001.

Conclusion

Although this study showed clearly that the most prevalent species identified was P. falciparum, the findings demonstrate the existence of non-falciparum malaria, especially P. malariae and P. vivax among children aged ≤ 5 years living both Kinshasa and North Kivu Provinces in DRC.

[Contribution of nested PCR in the diagnosis of imported malaria in southern Algeria]

The nested PCR was used to estimate its inputs in malaria diagnosis and in the performance of the microscope operators involved in the surveillance of malaria in remote areas of South Algeria. For the period 2010 to 2015, 112 patients (93 febrile and 19 asymptomatic) coming from sub-Saharan Africa were tested for malaria in the hospital of Tamanrasset. One part of the blood taken from fingertip was used for blood smears and the second part was absorbed in filter paper for molecular diagnosis. Overall, the infection was detected by nested PCR in 63 samples versus 53 by direct examination. In addition, 11 mixed infections and 6 positive asymptomatic cases not detected by microscopy were diagnosed by PCR. Moreover, two negative samples in nested PCR were tested positive by direct examination. The molecular tool is more sensitive than the direct examination in detecting infra-microscopic parasitaemia and mixed infections...

Enhanced Sensitivity for Detection of Plasmodium falciparum gametocytes by magnetic nanoparticles combined with enzyme substrate system

The highly sensitive and specific detection of Pfg377 gene of Plasmodium falciparum gametocyte using Magnetic Nanoparticles PCR Enzyme-Linked Gene Assay (MELGA) was successfully developed. The MELGA included amplification of the Pfg377 gene by polymerase chain reaction (PCR) using magnetic nanoparticles (MNPs)-conjugated forward primer and biotinylated reverse primer, followed by post-analytical process using horseradish peroxidase (HRP)-conjugated streptavidin (SA). The complexes of MELGA product were incubated with the peroxidase substrate and hydrogen peroxide to produce the signal for colorimetric measurement. Altogether, the MELGA technique provided a highly sensitive and specific detection at 1 P. falciparum gametocyte/µL, which was more efficient than that of microscopic examination and rapid diagnostic tests (RDTs). Additionally, the MELGA could detect target gene at femtogram level, which was greater sensitive than the conventional PCR, nested PCR and loop-mediated isothermal amplification (LAMP). The MELGA technique could become a novel and practical method that overcome limitation of traditional gametocyte detection.

Comparison of the Performances of Five Primer Sets for the Detection and Quantification of Plasmodium in Anopheline Vectors by Real-Time PCR

Quantitative real-time polymerase chain reaction (qrtPCR) has made a significant improvement for the detection of Plasmodium in anopheline vectors. A wide variety of primers has been used in different assays, mostly adapted from molecular diagnosis of malaria in human. However, such an adaptation can impact the sensitivity of the PCR. Therefore we compared the sensitivity of five primer sets with different molecular targets on blood stages, sporozoites and oocysts standards of Plasmodium falciparum (Pf) and P. vivax (Pv). Dilution series of standard DNA were used to discriminate between methods at low concentrations of parasite and to generate standard curves suitable for the absolute quantification of Plasmodium sporozoites. Our results showed that the best primers to detect blood stages were not necessarily the best ones to detect sporozoites. Absolute detection threshold of our qrtPCR assay varied between 3.6 and 360 Pv sporozoites and between 6 and 600 Pf sporozoites per mosquito according to the primer set used in the reaction mix. In this paper, we discuss the general performance of each primer set and highlight the need to use efficient detection methods for transmission studies.

External quality assurance of malaria nucleic acid testing for clinical trials and eradication surveillance

Nucleic acid testing (NAT) for malaria parasites is an increasingly recommended diagnostic endpoint in clinical trials of vaccine and drug candidates and is also important in surveillance of malaria control and elimination efforts. A variety of reported NAT assays have been described, yet no formal external quality assurance (EQA) program provides validation for the assays in use. Here, we report results of an EQA exercise for malaria NAT assays. Among five centers conducting controlled human malaria infection trials, all centers achieved 100% specificity and demonstrated limits of detection consistent with each laboratory's pre-stated expectations. Quantitative bias of reported results compared to expected results was generally <0.5 log₁₀ parasites/mL except for one laboratory where the EQA effort identified likely reasons for a general quantitative shift. The within-laboratory variation for all assays was low at <10% coefficient of variation across a range of parasite densities. Based on this study, we propose to create a Molecular Malaria Quality Assessment program that fulfills the need for EQA of malaria NAT assays worldwide.

Malaria diagnostics in clinical trials

Malaria diagnostics are widely used in epidemiologic studies to investigate natural history of disease and in drug and vaccine clinical trials to exclude participants or evaluate efficacy. The Malaria Laboratory Network (MLN), managed by the Office of HIV/AIDS Network Coordination, is an international working group with mutual interests in malaria disease and diagnosis and in human immunodeficiency virus/acquired immunodeficiency syndrome clinical trials. The MLN considered and studied the wide array of available malaria diagnostic tests for their suitability for screening trial participants and/or obtaining study endpoints for malaria clinical trials, including studies of HIV/malaria co-infection and other malaria natural history studies. The MLN provides recommendations on microscopy, rapid diagnostic tests, serologic tests, and molecular assays to guide selection of the most appropriate test(s) for specific research objectives. In addition, this report provides recommendations regarding quality management to ensure reproducibility across sites in clinical trials. Performance evaluation, quality control, and external quality assessment are critical processes that must be implemented in all clinical trials using malaria tests.

Increased detection of Plasmodium knowlesi in Sandakan division, Sabah as revealed by PlasmoNex™

BACKGROUND

Plasmodium knowlesi is a simian malaria parasite that is widespread in humans in Malaysian Borneo. However, little is known about the incidence and distribution of this parasite in the Sandakan division, Malaysian Borneo. Therefore, the aim of the present epidemiological study was to investigate the incidence and distribution of P. knowlesi as well as other Plasmodium species in this division based on a most recent developed hexaplex PCR system (PlasmoNex™).

METHODS

A total of 189 whole blood samples were collected from Telupid Health Clinic, Sabah, Malaysia, from 2008 to 2011. All patients who participated in the study were microscopically malaria positive before recruitment. Complete demographic details and haematological profiles were obtained from 85 patients (13 females and 72 males). Identification of Plasmodium species was conducted using PlasmoNex™ targeting the 18S ssu rRNA gene.

RESULTS

A total of 178 samples were positive for Plasmodium species by using PlasmoNex™. Plasmodium falciparum was identified in 68 samples (38.2%) followed by 64 cases (36.0%) of Plasmodium vivax, 42 (23.6%) cases of P. knowlesi, two (1.1%) cases of Plasmodium malariae and two (1.1%) mixed-species infections (i e, P. vivax/P. falciparum). Thirty-five PlasmoNex™ positive P. knowlesi samples were misdiagnosed as P. malariae by microscopy. Plasmodium knowlesi was detected in all four districts of Sandakan division with the highest incidence in the Kinabatangan district. Thrombocytopaenia and anaemia showed to be the most frequent malaria-associated haematological complications in this study.

CONCLUSIONS

The discovery of P. knowlesi in Sandakan division showed that prospective studies on the epidemiological risk factors and transmission dynamics of P. knowlesi in these areas are crucial in order to develop strategies for effective malaria control. The availability of advanced diagnostic tool PlasmoNex™ enhanced the accuracy and accelerated the speed in the diagnosis of malaria.

Stability of gametocyte-specific Pfs25-mRNA in dried blood spots on filter paper subjected to different storage conditions

BACKGROUND

Real-time quantitative nucleic acid sequence-based amplification (QT-NASBA) is a sensitive method for detection of sub-microscopic gametocytaemia by measuring gametocyte-specific mRNA. Performing analysis on fresh whole blood samples is often not feasible in remote and resource-poor areas. Convenient methods for sample storage and transport are urgently needed.

METHODS

Real-time QT-NASBA was performed on whole blood spiked with a dilution series of purified in-vitro cultivated gametocytes. The blood was either freshly processed or spotted on filter papers. Gametocyte detection sensitivity for QT-NASBA was determined and controlled by microscopy. Dried blood spot (DBS) samples were subjected to five different storage conditions and the loss of sensitivity over time was investigated. A formula to approximate the loss of Pfs25-mRNA due to different storage conditions and time was developed.

RESULTS

Pfs25-mRNA was measured in time to positivity (TTP) and correlated well with the microscopic counts and the theoretical concentrations of the dilution series. TTP results constantly indicated higher amounts of RNA in filter paper samples extracted after 24 hours than in immediately extracted fresh blood. Among investigated storage conditions freezing at -20°C performed best with 98.7% of the Pfs25-mRNA still detectable at day 28 compared to fresh blood samples. After 92 days, the RNA detection rate was only slightly decreased to 92.9%. Samples stored at 37°C showed most decay with only 64.5% of Pfs25-mRNA detectable after one month. The calculated theoretical detection limit for 24 h-old DBS filter paper samples was 0.0095 (95% CI: 0.0025 to 0.0380) per μl.

CONCLUSIONS

The results suggest that the application of DBS filter papers for quantification of Plasmodium falciparum gametocytes with real-time QT-NASBA is practical and recommendable. This method proved sensitive enough for detection of sub-microscopic densities even after prolonged storage. Decay rates can be predicted for different storage conditions as well as durations.

Real-time quantitative reverse transcription PCR for monitoring of blood-stage Plasmodium falciparum infections in malaria human challenge trials

To detect pre-patent parasitemia, we developed a real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR) for the asexual 18S ribosomal RNA (rRNAs) of Plasmodium falciparum. Total nucleic acids extracted from whole blood were combined with control RNA and tested by qRT-PCR. The assay quantified > 98.7% of parasite-containing samples to ±0.5 log(10) parasites/mL of the nominal value without false positives. The analytical sensitivity was ≥ 20 parasites/mL. The coefficient of variation was 0.6% and 1.8% within runs and 1.6% and 4.0% between runs for high and low parasitemia specimens, respectively. Using this assay, we determined that A-type 18S rRNAs are stably expressed at 1 × 10(4) copies per ring-stage parasite. When used to monitor experimental P. falciparum infection of human volunteers, the assay detected blood-stage infections 3.7 days earlier on average than thick blood smears. This validated, internally controlled qRT-PCR method also uses a small (50 μL) sample volume requiring minimal pre-analytical handling, making it useful for clinical trials.

Evaluation of three PCR-based diagnostic assays for detecting mixed Plasmodium infection

Background

One of the most commonly used molecular test for malaria diagnosis is the polymerase chain reaction (PCR)-based amplification of the 18S ribosomal DNA (rDNA) gene. Published diagnostic assays based on the 18S gene include the "gold standard" nested assay, semi-nested multiplex assay, and one tube multiplex assay. To our knowledge, no one has reported whether the two multiplex methods are better at detecting mixed Plasmodium infections compared to the nested assay using known quantities of DNA in experimentally mixed cocktails.

Findings

Here we evaluated three PCR assays (nested, semi-nested multiplex, and one-tube multiplex) for the simultaneous detection of human malaria parasites using experimentally mixed cocktails of known quantities of laboratory derived DNA. All three assays detected individual species with high sensitivity and specificity when DNA was from any one single species; however, experimentally mixed DNA cocktails with all four species present were correctly identified most consistently with the nested method. The other two methods failed to consistently identify all four species correctly, especially at lower concentrations of DNA -subclinical levels of malaria (DNA equivalent to or less than 10 parasites per microliter).

Conclusions

The nested PCR method remains the method of choice for the detection of mixed malaria infections and especially of sub-clinical infections. Further optimization and/or new molecular gene targets may improve the success rate of detecting multiple parasite species simultaneously using traditional PCR assays.

Application of molecular methods for monitoring transmission stages of malaria parasites

Recent technical advances in malaria research have allowed specific detection of mRNA of genes that are expressed exclusively in sexual stages (gametocytes) of malaria parasites. The specificity and sensitivity of these techniques were validated on cultured laboratory clones of both human malaria parasites (Plasmodium falciparum) and rodent parasites (P. chabaudi). More recently, quantitative molecular techniques have been developed to quantify these sexual stages and used to monitor gametocyte dynamics and their transmission to mosquitoes. Molecular techniques showed that the infectious reservoir for malaria is larger than expected from previous microscopic studies; individual parasite genotypes within an infection can simultaneously produce infectious gametocytes; gametocyte production can be sustained for several months, and is modulated by environmental factors. The above techniques have empowered approaches for in-depth analysis of the biology of the transmission stages of the parasite and epidemiology of malaria transmission.

Real-time PCR versus conventional PCR for malaria parasite detection in low-grade parasitemia

We have optimized a faster and cheaper real-time PCR and developed a conventional genus specific PCR based on 18S rRNA gene to detect malaria parasites in low-grade parasitemias. Additionally, we compared these PCRs to the OptiMAL-IT test. Since there is no consensus on choice of standard quantitative curve in real-time assays, we decided to investigate the performance of parasite DNA from three different sources: "genome", amplicon and plasmid. The amplicon curve showed the best efficiency in quantifying parasites. Both PCR assays detected 100% of the clinical samples tested; the sensitivity threshold was 0.5 parasite/mul and no PCR positive reaction occurred when malaria parasites were not present. Conversely, if OptiMAL-IT were employed for malaria diagnosis, 30% of false-negative results could be expected. We conclude that PCR assays have potential for detecting malaria parasites in asymptomatic infections, in evaluation of malaria vaccine molecule candidates, for screening blood donors, especially in endemic areas, or even in monitoring malaria therapy.

An update on malaria prevention, diagnosis and treatment for the returning traveller

The diagnosis of malaria needs to be considered for every returning traveller with a fever. Compliance with prevention, both pharmaceutical and non-pharmaceutical, is essential for every traveller. New tests for diagnosis are now available. Treatment options have recently expanded to include the artemisinin derivatives that used to be unavailable in the western countries.

Use of molecular beacon probes for real-time PCR detection of Plasmodium falciparum and other plasmodium species in peripheral blood specimens

We describe the development and evaluation of a novel pair of real-time, fluorescence-based PCR assays using molecular beacon probes for rapid, sensitive, and specific detection and quantification of Plasmodium falciparum and other Plasmodium species organisms.

In vitro antimalarial activity of flavonoid derivatives dehydrosilybin and 8-(1;1)-DMA-kaempferide

Multidrug-resistant Plasmodium falciparum strains are an increasing problem in endemic areas and are partly responsible for the worsening malaria situation around the world. New cheap and effective compounds active in combination with available drug in the field are urgently needed. The aim of this work was to explore the potential antiplasmodial effect of flavonoid derivatives on parasites growth in vitro. In vitro antiplasmodial activity of dehydrosilybin and 8-(1;1)-DMA-kaempferide has been evaluated by real time PCR for five P. falciparum strains. Both revealed significative antimalarial activity against the different strains. Since this drug family has been largely used and well-tolerated in humans, flavonoid derivatives could be in the near future associated with already available drugs in order to delay the spread of P. falciparum resistance.

Real-time PCR for detection and identification of Plasmodium spp

Rapid and accurate detection of malaria parasites in blood is needed to institute proper therapy. We developed and used a real-time PCR assay to detect and distinguish four Plasmodium spp. that cause human disease by using a single amplification reaction and melting curve analysis. Consensus primers were used to amplify a species-specific region of the multicopy 18S rRNA gene, and SYBR Green was used for detection in a LightCycler instrument. Patient specimens infected at 0.01 to 0.02% parasitemia densities were detected, and analytical sensitivity was estimated to be 0.2 genome equivalent per reaction. Melting curve analysis based on nucleotide variations within the amplicons provided a basis for accurate differentiation of Plasmodium falciparum, P. vivax, P. ovale, and P. malariae. For assay validation, 358 patient blood samples from the National University Hospital in Singapore and Evanston Northwestern Healthcare in Illinois were analyzed. Of 76 blinded patient samples with a microscopic diagnosis of P. falciparum, P. vivax, or P. ovale infection, 74 (97.4%) were detected by real-time PCR, including three specimens containing mixed P. falciparum-P. vivax infections. No Plasmodium DNA was amplified in any of the 82 specimens sent for malaria testing but that were microscopically negative for Plasmodium infection. In addition, 200 blood samples from patients whose blood was collected for reasons other than malaria testing were also determined to be negative by real-time PCR. Real-time PCR with melting curve analysis could be a rapid and objective supplement to the examination of Giemsa-stained blood smears and may replace microscopy following further validation.

Real-time nucleic acid sequence-based amplification is more convenient than real-time PCR for quantification of Plasmodium falciparum

Determination of the number of malaria parasites by routine or even expert microscopy is not always sufficiently sensitive for detailed quantitative studies on the population dynamics of Plasmodium falciparum, such as intervention or vaccine trials. To circumvent this problem, two more sensitive assays, real-time quantitative nucleic acid sequence-based amplification (QT-NASBA) and real-time quantitative PCR (QT-PCR) were compared for quantification of P. falciparum parasites. QT-NASBA was adapted to molecular beacon real-time detection technology, which enables a reduction of the time of analysis and of contamination risk while retaining the specificity and sensitivity of the original assay. Both QT-NASBA and QT-PCR have a sensitivity of 20 parasites/ml of blood, but QT-PCR requires a complicated DNA extraction procedure and the use of 500 microl of venous blood to achieve this sensitivity, compared to 50 microl of finger prick blood for real-time QT-NASBA. Both techniques show a significant correlation to microscopic parasite counts, and the quantification results of the two real-time assays are significantly correlated for in vitro as well as in vivo samples. However, in comparison to real-time QT-PCR, the results of real-time QT-NASBA can be obtained 12 h earlier, with relatively easy RNA extraction and use of finger prick blood samples. The prospective development of multiplex QT-NASBA for detection of various P. falciparum developmental stages increases the value of QT-NASBA for malaria studies. Therefore, for studies requiring sensitive and accurate detection of P. falciparum parasites in large numbers of samples, the use of real-time QT-NASBA is preferred over that of real-time QT-PCR.

Detection of four Plasmodium species in blood from humans by 18S rRNA gene subunit-based and species-specific real-time PCR assays

There have been reports of increasing numbers of cases of malaria among migrants and travelers. Although microscopic examination of blood smears remains the "gold standard" in diagnosis, this method suffers from insufficient sensitivity and requires considerable expertise. To improve diagnosis, a multiplex real-time PCR was developed. One set of generic primers targeting a highly conserved region of the 18S rRNA gene of the genus Plasmodium was designed; the primer set was polymorphic enough internally to design four species-specific probes for P. falciparum, P. vivax, P. malarie, and P. ovale. Real-time PCR with species-specific probes detected one plasmid copy of P. falciparum, P. vivax, P. malariae, and P. ovale specifically. The same sensitivity was achieved for all species with real-time PCR with the 18S screening probe. Ninety-seven blood samples were investigated. For 66 of them (60 patients), microscopy and real-time PCR results were compared and had a crude agreement of 86% for the detection of plasmodia. Discordant results were reevaluated with clinical, molecular, and sequencing data to resolve them. All nine discordances between 18S screening PCR and microscopy were resolved in favor of the molecular method, as were eight of nine discordances at the species level for the species-specific PCR among the 31 samples positive by both methods. The other 31 blood samples were tested to monitor the antimalaria treatment in seven patients. The number of parasites measured by real-time PCR fell rapidly for six out of seven patients in parallel to parasitemia determined microscopically. This suggests a role of quantitative PCR for the monitoring of patients receiving antimalaria therapy.

Development of a single tube hemi-nested PCR for genus-specific detection of Plasmodium in oligoparasitemic patients

Primers targeting the Plasmodium small-subunit (SSU) rDNA were designed to amplify DNA from P. vivax, P. falciparum, P. malariae, and P. ovale, using conventional PCR, two-step nested PCR (HNPCR), and single tube hemi-nested PCR (STHNPCR). The limit of detection of parasite DNA for the conventional PCR, HNPCR, and STHNPCR were 10 pg, 0.01 pg, and 0.1 pg, respectively, indicating that the STHNPCR is 100-fold more sensitive than conventional PCR, and only 10 times less sensitive than HNPCR. In addition, the detection limit was also defined using blood from a patient infected with P. falciparum. Using the saponin method, the detection limit of the conventional PCR, HNPCR, and STHNPCR were 70, 0.7, and 0.07 parasites/microl, respectively. Finally, the three techniques were evaluated using blood from 30 patients receiving antimalarial treatment, and negative by microscopy and conventional PCR. The HNPCR could still detect specific DNA in 16/30 patients, whereas STHNPCR detected parasite DNA in 10/30 patients, but the difference was not statistically significant. No significant correlation was found between presence of clinical manifestations and presence of parasite DNA, detected by either HNPCR or STHNPCR. We conclude that these sensitive molecular diagnostic systems can be used for the diagnosis of asymptomatic oligoparasitemic patients.

Simultaneous identification of the four human Plasmodium species and quantification of Plasmodium DNA load in human blood by real-time polymerase chain reaction

The incidence of imported malaria cases in travellers returning from endemic areas has considerably increased over the last few years. The microscopical examination of stained blood films is the gold standard method to confirm clinical suspicion of malaria but diagnosis is difficult in the case of mixed infections, low-grade parasitaemia, or forms altered by uncompleted treatment. We have developed a real-time polymerase chain reaction (PCR) for the simultaneous identification of the 4 human Plasmodium spp. and quantification of Plasmodium DNA in human blood. The rapid turnaround and reduction in the risk of PCR product carryover are major advantages compared with conventional PCR. In combination with conventional tests, this method could be a powerful tool for the diagnosis of malaria infections among travellers from endemic areas and during the follow-up of patients in reference centres involved in travel and tropical medicine. Quantitative real-time PCR could also be used for the follow-up of patients during drug resistance studies managed by national malaria programmes, the testing of new drugs, and vaccine trials.

Diagnosis of malaria aided by polymerase chain reaction in two cases with low-level parasitaemia

Light microscopy of thick and thin blood smears is the mainstay of malaria diagnosis. In situations of low-level parasitaemia such as drug-modified disease, however, this may be difficult making clinical management problematic. Polymerase chain reaction (PCR) methods have shown high sensitivity for the diagnosis of malaria and are able to differentiate the Plasmodium species involved. Two cases are presented in the present study, which illustrate how a PCR method can aid light microscopic malaria diagnosis and species differentiation in returned travellers with low-level parasitaemia. Plasmodium vivax was detected by PCR prior to the light microscopy becoming positive in one case, and in the second case Plasmodium malariae was detected when light microscopy was unable to speciate the causative Plasmodium species.

Comparative assessment of conventional PCR with multiplex real-time PCR using SYBR Green I detection for the molecular diagnosis of imported malaria

For the diagnosis of imported malaria, optical or immunochromatographic methods are known to be less sensitive and less specific than PCR-based methods, which are conversely more complicated and time-consuming. An original strategy, based upon the sequential use of a multiplex competitive real-time PCR detecting Plasmodium falciparum or Plasmodium spp. infection, followed by, if necessary, a single real-time PCR for species identification, was therefore performed and then tested versus conventional PCR in routine conditions. Conventional PCR has been used since October 1999 in the Department of Parasitology, University Hospitals in Toulouse, as a 2nd line diagnostic method. Out of 183 patients tested, 48 were found to be harbouring a falciparum infection by conventional microscopy, 60 by conventional PCR and 60 by multiplex competitive real-time PCR. Nine further patients had a non-falciparum infection, and concordant species identifications were obtained by both conventional PCR and single real-time PCR. The major value of PCR-based methods, when compared to microscopical techniques, was to ascertain the negativity of a suspect sample. Moreover, real-time PCR allows simplification of the operating procedure, with a diagnosis being made within 2 h.

Real-time PCR for chloroquine sensitivity assay and for pfmdr1-pfcrt single nucleotide polymorphisms in Plasmodium falciparum

Plasmodium falciparum drug resistance is a major problem in malaria endemic areas. Molecular markers and in vitro tests have been developed to study and monitor drug resistance. However, none, used alone, can provide sufficient data concerning the level of drug resistance and to issue precise guidelines for drug use policies in endemic areas. We propose real-time PCR for the simultaneous detection of pfcrt and pfmdr1 genes mutations and to determine the half-maximal inhibitory response (IC(50)) of antimalarial drug. Using hybridization probes and SybrGreen technology on LightCycler instrument, point mutations of pfcrt and pfmdr1 genes have been successfully detected in 161 human blood samples and determination of IC values was applied to chloroquine-sensitive and chloroquine-resistant strains. Moreover, mixed infections caused by P. falciparum clones with wild-type or mutant alleles could be efficiency separated. The aim of this study was not to provide definitive data concerning the rate of mutations in an endemic area, but to describe a powerful method allowing the quantification of DNA for IC(50) determination and the detection of major pfmdr1 and pfcrt mutations.

A multiplex polymerase chain reaction for a differential diagnosis of Plasmodium falciparum and Plasmodium vivax

A multiplex PCR was designed for the differential diagnosis of the two parasite species by targeting the 18S rRNA gene with a set of primer combinations, amplifying DNA fragments of 1451-bp and 833-bp for P. falciparum and P. vivax, respectively. The sensitivity of this PCR test was high, as minimal as 0.1 parasite per one microliter of blood sample and a minimum of four copies of the target gene could be detected. For the diagnosis of mixed infection of two Plasmodium spp., there were no apparent competition or cross-reaction between the majority and minority Plasmodium species. The multiplex PCR was evaluated on 210 clinical samples and 60 normal controls. The PCR test yielded highly concordant results with microscopic examination, with the only one exception of a mixed (P. falciparum plus P. vivax) infection case, which was diagnosed as a single infection of P. falciparum by microscopy. We propose that the multiplex PCR is a sensitive, specific, and rapid tool that can serve as a useful differential diagnostic tool for detecting P. falciparum and P. vivax.

Use of a rapid, single-round, multiplex PCR to detect malarial parasites and identify the species present

A new, rapid assay, based on a single-round, multiplex PCR, can be used to detect Plasmodium falciparum, P. vivax, P. malariae or P. ovale in human blood. The PCR, which targets the conserved 18S small-subunit RNA genes of the parasites, not only permits a malarial infection to be detected but also allows each Plasmodium species present to be identified, even in cases of mixed infection.

[Diagnosis of imported malaria with multiplex PCR on LighCycler apparatus]

For the diagnosis of imported malaria, a competitive multiplex PCR using LightCycler was developed and compared for 3 months with a traditional PCR method. Hundred eighty three patients were tested by these 2 techniques of molecular diagnosis: 60 were positive for P. falciparum, 9 were positive for other Plasmodium species and 114 were negative. The LightCycler method was found in total agreement with the traditional PCR.

How useful is PCR in the diagnosis of malaria?

Polymerase chain reaction (PCR) assays are the most sensitive and specific method to detect malaria parasites, and have acknowledged value in research settings. However, the time lag between sample collection, transportation and processing, and dissemination of results back to the physician limits the usefulness of PCR in routine clinical practice. Furthermore, in most areas with malaria transmission, factors such as limited financial resources, persistent subclinical parasitaemia, inadequate laboratory infrastructures in the poorer, remote rural areas preclude PCR as a diagnostic method. Even in affluent, non-endemic countries, PCR is not a suitable method for routine use. Nonetheless, PCR could be clinically useful in selected situations.

Potential value of Plasmodium falciparum-associated antigen and antibody detection for screening of blood donors to prevent transfusion-transmitted malaria

BACKGROUND

Malaria antibody detection is a valuable tool in the prevention of transfusion-transmitted malaria in countries with a high proportion of donors with travel exposure to malaria. The immunofluorescent antibody test (IFAT) is still the reference method, but it is not suitable for screening of blood donors. ELISA would be an interesting alternative to the IFAT, but it lacks sensitivity.

STUDY DESIGN AND METHODS

To evaluate the potential value of a combined screening strategy based on malaria antigen and antibody detection, plasma samples from 203 patients infected with Plasmodium falciparum were tested with an ELISA for the detection of malaria antibodies (Malaria IgG CELISA, Cellabs) and a P. falciparum histidine-rich protein-2 kit (Malaria P.f., ICT Diagnostics) for the detection of malaria antigens.

RESULTS

Among patients with positive IFAT results, CELISA had a sensitivity of 71 percent, whereas the combined screening tests (CELISA and Malaria P.f.) had a sensitivity of 88 percent (p < 0.001). Sequential samples from 50 patients were tested. The combined screening tests shortened the detection of seroconversion from 11.4 +/- 1.6 to 5.3 +/- 1.1 days (p < 0.001).

CONCLUSION

Combined malaria antigen and antibody detection, with methods compatible with mass screening, may constitute an attractive alternative to the IFAT for blood donor screening.

Comparison of a nested polymerase chain reaction – restriction fragment length polymorphism method, the PATH antigen detection method, and microscopy for the detection and identification of malaria parasites

A nested polymerase chain reaction – restriction fragment length polymorphism (PCR–RFLP) method, the PATH antigen detection method, and light microscopy were compared for their capacity to detect and identify Plasmodium species. One hundred and thirty-six blood specimens obtained from patients suspected of having malaria were examined by each of the three methods. Forty-four specimens were positive for malaria using microscopy as the "gold standard". The sensitivity for nested PCR was 100%, and the specificity was 98%. For the detection of Plasmodium falciparum, the antigen detection method had a sensitivity of 100% and a specificity of 97%. Species identification obtained using PCR–RFLP was identical or superior to light microscopy in 42 cases (96%). Although the nested PCR–RFLP method was more sensitive and specific, the rapid turnaround time and high sensitivity of the antigen detection method makes it a useful adjunct to standard microscopy.Key words: malaria, PCR–RFLP, antigen detection.

PCR-based ELISA technique for malaria diagnosis of specimens from Thailand

We performed a field evaluation of polymerase chain reaction (PCR)-based enzyme-linked immuno-sorbent assays (ELISA) for the diagnosis of malaria. A commercially available PCR-ELISA microplate hybridization (MPH) assay was used. Blood specimens were collected from 300 volunteers seeking care at malaria clinics in Thailand. Examination of 200 high power fields by Giemsa-stained thick and thin smear (GTTS) revealed 51 P. falciparum (Pf), 45 P. vivax (Pv), seven mixed Pf-Pv infections. These plus a random sample of 48 GTTS-negative specimens were selected for this study. All 151 specimens were processed for parasite DNA extraction and assayed by PCR-MPH. The target DNA sequence of the 18S small subunit ribosomal RNA (SSUrRNA) gene was amplified by PCR and hybridized with species-specific probes for Pf, Pv, P. malariae (Pm) and P. ovale (Po) immobilized in the wells of the microtiter plate and detected by colorimetric assay. Colour development was assessed at an optical density (OD) of 405 nm. An absorbance reading of > or = 0.1 was used as a positive cut-off. In comparison with GTTS results, PCR-MPH sensitivity was 91.4% (53/58, 95% CI 84.2-98.6) for Pf, 94.2% (49/52, 87.9-100) for Pv and specificity was 95.8% (46/48, 95% CI 90.2-100). There was statistically significant positive correlation between parasite densities < or = 7000/microl blood and absorbance reading, suggestive of PCR-MPH being semiquantitative. PCR-MPH also detected additional Pf and Pv cases as well as Pm and Po.

Detection and quantification of Plasmodium falciparum in blood samples using quantitative nucleic acid sequence-based amplification

A quantitative nucleic acid sequence-based amplification (QT-NASBA) assay for the detection of Plasmodium parasites has been developed. Primers and probes were selected on the basis of the sequence of the small-subunit rRNA gene. Quantification was achieved by coamplification of the RNA in the sample with one modified in vitro RNA as a competitor in a single-tube NASBA reaction. Parasite densities ranging from 10 to 10(8) Plasmodium falciparum parasites per ml could be demonstrated and quantified in whole blood. This is approximately 1,000 times more sensitive than conventional microscopy analysis of thick blood smears. Comparison of the parasite densities obtained by microscopy and QT-NASBA with 120 blood samples from Kenyan patients with clinical malaria revealed that for 112 of 120 (93%) of the samples results were within a 1-log difference. QT-NASBA may be especially useful for the detection of low parasite levels in patients with early-stage malaria and for the monitoring of the efficacy of drug treatment.