Mature beyond their years: young children who escape detection of parasitemia despite living in settings of intense malaria transmission

Despite having the highest risk of progressing to severe disease due to lack of acquired immunity, the youngest children living in areas of highly intense malaria transmission have long been observed to be infected at lower rates than older children. Whether this observation is due to reduced exposure to infectious mosquito bites from behavioral and biological factors, maternally transferred immunity, genetic factors, or enhanced innate immunity in the young child has intrigued malaria researchers for over half a century. Recent evidence suggests that maternally transferred immunity may be limited to early infancy and that the young child's own immune system may contribute to control of malarial symptoms early in life and prior to the development of more effective adaptive immunity. Prospective studies of active and passive detection of Plasmodium falciparum blood-stage infections have identified young children (<5 years old) who remain uninfected through a defined surveillance period despite living in settings of highly intense malaria transmission. Yet, little is known about the potential immunological basis for this 'aparasitemic' phenotype. In this review, we summarize the observational evidence for this phenotype in field studies and examine potential reasons why these children escape detection of parasitemia, covering factors that are either extrinsic or intrinsic to their developing immune system. We discuss the challenges of distinguishing malaria protection from lack of malaria exposure in field studies. We also identify gaps in our knowledge regarding cellular immunity in the youngest age group and propose directions that researchers may take to address these gaps.

Low Prevalence of Antimalarial Resistance Mutations in India During 2014-2015: Impact of Combining First-line Therapy With Primaquine

BACKGROUND

Antimalarial drug resistance surveillance and containment are crucial for countries aiming to eliminate malaria. Monitoring resistance evolution through studies before and after treatment policy changes is essential.

METHODS

A total of 939 Plasmodium falciparum-positive blood samples were collected between 2014 and 2015 across 10 sites in India, categorized into 4 geographic clusters. Polymerase chain reaction-amplified products were sequenced to identify point mutations at drug resistance-conferring genes (Pfdhfr, Pfdhps, Pfmdr1, and Pfk13).

RESULTS

Triple Pfdhfr mutants were found only in northeast India bordering Myanmar, while the wild type (WT) was dominant in central India. Pfdhps WTs were prevalent in all areas, and no double mutants were found. Except in northwest India, Pfmdr1 WT was dominant in all clusters. Nonsynonymous double mutations were only found in northwest India. Only synonymous mutations occurred in Pfk13. These were found in central India at a low frequency. The pattern of linkage disequilibrium and principal component analysis reflects low pressure for drug resistance and heterogeneity between the geographic clusters.

CONCLUSIONS

Resistance levels were highest in northeast India, close to the Myanmar border, where resistance is common. Primaquine has been widely used as a gametocidal and schizonticidal drug, likely contributing to maintaining low drug resistance levels and preventing strong selection for resistance.

Malaria Species Positivity Rates Among Symptomatic Individuals Across Regions of Differing Transmission Intensities in Mainland Tanzania

BACKGROUND

Recent data indicate that non-Plasmodium falciparum species may be more prevalent than thought in sub-Saharan Africa. Although Plasmodium malariae, Plasmodium ovale spp., and Plasmodium vivax are less severe than P. falciparum, treatment and control are more challenging, and their geographic distributions are not well characterized.

METHODS

We randomly selected 3284 of 12 845 samples collected from cross-sectional surveys in 100 health facilities across 10 regions of Mainland Tanzania and performed quantitative real-time PCR to determine presence and parasitemia of each malaria species.

RESULTS

P. falciparum was most prevalent, but P. malariae and P. ovale were found in all but 1 region, with high levels (>5%) of P. ovale in 7 regions. The highest P. malariae positivity rate was 4.5% in Mara and 8 regions had positivity rates ≥1%. We only detected 3 P. vivax infections, all in Kilimanjaro. While most nonfalciparum malaria-positive samples were coinfected with P. falciparum, 23.6% (n = 13 of 55) of P. malariae and 14.7% (n = 24 of 163) of P. ovale spp. were monoinfections.

CONCLUSIONS

P. falciparum remains by far the largest threat, but our data indicate that malaria elimination efforts in Tanzania will require increased surveillance and improved understanding of the biology of nonfalciparum species.

Risk of Malaria Following Untreated Subpatent Plasmodium falciparum Infections: Results Over 4 Years From a Cohort in a High-Transmission Area in Western Kenya

BACKGROUND

People with suspected malaria may harbor Plasmodium falciparum undetected by rapid diagnostic test (RDT). The impact of these subpatent infections on the risk of developing clinical malaria is not fully understood.

METHODS

We analyzed subpatent P. falciparum infections using a longitudinal cohort in a high-transmission site in Kenya. Weighted Kaplan-Meier models estimated the risk difference (RD) for clinical malaria during the 60 days following a symptomatic subpatent infection. Stratum-specific estimates by age and transmission season assessed modification.

RESULTS

Over 54 months, we observed 1128 symptomatic RDT-negative suspected malaria episodes, of which 400 (35.5%) harbored subpatent P. falciparum. Overall, the 60-day risk of developing clinical malaria was low following all episodes (8.6% [95% confidence interval, 6.7%-10.4%]). In the low-transmission season, the risk of clinical malaria was slightly higher in those with subpatent infection, whereas the opposite was true in the high-transmission season (low-transmission season RD, 2.3% [95% confidence interval, .4%-4.2%]; high-transmission season RD, -4.8% [-9.5% to -.05%]).

CONCLUSIONS

The risk of developing clinical malaria among people with undetected subpatent infections is low. A slightly elevated risk in the low-transmission season may merit alternate management, but RDTs identify clinically relevant infections in the high-transmission season.

Subclinical inflammation in asymptomatic schoolchildren with Plasmodium falciparum parasitemia correlates with impaired cognition

BACKGROUND

Subclinical inflammation and cognitive deficits have been separately associated with asymptomatic Plasmodium falciparum infections in schoolchildren. However, whether parasite-induced inflammation is associated with worse cognition has not been addressed. We conducted a cross-sectional pilot study to better assess the effect of asymptomatic P. falciparum parasitemia and inflammation on cognition in Kenyan schoolchildren.

METHODS

We enrolled 240 children aged 7-14 years residing in high malaria transmission in Western Kenya. Children performed five fluid cognition tests from a culturally adapted NIH toolbox and provided blood samples for blood smears and laboratory testing. Parasite densities and plasma concentrations of 14 cytokines were determined by quantitative PCR and multiplex immunoassay, respectively. Linear regression models were used to determine the effects of parasitemia and plasma cytokine concentrations on each of the cognitive scores as well as a composite cognitive score while controlling for age, gender, maternal education, and an interaction between age and P. falciparum infection status.

RESULTS

Plasma concentrations of TNF, IL-6, IL-8, and IL-10 negatively correlated with the composite score and at least one of the individual cognitive tests. Parasite density in parasitemic children negatively correlated with the composite score and measures of cognitive flexibility and attention. In the adjusted model, parasite density and TNF, but not P. falciparum infection status, independently predicted lower cognitive composite scores. By mediation analysis, TNF significantly mediated ~29% of the negative effect of parasitemia on cognition.

CONCLUSION

Among schoolchildren with PCR-confirmed asymptomatic P. falciparum infections, the negative effect of parasitemia on cognition could be mediated, in part, by subclinical inflammation. Additional studies are needed to validate our findings in settings of lower malaria transmission and address potential confounders that could affect both inflammation and cognitive performance.

RTS,S/AS02A Malaria Vaccine-Induced IgG Responses Equally Recognize Native-Like Fucosylated and Nonfucosylated Plasmodium falciparum Circumsporozoite Proteins

The RTS,S/AS02A malaria vaccine is based on the Plasmodium falciparum circumsporozoite protein (PfCSP), which is O-fucosylated on the sporozoite surface. We determined whether RTS,S/AS02A-induced immunoglobulin G (IgG) antibodies recognize vaccine-like nonfucosylated PfCSP better than native-like fucosylated PfCSP. Similar to previous vaccine trials, RTS,S/AS02A vaccination induced high anti-PfCSP IgG levels associated with malaria protection. IgG recognition of nonfucosylated and fucosylated PfCSP was equivalent, suggesting that PfCSP fucosylation does not affect antibody recognition. Clinical Trials Registration. NCT00197041.

Treatment Failure in a UK Malaria Patient Harboring Genetically Variant Plasmodium falciparum From Uganda With Reduced In Vitro Susceptibility to Artemisinin and Lumefantrine

BACKGROUND

Recent cases of clinical failure in malaria patients in the United Kingdom (UK) treated with artemether-lumefantrine have implications for malaria chemotherapy worldwide.

METHODS

Parasites were isolated from an index case of confirmed Plasmodium falciparum treatment failure after standard treatment, and from comparable travel-acquired UK malaria cases. Drug susceptibility in vitro and genotypes at 6 resistance-associated loci were determined for all parasite isolates and compared with clinical outcomes for each parasite donor.

RESULTS

A traveler, who returned to the UK from Uganda in 2022 with Plasmodium falciparum malaria, twice failed treatment with full courses of artemether-lumefantrine. Parasites from the patient exhibited significantly reduced susceptibility to artemisinin (ring-stage survival, 17.3% [95% confidence interval {CI}, 13.6%-21.1%]; P < .0001) and lumefantrine (effective concentration preventing 50% of growth = 259.4 nM [95% CI, 130.6-388.2 nM]; P = .001). Parasite genotyping identified an allele of pfk13 encoding both the A675V variant in the Pfk13 propeller domain and a novel L145V nonpropeller variant. In vitro susceptibility testing of 6 other P. falciparum lines of Ugandan origin identified reduced susceptibility to artemisinin and lumefantrine in 1 additional line, also from a 2022 treatment failure case. These parasites did not harbor a pfk13 propeller domain variant but rather the novel nonpropeller variant T349I. Variant alleles of pfubp1, pfap2mu, and pfcoronin were also identified among the 7 parasite lines.

CONCLUSIONS

We confirm, in a documented case of artemether-lumefantrine treatment failure imported from Uganda, the presence of pfk13 mutations encoding L145V and A675V. Parasites with reduced susceptibility to both artemisinin and lumefantrine may be emerging in Uganda.

Recombinant full-length Plasmodium falciparum circumsporozoite protein-based vaccine adjuvanted with GLA-LSQ: Results of Phase 1 testing with malaria challenge

INTRODUCTION

Malaria is preventable yet causes >600,000 deaths annually. RTS, S, the first marketed malaria vaccine, has modest efficacy, but improvements are needed for eradication.

METHODS

We conducted an open-label, dose escalation Phase 1 study of a recombinant, full-length circumsporozoite protein vaccine (rCSP) administered with adjuvant GLA-LSQ on days 1, 29, and 85 or 1 and 490 to healthy, malaria-naïve adults. Primary endpoints were safety and reactogenicity. Secondary endpoints were antibody responses and Plasmodium falciparum parasitemia after homologous controlled human malaria infection (CHMI).

RESULTS

Participants were enrolled into four groups receiving rCSP/GLA-LSQ: 10 µg x 3 (n = 20), 30 µg x 3 (n = 10), 60 µg x 3 (n = 10) or 60 µg x 2 (n = 9); ten participants received 30 µg rCSP alone x 3; and six infectivity controls. Participants experienced no serious adverse events. Rates of solicited and unsolicited adverse events were similar among groups. All 26 participants who underwent CHMI 28 days after final vaccinations developed malaria. Increasing vaccine doses induced higher IgG titers, but did not achieve previously established RTS, S benchmarks.

CONCLUSIONS

rCSP/GLA-LSQ had favorable safety results. However, tested regimens did not induce protective immunity. Further investigation could assess if adjuvant or schedule adjustments improve efficacy.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier NCT03589794.

Artesunate-mefloquine therapy for uncomplicated Plasmodium falciparum malaria: an updated systematic review and meta-analysis of efficacy and safety

To provide a continuous update on the safety and efficacy of artesunate-mefloquine (ASMQ) compared with other artemisinin combination therapy (ACT) schemes used in the treatment of uncomplicated malaria caused by Plasmodium falciparum, this study updated and expanded the results of the systematic literature review published in 2016. Only randomised controlled clinical trials published from 1 January 2001 to 12 June 2023 from five databases were included in this study. The results related to efficacy, expressed through RR, were summarized in meta-analyses, performed according to the compared ACTs and with the intention-to-treat and per-protocol analyses. The results related to safety were synthesized in a descriptive manner. Thirty-two studies were included, of which 24 had been analysed in the 2016 review and eight new ones were added. Although the methodological quality of most studies was considered moderate, the body of evidence gathered indicates that ASMQ continues to be safe and effective for the treatment of uncomplicated infections caused by P. falciparum compared with other ACTs. However, the inclusion of two new studies, which identified failure rates exceeding 10%, suggests a possible reduction in the efficacy of ASMQ in the analysed locations. The incidence of serious adverse effects, such as seizure, encephalopathy and cardiac arrhythmia, was infrequent in both the ASMQ group and the comparison groups. After including new evidence, ASMQ is still recommended as a first-line treatment of uncomplicated malaria caused by P. falciparum, although local aspects need to be considered.

The Genome of Plasmodium gonderi: Insights into the Evolution of Human Malaria Parasites

Plasmodium species causing malaria in humans are not monophyletic, sharing common ancestors with nonhuman primate parasites. Plasmodium gonderi is one of the few known Plasmodium species infecting African old-world monkeys that are not found in apes. This study reports a de novo assembled P. gonderi genome with complete chromosomes. The P. gonderi genome shares codon usage, syntenic blocks, and other characteristics with the human parasites Plasmodium ovale s.l. and Plasmodium malariae, also of African origin, and the human parasite Plasmodium vivax and species found in nonhuman primates from Southeast Asia. Using phylogenetically aware methods, newly identified syntenic blocks were found enriched with conserved metabolic genes. Regions outside those blocks harbored genes encoding proteins involved in the vertebrate host-Plasmodium relationship undergoing faster evolution. Such genome architecture may have facilitated colonizing vertebrate hosts. Phylogenomic analyses estimated the common ancestor between P. vivax and an African ape parasite P. vivax-like, within the Asian nonhuman primates parasites clade. Time estimates incorporating P. gonderi placed the P. vivax and P. vivax-like common ancestor in the late Pleistocene, a time of active migration of hominids between Africa and Asia. Thus, phylogenomic and time-tree analyses are consistent with an Asian origin for P. vivax and an introduction of P. vivax-like into Africa. Unlike other studies, time estimates for the clade with Plasmodium falciparum, the most lethal human malaria parasite, coincide with their host species radiation, African hominids. Overall, the newly assembled genome presented here has the quality to support comparative genomic investigations in Plasmodium.

Tafenoquine-Atovaquone Combination Achieves Radical Cure and Confers Sterile Immunity in Experimental Models of Human Babesiosis

Human babesiosis is a potentially fatal tick-borne disease caused by intraerythrocytic Babesia parasites. The emergence of resistance to recommended therapies highlights the need for new and more effective treatments. Here we demonstrate that the 8-aminoquinoline antimalarial drug tafenoquine inhibits the growth of different Babesia species in vitro, is highly effective against Babesia microti and Babesia duncani in mice and protects animals from lethal infection caused by atovaquone-sensitive and -resistant B. duncani strains. We further show that a combination of tafenoquine and atovaquone achieves cure with no recrudescence in both models of human babesiosis. Interestingly, elimination of B. duncani infection in animals following drug treatment also confers immunity to subsequent challenge. Altogether, the data demonstrate superior efficacy of tafenoquine plus atovaquone combination over current therapies for the treatment of human babesiosis and highlight its potential in providing protective immunity against Babesia following parasite clearance.

The association of intraleucocytic malaria pigment and disease severity in Papua New Guinean children with severe P. falciparum malaria

BACKGROUND

Plasmodium falciparum pigment-containing leucocytes (PCLs) are associated with adverse clinical manifestations of severe malaria in African children. However, limited data exist on the association of PCLs in settings outside of Africa.

METHODS

Thin films on peripheral blood slides obtained from children ages 6 months-10 y with severe malaria were examined for PCLs. The intraleucocytic pigment data were correlated with clinical phenotypic data such as severe anaemia, metabolic acidosis and coma to determine the association of PCLs with clinical phenotypes of severe malaria and outcome.

RESULTS

Of the 169 children with severe P. falciparum malaria confirmed by microscopy, 76% (129/169) had PCLs. Compared with children without PCLs, the presence (adjusted odds ratio [AOR] 3.2 [95% confidence interval {CI} 1.5 to 6.9], p≤0.01) and quantity (AOR 1.0 [95% CI 1.0 to 1.1], p=0.04) of pigment-containing monocytes (PCMs) was significantly associated with severe anaemia, while the quantity of both PCMs (AOR 1.0 [95% CI 1.0 to 1.1], p≤0.01) and pigment-containing neutrophils (AOR 1.0 [95% CI 1.0 to 1.1], p=0.01) was significantly associated with metabolic acidosis. Plasma P. falciparum histidine-rich protein-2 level negatively correlated with the platelet count (r=-0.5, p≤0.01) in patients with PCLs and no PCLs.

CONCLUSIONS

In Papua New Guinean children with severe P. falciparum malaria, the presence and quantity of PCLs are predictors of disease severity, severe anaemia and metabolic acidosis.

Time-resolved proximity biotinylation implicates a porin protein in export of transmembrane malaria parasite effectors

The malaria-causing parasite, Plasmodium falciparum completely remodels its host red blood cell (RBC) through the export of several hundred parasite proteins, including transmembrane proteins, across multiple membranes to the RBC. However, the process by which these exported membrane proteins are extracted from the parasite plasma membrane for export remains unknown. To address this question, we fused the exported membrane protein, skeleton binding protein 1 (SBP1), with TurboID, a rapid, efficient and promiscuous biotin ligase (SBP1TbID). Using time-resolved proximity biotinylation and label-free quantitative proteomics, we identified two groups of SBP1TbID interactors - early interactors (pre-export) and late interactors (post-export). Notably, two promising membrane-associated proteins were identified as pre-export interactors, one of which possesses a predicted translocon domain, that could facilitate the export of membrane proteins. Further investigation using conditional mutants of these candidate proteins showed that these proteins were essential for asexual growth and localize to the host-parasite interface during early stages of the intraerythrocytic cycle. These data suggest that they might play a role in ushering membrane proteins from the parasite plasma membrane for export to the host RBC.

Severe Falciparum and Vivax Malaria on the Thailand-Myanmar Border: A Review of 1503 Cases

BACKGROUND

The northwestern border of Thailand is an area of low seasonal malaria transmission. Until recent successful malaria elimination activities, malaria was a major cause of disease and death. Historically the incidences of symptomatic Plasmodium falciparum and Plasmodium vivax malaria were approximately similar.

METHODS

All malaria cases managed in the Shoklo Malaria Research Unit along the Thailand-Myanmar border between 2000 and 2016 were reviewed.

RESULTS

There were 80 841 consultations for symptomatic P. vivax and 94 467 for symptomatic P. falciparum malaria. Overall, 4844 (5.1%) patients with P. falciparum malaria were admitted to field hospitals, of whom 66 died, compared with 278 (0.34%) with P. vivax malaria, of whom 4 died (3 had diagnoses of sepsis, so the contribution of malaria to their fatal outcomes is uncertain). Applying the 2015 World Health Organization severe malaria criteria, 68 of 80 841 P. vivax admissions (0.08%) and 1482 of 94 467 P. falciparum admissions (1.6%) were classified as severe. Overall, patients with P. falciparum malaria were 15 (95% confidence interval, 13.2-16.8) times more likely than those with P. vivax malaria to require hospital admission, 19 (14.6-23.8) times more likely to develop severe malaria, and ≥14 (5.1-38.7) times more likely to die.

CONCLUSIONS

In this area, both P. falciparum and P. vivax infections were important causes of hospitalization, but life-threatening P. vivax illness was rare.

The Rapid and Spontaneous Postpartum Clearance of Plasmodium falciparum Is Related to Expulsion of the Placenta

Parasitemia among pregnant women with protective immunity to Plasmodium falciparum malaria is often dominated by VAR2CSA-positive infected erythrocytes (IEs). VAR2CSA mediates sequestration of IEs in the placenta. We hypothesized that the previously observed spontaneous postpartum clearance of parasitemia in such women is related to the expulsion of the placenta, which removes the sequestration focus of VAR2CSA-positive IEs. We assessed parasitemias and gene transcription before and shortly after delivery in 17 Ghanaian women. The precipitous decline in parasitemia postpartum was accompanied by selective reduction in transcription of the gene encoding VAR2CSA. Our findings provide a mechanistic explanation for the earlier observation.

Characterizing the Blood-Stage Antimalarial Activity of Tafenoquine in Healthy Volunteers Experimentally Infected With Plasmodium falciparum

BACKGROUND

The long-acting 8-aminoquinoline tafenoquine may be a good candidate for mass drug administration if it exhibits sufficient blood-stage antimalarial activity at doses low enough to be tolerated by glucose 6-phosphate dehydrogenase (G6PD)-deficient individuals.

METHODS

Healthy adults with normal levels of G6PD were inoculated with Plasmodium falciparum 3D7-infected erythrocytes on day 0. Different single oral doses of tafenoquine were administered on day 8. Parasitemia and concentrations of tafenoquine and the 5,6-orthoquinone metabolite in plasma/whole blood/urine were measured and standard safety assessments performed. Curative artemether-lumefantrine therapy was administered if parasite regrowth occurred, or on day 48 ± 2. Outcomes were parasite clearance kinetics, pharmacokinetic and pharmacokinetic/pharmacodynamic (PK/PD) parameters from modelling, and dose simulations in a theoretical endemic population.

RESULTS

Twelve participants were inoculated and administered 200 mg (n = 3), 300 mg (n = 4), 400 mg (n = 2), or 600 mg (n = 3) tafenoquine. The parasite clearance half-life with 400 mg or 600 mg (5.4 hours and 4.2 hours, respectively) was faster than with 200 mg or 300 mg (11.8 hours and 9.6 hours, respectively). Parasite regrowth occurred after dosing with 200 mg (3/3 participants) and 300 mg (3/4 participants) but not after 400 mg or 600 mg. Simulations using the PK/PD model predicted that 460 mg and 540 mg would clear parasitaemia by a factor of 106 and 109, respectively, in a 60-kg adult.

CONCLUSIONS

Although a single dose of tafenoquine exhibits potent P. falciparum blood-stage antimalarial activity, the estimated doses to effectively clear asexual parasitemia will require prior screening to exclude G6PD deficiency. Clinical Trials Registration. Australian and New Zealand Clinical Trials Registry (ACTRN12620000995976).

A diagnostic accuracy study to evaluate standard rapid diagnostic test (RDT) alone to safely rule out imported malaria in children presenting to UK emergency departments

BACKGROUND

Microscopy is the gold standard for malaria diagnosis but is dependent on trained personnel. Rapid diagnostic tests (RDT) form the mainstay of diagnosis in endemic areas without access to high quality microscopy. We aimed to evaluate whether RDT alone could rule out imported malaria in children presenting to UK Emergency Departments (EDs).

METHODS

UK-based, multi-centre, retrospective, diagnostic accuracy study. Included: any child <16 years presenting to ED with history of fever and travel to a malaria-endemic country, between 01/01/2016 and 31/12/2017. Diagnosis: microscopy for malarial parasites (clinical reference standard) and RDT (index test). UK Health Research Authority approval: 20/HRA/1341.

RESULTS

There were 47 cases of malaria out of 1,414 eligible cases (prevalence 3·3%) in a cohort of children whose median age was 4 years (IQR 2-9), of whom 43% were female. Cases of Plasmodium falciparum totalled 36 (77%, prevalence 2·5% ).The sensitivity of RDT alone to detect malaria infection due to any Plasmodium species was 93·6% (95% CI 82·5% to 98·7%), specificity 99·4% (95% CI 98·9% to 99·7%), positive predictive value 84·6% (95% CI 71·9% to 93·1%) and negative predictive value 99·8% (95% CI 99·4% to 100·0%). Sensitivity of RDT to detect P. falciparum infection was 100% (90·3% to 100%), specificity 98·8% (98·1% to 99·3%), positive predictive value 69·2% (54·9% to 81·2%, n=46/52) and negative predictive value 100% (99·7% to 100%, n=1362/1362).

CONCLUSIONS

RDTs were 100% sensitive in detecting P. falciparum malaria. However, lower sensitivity for other malaria species and the rise of pfhrp2 and pfhrp3 (pfhrp2/3) gene deletions in the P. falciparum parasite mandate the continued use of microscopy for diagnosing malaria.

Population genomic evidence of adaptive response during the invasion history of Plasmodium falciparum in the Americas

Plasmodium falciparum, the most virulent agent of human malaria, spread from Africa to all continents following the out-of-Africa human migrations. During the transatlantic slave trade between the 16th and 19th centuries, it was introduced twice independently to the Americas where it adapted to new environmental conditions (new human populations and mosquito species). Here, we analyzed the genome-wide polymorphisms of 2,635 isolates across the current P. falciparum distribution range in Africa, Asia, Oceania, and the Americas to investigate its genetic structure, invasion history, and selective pressures associated with its adaptation to the American environment. We confirmed that American populations originated from Africa with at least two independent introductions that led to two genetically distinct clusters, one in the North (Haiti and Colombia) and one in the South (French Guiana and Brazil), and the admixed Peruvian group. Genome scans revealed recent and more ancient signals of positive selection in the American populations. Particularly, we detected positive selection signals in genes involved in interactions with hosts (human and mosquito) cells and in genes involved in resistance to malaria drugs in both clusters. We found that some genes were under selection in both clusters. Analyses suggested that for five genes, adaptive introgression between clusters or selection on standing variation was at the origin of this repeated evolution. This study provides new genetic evidence on P. falciparum colonization history and on its local adaptation in the Americas.

Return to Travel in the Coronavirus Disease 2019 Pandemic Recovery Period and Implications for Imported Malaria: Reinforcing Prevention, Early Diagnosis, and Appropriate Treatment of Malaria

Return to international travel in the COVID-19 pandemic recovery period is expected to increase the number of patients with imported malaria in the United States (US). Malaria prevention in travelers and preparedness for timely diagnosis and appropriate treatment are key to minimize imported malaria morbidity and mortality. Intravenous artesunate (IVAS) is now available from commercial distributors in the US for the treatment of severe malaria. Hospitals and pharmacists should have a plan for malaria treatment, including stocking artemether-lumefantrine for uncomplicated malaria, and stocking or planning for rapid procurement of IVAS for the treatment of severe malaria.

Malaria Transmission Dynamics in a High-Transmission Setting of Western Kenya and the Inadequate Treatment Response to Artemether-Lumefantrine in an Asymptomatic Population

BACKGROUND

Assessing the infectious reservoir is critical in malaria control and elimination strategies. We conducted a longitudinal epidemiological study in a high-malaria-burden region in Kenya to characterize transmission in an asymptomatic population.

METHODS

488 study participants encompassing all ages in 120 households within 30 clusters were followed for 1 year with monthly sampling. Malaria was diagnosed by microscopy and molecular methods. Transmission potential in gametocytemic participants was assessed using direct skin and/or membrane mosquito feeding assays, then treated with artemether-lumefantrine. Study variables were assessed using mixed-effects generalized linear models.

RESULTS

Asexual and sexual parasite data were collected from 3792 participant visits, with 903 linked with feeding assays. Univariate analysis revealed that the 6-11-year-old age group was at higher risk of harboring asexual and sexual infections than those <6 years old (odds ratio [OR] 1.68, P < .001; and OR 1.81, P < .001), respectively. Participants with submicroscopic parasitemia were at a lower risk of gametocytemia compared with microscopic parasitemia (OR 0.04, P < .001), but they transmitted at a significantly higher rate (OR 2.00, P = .002). A large proportion of the study population who were infected at least once remained infected (despite treatment) with asexual (71.7%, 291/406) or sexual (37.4%, 152/406) parasites. 88.6% (365/412) of feeding assays conducted in individuals who failed treatment the previous month resulted in transmissions.

CONCLUSIONS

Individuals with asymptomatic infection sustain the transmission cycle, with the 6-11-year age group serving as an important reservoir. The high rates of artemether-lumefantrine treatment failures suggest surveillance programs using molecular methods need to be expanded for accurate monitoring and evaluation of treatment outcomes.

Transmission Blocking Activity of Low-dose Tafenoquine in Healthy Volunteers Experimentally Infected With Plasmodium falciparum

BACKGROUND

Blocking the transmission of parasites from humans to mosquitoes is a key component of malaria control. Tafenoquine exhibits activity against all stages of the malaria parasite and may have utility as a transmission blocking agent. We aimed to characterize the transmission blocking activity of low-dose tafenoquine.

METHODS

Healthy adults were inoculated with Plasmodium falciparum 3D7-infected erythrocytes on day 0. Piperaquine was administered on days 9 and 11 to clear asexual parasitemia while allowing gametocyte development. A single 50-mg oral dose of tafenoquine was administered on day 25. Transmission was determined by enriched membrane feeding assays predose and at 1, 4, and 7 days postdose. Artemether-lumefantrine was administered following the final assay. Outcomes were the reduction in mosquito infection and gametocytemia after tafenoquine and safety parameters.

RESULTS

Six participants were enrolled, and all were infective to mosquitoes before tafenoquine, with a median 86% (range, 22-98) of mosquitoes positive for oocysts and 57% (range, 4-92) positive for sporozoites. By day 4 after tafenoquine, the oocyst and sporozoite positivity rate had reduced by a median 35% (interquartile range [IQR]: 16-46) and 52% (IQR: 40-62), respectively, and by day 7, 81% (IQR 36-92) and 77% (IQR 52-98), respectively. The decline in gametocyte density after tafenoquine was not significant. No significant participant safety concerns were identified.

CONCLUSIONS

Low-dose tafenoquine (50 mg) reduces P. falciparum transmission to mosquitoes, with a delay in effect.

Malaria in Refugee Children Resettled to a Holoendemic Area of Sub-Saharan Africa

BACKGROUND

Malaria is a leading cause of morbidity and mortality in refugee children in high-transmission parts of Africa. Characterizing the clinical features of malaria in refugees can inform approaches to reduce its burden.

METHODS

The study was conducted in a high-transmission region of northern Zambia hosting Congolese refugees. We analyzed surveillance data and hospital records of children with severe malaria from refugee and local sites using multivariable regression models and geospatial visualization.

RESULTS

Malaria prevalence in the refugee settlement was similar to the highest burden areas in the district, consistent with the local ecology and leading to frequent rapid diagnostic test stockouts. We identified 2197 children hospitalized for severe malaria during the refugee crisis in 2017 and 2018. Refugee children referred from a refugee transit center (n = 63) experienced similar in-hospital mortality to local children and presented with less advanced infection. However, refugee children from a permanent refugee settlement (n = 110) had more than double the mortality of local children (P < .001), had lower referral rates, and presented more frequently with advanced infection and malnutrition. Distance from the hospital was an important mediator of the association between refugee status and mortality but did not account for all of the increased risk.

CONCLUSIONS

Malaria outcomes were more favorable in refugee children referred from a highly outfitted refugee transit center than those referred later from a permanent refugee settlement. Refugee children experienced higher in-hospital malaria mortality due in part to delayed presentation and higher rates of malnutrition. Interventions tailored to the refugee context are required to ensure capacity for rapid diagnosis and referral to reduce malaria mortality.

A novel class of sulphonamides potently block malaria transmission by targeting a Plasmodium vacuole membrane protein

Phenotypic cell-based screens are critical tools for discovering candidate drugs for development, yet identification of the cellular target and mode of action of a candidate drug is often lacking. Using an imaging-based screen, we recently discovered an N-[(4-hydroxychroman-4-yl)methyl]-sulphonamide (N-4HCS) compound, DDD01035881, that blocks male gamete formation in the malaria parasite life cycle and subsequent transmission of the parasite to the mosquito with nanomolar activity. To identify the target(s) of DDD01035881, and of the N-4HCS class of compounds more broadly, we synthesised a photoactivatable derivative, probe 2. Photoaffinity labelling of probe 2 coupled with mass spectrometry identified the 16 kDa Plasmodium falciparum parasitophorous vacuole membrane protein Pfs16 as a potential parasite target. Complementary methods including cellular thermal shift assays confirmed that the parent molecule DDD01035881 stabilised Pfs16 in lysates from activated mature gametocytes. Combined with high-resolution, fluorescence and electron microscopy data, which demonstrated that parasites inhibited with N-4HCS compounds phenocopy the targeted deletion of Pfs16 in gametocytes, these data implicate Pfs16 as a likely target of DDD01035881. This finding establishes N-4HCS compounds as being flexible and effective starting candidates from which transmission-blocking antimalarials can be developed in the future.

A Plasmodium falciparum ubiquitin-specific protease (PfUSP) is essential for parasite survival and its disruption enhances artemisinin efficacy

Proteins associated with ubiquitin-proteasome system (UPS) are potential drug targets in the malaria parasite. The ubiquitination and deubiquitination are key regulatory processes for the functioning of UPS. In this study, we have characterized the biochemical and functional role of a novel ubiquitin-specific protease (USP) domain-containing protein of the human malaria parasite Plasmodium falciparum (PfUSP). We have shown that the PfUSP is an active deubiquitinase associated with parasite endoplasmic reticulum (ER). Selection linked integration (SLI) method for C-terminal tagging and GlmS-ribozyme mediated inducible knock-down (iKD) of PfUSP was utilized to assess its functional role. Inducible knockdown of PfUSP resulted in a remarkable reduction in parasite growth and multiplication; specifically, PfUSP-iKD disrupted ER morphology and development, blocked the development of healthy schizonts, and hindered proper merozoite development. PfUSP-iKD caused increased ubiquitylation of specific proteins, disrupted organelle homeostasis and reduced parasite survival. Since the mode of action of artemisinin and the artemisinin-resistance are shown to be associated with the proteasome machinery, we analyzed the effect of dihydroartemisinin (DHA) on PfUSP-iKD parasites. Importantly, the PfUSP-knocked-down parasite showed increased sensitivity to dihydroartemisinin (DHA), whereas no change in chloroquine sensitivity was observed, suggesting a role of PfUSP in combating artemisinin-induced cellular stress. Together, the results show that Plasmodium PfUSP is an essential protease for parasite survival, and its inhibition increases the efficacy of artemisinin-based drugs. Therefore, PfUSP can be targeted to develop novel scaffolds for developing new antimalarials to combat artemisinin resistance.

Comparative analysis of glass and Hemotek membrane feeding systems for malaria transmission research

BACKGROUND

Glass membrane feeders are used in malaria research for artificial blood feeding. This study investigates the use of Hemotek membrane feeders as a standardized alternative feeding system.

METHODS

Hemotek feeders were compared with glass feeders by assessing mosquito feeding rate, imbibed blood meal volume and Plasmodium falciparum infection intensity on mosquito guts.

RESULTS

While mosquito feeding rate and blood meal volume were comparable between Hemotek and glass feeders, a loss in transmission was observed using the Hemotek feeder with a conventional collagen membrane. There was no difference in transmission between both feeders when Parafilm was used as the membrane.

CONCLUSIONS

Hemotek feeders with a Parafilm membrane can be used as an alternative feeding system for malaria transmission research.

and Anopheles funestus (Diptera: Culicidae) to Pyrethroid Insecticides in the North-Western and South-Eastern, Tanzania

The rapid development of insecticide resistance in malaria vectors threatens insecticide-based interventions. It is hypothesized that infection of insecticide-resistant vectors with Plasmodium parasites increases their vulnerability to insecticides, thus assuring the effectiveness of insecticide-based strategies for malaria control. Nonetheless, there is limited field data to support this. We investigated the effect of the Plasmodium falciparum infection on the susceptibility of Anopheles gambiae s.l. and Anopheles funestus to pyrethroids in south-eastern (Kilombero) and north-western (Muleba), Tanzania. The wild-collected mosquitoes were tested against 0.05% deltamethrin and 0.75% permethrin, then assessed for sporozoite rate and resistant gene (kdr) mutations. All Anopheles gambiae s.l. from Kilombero were An. arabiensis (Patton, 1905) while those from Muleba were 87% An. gambiae s.s (Giles, 1902) and 13% An. Arabiensis. High levels of pyrethroid resistance were observed in both areas studied. The kdr mutation was only detected in An. gambiae s.s. at the frequency of 100% in survivors and 97% in dead mosquitoes. The P. falciparum sporozoite rates were slightly higher in susceptible than in resistant mosquitoes. In Muleba, sporozoite rates in An. gambiae s.l. were 8.1% and 6.4% in dead mosquitoes and survivors, respectively (SRR = 1.28, p = 0.19). The sporozoite rates in Kilombero were 1.3% and 0.7% in the dead and survived mosquitoes, respectively (sporozoite rate ratio (SRR) = 1.9, p = 0.33). In An. funestus group sporozoite rates were 6.2% and 4.4% in dead and survived mosquitoes, respectively (SRR = 1.4, p = 0.54). These findings indicate that insecticides might still be effective in malaria control despite the rapid development of insecticide resistance in malaria vectors.

Nanobodies against Pfs230 block Plasmodium falciparum transmission

Transmission blocking interventions can stop malaria parasite transmission from mosquito to human by inhibiting parasite infection in mosquitos. One of the most advanced candidates for a malaria transmission blocking vaccine is Pfs230. Pfs230 is the largest member of the 6-cysteine protein family with 14 consecutive 6-cysteine domains and is expressed on the surface of gametocytes and gametes. Here, we present the crystal structure of the first two 6-cysteine domains of Pfs230. We identified high affinity Pfs230-specific nanobodies that recognized gametocytes and bind to distinct sites on Pfs230, which were isolated from immunized alpacas. Using two non-overlapping Pfs230 nanobodies, we show that these nanobodies significantly blocked P. falciparum transmission and reduced the formation of exflagellation centers. Crystal structures of the transmission blocking nanobodies with the first 6-cysteine domain of Pfs230 confirm that they bind to different epitopes. In addition, these nanobodies bind to Pfs230 in the absence of the prodomain, in contrast with the binding of known Pfs230 transmission blocking antibodies. These results provide additional structural insight into Pfs230 domains and elucidate a mechanism of action of transmission blocking Pfs230 nanobodies.

Limited genetic diversity and expression profile of Plasmodium falciparum haem detoxification protein: a possible diagnostic target

BACKGROUND

Haem detoxification protein (HDP) is a significant protein in the erythrocytic stage of the Plasmodium lifecycle. HDP could be of paramount interest as a diagnostic biomarker for accurate diagnosis of malaria. We thus explored HDP genetic variation, expression levels of HDP and immune response.

METHODS

Phylogenetic analysis was carried out using Pfhdp orthologues sequences of various Plasmodium species. Blood samples were collected from patients in central India. Pfhdp gene was amplified, and sequenced by sanger DNA sequencing. B-cell epitopes were identified in PfHDP using Bepipred Linear Epitope Prediction 2.0, and median-joining network was constructed using global PfHDP sequences. Pfhdp expression levels during erythrocytic stage were assessed using real-time qPCR at 4-h intervals. An IgG immune response against synthetic PfHDP peptides was analysed using ELISA.

RESULTS

Phylogenetic analysis revealed the conserved nature of Pfhdp gene. Diversity analysis revealed one non-synonymous mutation (F91L) among all isolates. Neutrality tests indicated negative selection for Pfhdp gene. HDP was expressed throughout the erythrocytic cycle, and comparatively, high expression was observed in the late trophozoite and schizont stages. High IgG response against both peptides was observed, and no polymorphism was seen in any of the seven predicted B-cell epitopes.

CONCLUSIONS

Findings of the present study indicate the possibility of HDP being exploited as a diagnostic biomarker for Plasmodium falciparum malaria after proteomic validation studies.

Whole Blood Transfusion for Severe Malarial Anemia in a High Plasmodium falciparum Transmission Setting

BACKGROUND

Severe malaria resulting from Plasmodium falciparum infection is the leading parasitic cause of death in children worldwide, and severe malarial anemia (SMA) is the most common clinical presentation. The evidence in support of current blood transfusion guidelines for patients with SMA is limited.

METHODS

We conducted a retrospective cohort study of 911 hospitalized children with SMA in a holoendemic region of Zambia to examine the association of whole blood transfusion with in-hospital survival. Data were analyzed in adjusted logistic regression models using multiple imputation for missing data.

RESULTS

The median age of patients was 24 months (interquartile range, 16-30) and overall case fatality was 16%. Blood transfusion was associated with 35% reduced odds of death in children with SMA (odds ratio, 0.65; 95% confidence interval, .52-.81; P = .0002) corresponding to a number-needed-to-treat (NNT) of 14 patients. Children with SMA complicated by thrombocytopenia were more likely to benefit from transfusion than those without thrombocytopenia (NNT = 5). Longer storage time of whole blood was negatively associated with survival and with the posttransfusion rise in the platelet count but was not associated with the posttransfusion change in hemoglobin concentration.

CONCLUSIONS

Whole blood given to pediatric patients with SMA was associated with improved survival, mainly among those with thrombocytopenia who received whole blood stored for <4 weeks. These findings point to a potential use for incorporating thrombocytopenia into clinical decision making and management of severe malaria, which can be further assessed in prospective studies, and underline the importance of maintaining reliable blood donation networks in areas of high malaria transmission.

Mutant PfCRT Can Mediate Piperaquine Resistance in African Plasmodium falciparum With Reduced Fitness and Increased Susceptibility to Other Antimalarials

BACKGROUND

Additional therapeutic strategies could benefit efforts to reverse the recent increase in malaria cases in sub-Saharan Africa, which mostly affects young children. A primary candidate is dihydroartemisinin + piperaquine (DHA + PPQ), which is effective for uncomplicated malaria treatment, seasonal malaria chemoprevention, and intermittent preventive treatment. In Southeast Asia, Plasmodium falciparum parasites acquired PPQ resistance, mediated primarily by mutations in the P falciparum chloroquine resistance transporter PfCRT. The recent emergence in Africa of DHA-resistant parasites creates an imperative to assess whether PPQ resistance could emerge in African parasites with distinct PfCRT isoforms.

METHODS

We edited 2 PfCRT mutations known to mediate high-grade PPQ resistance in Southeast Asia into GB4 parasites from Gabon. Gene-edited clones were profiled in antimalarial concentration-response and fitness assays.

RESULTS

The PfCRT F145I mutation mediated moderate PPQ resistance in GB4 parasites but with a substantial fitness cost. No resistance was observed with the PfCRT G353V mutant. Both edited clones became significantly more susceptible to amodiaquine, chloroquine, and quinine.

CONCLUSIONS

A single PfCRT mutation can mediate PPQ resistance in GB4 parasites, but with a growth defect that may preclude its spread without further genetic adaptations. Our findings support regional use of drug combinations that exert opposing selective pressures on PfCRT.

Fitness Costs of pfhrp2 and pfhrp3 Deletions Underlying Diagnostic Evasion in Malaria Parasites

BACKGROUND

Rapid diagnostic tests based on detection of histidine-rich proteins (HRPs) are widely used for malaria diagnosis, but parasites carrying pfhrp deletions can evade detection and are increasing in frequency in some countries. Models aim to predict conditions under which pfhrp2 and/or pfhrp3 deletions will increase, but a key parameter-the fitness cost of deletions-is unknown.

METHODS

We removed pfhrp2 and/or pfhrp3 from a Malawian parasite clone using gene editing approaches) and measured fitness costs by conducting pairwise competition experiments.

RESULTS

We observed significant fitness costs of 0.087 ± 0.008 (1 standard error) per asexual cycle for pfhrp2 deletion and 0.113 ± 0.008 for the pfhrp2/3 double deletion, relative to the unedited progenitor parasite. Selection against deletions is strong and comparable to that resulting from drug resistance mutations.

CONCLUSIONS

Prior modeling suggested that diagnostic selection may drive increased frequency of pfhrp deletions only when fitness costs are mild. Our experiments show that costs of pfhrp deletions are higher than these thresholds, but modeling and empirical results can be reconciled if the duration of infection is short. These results may inform future modeling to understand why pfhrp2/3 deletions are increasing in some locations (Ethiopia and Eritrea) but not in others (Mekong region).

Factors Associated With Human IgG Antibody Response to Anopheles albimanus Salivary Gland Extract, Artibonite Department, Haiti, 2017

Serological data can provide estimates of human exposure to both malaria vector and parasite based on antibody responses. A multiplex bead-based assay was developed to simultaneously detect IgG to Anopheles albimanus salivary gland extract (SGE) and 23 Plasmodium falciparum antigens among 4185 participants enrolled in Artibonite department, Haiti in 2017. Logistic regression adjusted for participant- and site-level covariates and found children under 5 years and 6-15 years old had 3.7- and 5.4-fold increase in odds, respectively, of high anti-SGE IgG compared to participants >15 years. Seropositivity to P. falciparum CSP, Rh2_2030, and SEA-1 antigens was significantly associated with high IgG response against SGE, and participant enrolment at elevations under 200 m was associated with higher anti-SGE IgG levels. The ability to approximate population exposure to malaria vectors through SGE serology data is very dependent by age categories, and SGE antigens can be easily integrated into a multiplex serological assay.

Association between indoor residual spraying and pregnancy outcomes: a quasi-experimental study from Uganda

BACKGROUND

Malaria is a risk factor for adverse pregnancy outcomes. Indoor residual spraying with insecticide (IRS) reduces malaria infections, yet the effects of IRS on pregnancy outcomes are not well established. We evaluated the impact of a large-scale IRS campaign on pregnancy outcomes in Eastern Uganda.

METHODS

Birth records (n = 59 992) were obtained from routine surveillance data at 25 health facilities from five districts that were part of the IRS campaign and six neighbouring control districts ∼27 months before and ∼24 months after the start of the campaign (January 2013-May 2017). Campaign effects on low birthweight (LBW) and stillbirth incidence were estimated using the matrix completion method (MC-NNM), a machine-learning approach to estimating potential outcomes, and compared with the difference-in-differences (DiD) estimator. Subgroup analyses were conducted by HIV and gravidity.

RESULTS

MC-NNM estimates indicated that the campaign was associated with a 33% reduction in LBW incidence: incidence rate ratio (IRR) = 0.67 [95% confidence interval (CI): 0.49-0.93)]. DiD estimates were similar to MC-NNM [IRR = 0.69 (0.47-1.01)], despite a parallel trends violation during the pre-IRS period. The campaign was not associated with substantial reductions in stillbirth incidence [IRRMC-NNM = 0.94 (0.50-1.77)]. HIV status modified the effects of the IRS campaign on LBW [βIRSxHIV = 0.42 (0.05-0.78)], whereby HIV-negative women appeared to benefit from the campaign [IRR = 0.70 (0.61-0.81)], but not HIV-positive women [IRR = 1.12 (0.59-2.12)].

CONCLUSIONS

Our results support the effectiveness of the campaign in Eastern Uganda based on its benefit to LBW prevention, though HIV-positive women may require additional interventions. The IRS campaign was not associated with a substantively lower stillbirth incidence, warranting further research.

Plasmodium falciparum in Aotus nancymaae: A New Model for Placental Malaria

Plasmodium falciparum-infected erythrocytes that display the variant surface antigen VAR2CSA bind chondroitin sulfate A (CSA) to sequester in placental intervillous spaces, causing severe sequelae for mother and offspring. Here, we establish a placental malaria (PM) monkey model. Pregnant Aotus infected with CSA-binding P. falciparum CS2 parasites during the third trimester developed pronounced sequestration of late-stage parasites in placental intervillous spaces that express VAR2CSA and bind specifically to CSA. Similar to immune multigravid women, a monkey infected with P. falciparum CS2 parasites over successive pregnancies acquired antibodies against VAR2CSA, with potent functional activity that was boosted upon subsequent pregnancy infections. Aotus also developed functional antibodies after multiple acute PM episodes and subsequent VAR2CSA immunization. In summary, P. falciparum infections in pregnant Aotus monkeys recapitulate all the prominent features of human PM infection and immunity, and this model can be useful for basic mechanistic studies and preclinical studies to qualify candidate PM vaccines. Clinical Trials Registration: NCT02471378.

Post malaria acute motor axonal neuropathy

We report herein the first case of acute motor axonal neuropathy syndrome after severe Plasmodium falciparum malaria in a traveller, diagnosed through neurophysiological findings and high level of neurofilaments light chain in cerebrospinal fluid analysis, with negative testing for anti-ganglioside antibodies.

Cerebral malaria - modelling interactions at the blood-brain barrier in vitro

The blood–brain barrier (BBB) is a continuous endothelial barrier that is supported by pericytes and astrocytes and regulates the passage of solutes between the bloodstream and the brain. This structure is called the neurovascular unit and serves to protect the brain from blood-borne disease-causing agents and other risk factors. In the past decade, great strides have been made to investigate the neurovascular unit for delivery of chemotherapeutics and for understanding how pathogens can circumvent the barrier, leading to severe and, at times, fatal complications. One such complication is cerebral malaria, in which Plasmodium falciparum-infected red blood cells disrupt the barrier function of the BBB, causing severe brain swelling. Multiple in vitro models of the BBB are available to investigate the mechanisms underlying the pathogenesis of cerebral malaria and other diseases. These range from single-cell monolayer cultures to multicellular BBB organoids and highly complex cerebral organoids. Here, we review the technologies available in malaria research to investigate the interaction between P. falciparum-infected red blood cells and the BBB, and discuss the advantages and disadvantages of each model.

Summary: This Review discusses the available in vitro models to investigate the impact of adhesion of Plasmodium falciparum-infected red blood cells on the blood–brain barrier, a process associated with cerebral malaria.

Impact of Oxidative Stress on Risk of Death and Readmission in African Children With Severe Malaria: A Prospective Observational Study

BACKGROUND

We hypothesized that oxidative stress in Ugandan children with severe malaria is associated with mortality.

METHODS

We evaluated biomarkers of oxidative stress in children with cerebral malaria (CM, n = 77) or severe malarial anemia (SMA, n = 79), who were enrolled in a randomized clinical trial of immediate vs delayed iron therapy, compared with community children (CC, n = 83). Associations between admission biomarkers and risk of death during hospitalization or risk of readmission within 6 months were analyzed.

RESULTS

Nine children with CM and none with SMA died during hospitalization. Children with CM or SMA had higher levels of heme oxygenase-1 (HO-1) (P < .001) and lower superoxide dismutase (SOD) activity than CC (P < .02). Children with CM had a higher risk of death with increasing HO-1 concentration (odds ratio [OR], 6.07 [95% confidence interval {CI}, 1.17-31.31]; P = .03) but a lower risk of death with increasing SOD activity (OR, 0.02 [95% CI, .001-.70]; P = .03). There were no associations between oxidative stress biomarkers on admission and risk of readmission within 6 months of enrollment.

CONCLUSIONS

Children with CM or SMA develop oxidative stress in response to severe malaria. Oxidative stress is associated with higher mortality in children with CM but not with SMA.

CLINICAL TRIALS REGISTRATION

NCT01093989.

CRISPR/Cas9 and genetic screens in malaria parasites: small genomes, big impact

The ∼30 Mb genomes of the Plasmodium parasites that cause malaria each encode ∼5000 genes, but the functions of the majority remain unknown. This is due to a paucity of functional annotation from sequence homology, which is compounded by low genetic tractability compared with many model organisms. In recent years technical breakthroughs have made forward and reverse genome-scale screens in Plasmodium possible. Furthermore, the adaptation of Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR-Associated protein 9 (CRISPR/Cas9) technology has dramatically improved gene editing efficiency at the single gene level. Here, we review the arrival of genetic screens in malaria parasites to analyse parasite gene function at a genome-scale and their impact on understanding parasite biology. CRISPR/Cas9 screens, which have revolutionised human and model organism research, have not yet been implemented in malaria parasites due to the need for more complex CRISPR/Cas9 gene targeting vector libraries. We therefore introduce the reader to CRISPR-based screens in the related apicomplexan Toxoplasma gondii and discuss how these approaches could be adapted to develop CRISPR/Cas9 based genome-scale genetic screens in malaria parasites. Moreover, since more than half of Plasmodium genes are required for normal asexual blood-stage reproduction, and cannot be targeted using knockout methods, we discuss how CRISPR/Cas9 could be used to scale up conditional gene knockdown approaches to systematically assign function to essential genes.

Asymptomatic School-Aged Children Are Important Drivers of Malaria Transmission in a High Endemicity Setting in Uganda

Achieving malaria elimination requires a better understanding of the transmissibility of human infections in different transmission settings. This study aimed to characterize the human infectious reservoir in a high endemicity setting in eastern Uganda, using gametocyte quantification and mosquito feeding assays. In asymptomatic infections, gametocyte densities were positively associated with the proportion of infected mosquitoes (β = 1.60; 95% CI, 1.32-1.92; P < .0001). Combining transmissibility and abundance in the population, symptomatic and asymptomatic infections were estimated to contribute to 5.3% and 94.7% of the infectious reservoir, respectively. School-aged children (5-15 years old) contributed to 50.4% of transmission events and were important drivers of malaria transmission.

Specific components associated with the endothelial glycocalyx are lost from brain capillaries in cerebral malaria

BACKGROUND

Cerebral malaria (CM) is a rare, but severe and frequently fatal outcome of infections with Plasmodium falciparum. Pathogenetic mechanisms include endothelial activation and sequestration of parasitized erythrocytes in the cerebral microvessels. Increased concentrations of glycosaminoglycans in urine and plasma of malaria patients have been described, suggesting involvement of endothelial glycocalyx.

METHODS

We used lectin histochemistry on postmortem samples to compare the distribution of multiple sugar epitopes on cerebral capillaries in children who died from CM and from non-malarial comas.

RESULTS

N-acetyl glucosamine residues detected by tomato lectin are generally reduced in children with CM compared to controls. We used the vascular expression of intercellular adhesion molecule-1 and mannose residues on brain capillaries of CM as evidence of local vascular inflammation, and both were expressed more highly in CM patients than controls. Sialic acid residues were found to be significantly reduced in patients with CM. By contrast, the levels of other sugar epitopes regularly detected on the cerebral vasculature were unchanged, and this suggests specific remodeling of cerebral microvessels in CM patients.

CONCLUSIONS

Our findings support and expand upon earlier reports of disruptions of the endothelial glycocalyx in children with severe malaria.

Brain Magnetic Resonance Imaging Reveals Different Courses of Disease in Pediatric and Adult Cerebral Malaria

BACKGROUND

Cerebral malaria is a common presentation of severe Plasmodium falciparum infection and remains an important cause of death in the tropics. Key aspects of its pathogenesis are still incompletely understood, but severe brain swelling identified by magnetic resonance imaging (MRI) was associated with a fatal outcome in African children. In contrast, neuroimaging investigations failed to identify cerebral features associated with fatality in Asian adults.

METHODS

Quantitative MRI with brain volume assessment and apparent diffusion coefficient (ADC) histogram analyses were performed for the first time in 65 patients with cerebral malaria to compare disease signatures between children and adults from the same cohort, as well as between fatal and nonfatal cases.

RESULTS

We found an age-dependent decrease in brain swelling during acute cerebral malaria, and brain volumes did not differ between fatal and nonfatal cases across both age groups. In nonfatal disease, reversible, hypoxia-induced cytotoxic edema occurred predominantly in the white matter in children, and in the basal ganglia in adults. In fatal cases, quantitative ADC histogram analyses also demonstrated different end-stage patterns between adults and children: Severe hypoxia, evidenced by global ADC decrease and elevated plasma levels of lipocalin-2 and microRNA-150, was associated with a fatal outcome in adults. In fatal pediatric disease, our results corroborate an increase in brain volume, leading to augmented cerebral pressure, brainstem herniation, and death.

CONCLUSIONS

Our findings suggest distinct pathogenic patterns in pediatric and adult cerebral malaria with a stronger cytotoxic component in adults, supporting the development of age-specific adjunct therapies.

SELECTION OF PFCRT 76T AND PFMDR1 86Y MUTANT PLASMODIUM FALCIPARUM AFTER TREATMENT OF UNCOMPLICATED MALARIA WITH ARTESUNATE-AMODIAQUINE IN REPUBLIC OF GUINEA

The use of Amodiaquine monotherapy is associated with the selection of molecular markers of Plasmodium falciparum resistance to chloroquine (pfcrt and pfmdr1). The decrease in sensitivity and the emergence of P. falciparum resistant to artemisinin-based combination therapy have been reported. Therefore, it is important to assess the impact of treatment of uncomplicated malaria with Artesunate-Amodiaquine (AS+AQ) on molecular markers of antimalarial resistance. We used standard World Health Organization (WHO) protocols to determine the in vivo efficacy of the combination (AS+AQ). In total, 170 subjects were included in the study. The molecular analysis focused on 168 dried blood spots. The aims were to determine the frequency of pfcrt 76T and pfmdr1 86Y mutations and the rates of reinfection using polymorphism markers msp1, msp2, and microsatellite markers (CA1, Ta87, TA99). Nested-PCR was used, followed in some cases by a restriction digestion. The level of P. falciparum clinical response was 92.9% (156/168) of Adequate Clinical and Parasitological Response (ACPR) before molecular correction and 97.0% (163/168) after molecular correction (P = 0.089). The frequency of mutation point pfcrt 76T was 76.2% (128/168) before treatment and 100% (7/7) after treatment (P = 0.1423). For the pfmdr1 mutation, the frequency was 28% (47/168) before treatment and 60% (6/10) after treatment (P = 0.1124). The rate of pfcrt 76T + pfmdr1 86Y was 22% (37/168) before and 50% (6/12) after treatment (P = 0.1465). Despite the presence of AS in the combination, AS+AQ selects for pfcrt 76T and pfmdr1 86Y mutant P. falciparum in Guinea.

Decreased Susceptibility to Dihydrofolate Reductase Inhibitors Associated With Genetic Polymorphisms in Ugandan Plasmodium falciparum Isolates

BACKGROUND

The Plasmodium falciparum dihydrofolate reductase (PfDHFR) inhibitors pyrimethamine and cycloguanil (the active metabolite of proguanil) have important roles in malaria chemoprevention, but drug resistance challenges their efficacies. A new compound, P218, was designed to overcome resistance, but drug-susceptibility data for P falciparum field isolates are limited.

METHODS

We studied ex vivo PfDHFR inhibitor susceptibilities of 559 isolates from Tororo and Busia districts, Uganda, from 2016 to 2020, sequenced 383 isolates, and assessed associations between genotypes and drug-susceptibility phenotypes.

RESULTS

Median half-maximal inhibitory concentrations (IC50s) were 42 100 nM for pyrimethamine, 1200 nM for cycloguanil, 13000 nM for proguanil, and 0.6 nM for P218. Among sequenced isolates, 3 PfDHFR mutations, 51I (100%), 59R (93.7%), and 108N (100%), were very common, as previously seen in Uganda, and another mutation, 164L (12.8%), had moderate prevalence. Increasing numbers of mutations were associated with decreasing susceptibility to pyrimethamine, cycloguanil, and P218, but not proguanil, which does not act directly against PfDHFR. Differences in P218 susceptibilities were modest, with median IC50s of 1.4 nM for parasites with mixed genotype at position 164 and 5.7 nM for pure quadruple mutant (51I/59R/108N/164L) parasites.

CONCLUSIONS

Resistance-mediating PfDHFR mutations were common in Ugandan isolates, but P218 retained excellent activity against mutant parasites.

Artemisinin resistance surveillance in African Plasmodium falciparum isolates from imported malaria cases to Italy

BACKGROUND

Plasmodium falciparum (P. falciparum) malaria is a significant public health problem in returning travellers, and artemisinin combination therapy (ACT) remains the first choice for treatment. Several single nucleotide polymorphisms (SNPs) in the P. falciparum kelch 13 (Pfk13) gene have been associated with artemisinin (ART) resistance. Moreover, the increase in the P. falciparum plasmepsin 2 (Pfpm2) gene copy number was shown to be linked with reduced susceptibility of P. falciparum to piperaquine (PPQ), a partner drug in an ACT regimen. Active molecular surveillance for imported drug-resistant malaria parasites is a pivotal activity to provide adequate chemoprophylaxis and treatment guidelines.

METHODS

A retrospective study to review imported P. falciparum malaria in patients admitted to Spallanzani Institute between 2014 and 2015 was conducted. Information collected included clinic and epidemiological characteristics such as age, gender, country of origin, time since arrival to our country, travel history. All P.falciparum isolates were analysed for SNPs in the Pfk13 gene and for copy number variations in the Pfpm2 gene.

RESULTS

P. falciparum malaria was identified in 54 travellers. The mean age was 37 years, 44 were males. All cases were imported from non-EU countries. In the Pfk13 gene two mutations (R561R and F673L) were detected. Six P. falciparum isolates carried two copies of Pfpm2 gene, and one three copies, representing ≈16% of the analysed isolates.

CONCLUSIONS

None of the SNPs known to be associated with ART resistance were detected in the examined parasites. Our results provide evidence that Pfpm2 duplications (associated with piperaquine resistance) occur in Africa, emphasizing the necessity to better decode the genetic background associated with PPQ resistance. Further epidemiological investigations in Pfpm2 amplification along with mutations in the Pfk13 gene will be useful for developing and updating anti-malarial guidance in travellers.

The Burden of Malaria in the Democratic Republic of the Congo

Despite evidence that older children and adolescents bear the highest burden of malaria, large malaria surveys focus on younger children. We used polymerase chain reaction data from the 2013-2014 Demographic and Health Survey in the Democratic Republic of Congo (including children aged <5 years and adults aged ≥15 years) and a longitudinal study in Kinshasa Province (participants aged 6 months to 98 years) to estimate malaria prevalence across age strata. We fit linear models and estimated prevalences for each age category; adolescents aged 10-14 years had the highest prevalence. We estimate approximately 26 million polymerase chain reaction-detectable infections nationally. Adolescents and older children should be included in surveillance studies.

Changing Prevalence of Potential Mediators of Aminoquinoline, Antifolate, and Artemisinin Resistance Across Uganda

BACKGROUND

In Uganda, artemether-lumefantrine is recommended for malaria treatment and sulfadoxine-pyrimethamine for chemoprevention during pregnancy, but drug resistance may limit efficacies.

METHODS

Genetic polymorphisms associated with sensitivities to key drugs were characterized in samples collected from 16 sites across Uganda in 2018 and 2019 by ligase detection reaction fluorescent microsphere, molecular inversion probe, dideoxy sequencing, and quantitative polymerase chain reaction assays.

RESULTS

Considering transporter polymorphisms associated with resistance to aminoquinolines, the prevalence of Plasmodium falciparum chloroquine resistance transporter (PfCRT) 76T decreased, but varied markedly between sites (0-46% in 2018; 0-23% in 2019); additional PfCRT polymorphisms and plasmepsin-2/3 amplifications associated elsewhere with resistance to piperaquine were not seen. For P. falciparum multidrug resistance protein 1, in 2019 the 86Y mutation was absent at all sites, the 1246Y mutation had prevalence ≤20% at 14 of 16 sites, and gene amplification was not seen. Considering mutations associated with high-level sulfadoxine-pyrimethamine resistance, prevalences of P. falciparum dihydrofolate reductase 164L (up to 80%) and dihydropteroate synthase 581G (up to 67%) were high at multiple sites. Considering P. falciparum kelch protein propeller domain mutations associated with artemisinin delayed clearance, prevalence of the 469Y and 675V mutations has increased at multiple sites in northern Uganda (up to 23% and 41%, respectively).

CONCLUSIONS

We demonstrate concerning spread of mutations that may limit efficacies of key antimalarial drugs.

High Prevalence of Molecular Markers of Plasmodium falciparum Resistance to Sulphadoxine-Pyrimethamine in Parts of Ghana: A Threat to ITPTp-SP?

Malaria in pregnancy is a huge public health problem as it is the cause of maternal anaemia, still birth, premature delivery, low birth weight among others. To tackle this problem, WHO recommended the administration, during pregnancy, of intermittent preventive treatment with sulphadoxine-pyrimethamine (IPTp-SP). The introduction of this policy is likely to create SP drug pressure which may lead to the emergence of parasite strains resistant to the drug. This study investigated the prevalence of the molecular markers of SP resistance as pointers to potential failure of IPTp-SP among pregnant women attending antenatal clinic, women at the point of baby delivery and out patients department (OPD) attendees. The study was conducted in health facilities located in parts of Ghana. Prevalence of mutations in dhfr and dhps genes of Plasmodium falciparum was determined using the method described by Duraisingh et al. The outcome of the study indicated the presence of high prevalence of strains of P.falciparum with the resistant alleles of the dhfr or dhps genes in the three categories of participants. There was a high prevalence of triple mutations (IRN) in the dhfr gene of P.falciparum isolates: 71.4% in peripheral blood of antenatal attendees; 74.1% in placenta cord blood of delivering mothers and 71.1% in OPD attendees. Quintuple mutations were only found in 2 (0.5%) isolates from OPD attendees. This observation might have occurred due to the increased use of SP for IPTp among others. There is the need for an interventional measure in order to protect pregnant women and their unborn children. Lay summaryWhen pregnant women get infected with the malaria parasites they are exposed to all manner of dangers including pre-term delivery, still birth, maternal anaemia and low birth weight. Taking sulphadoxine-pyrimethamine (SP) at predetermined periods during pregnancy, referred to as 'intermittent preventive treatment with SP' (IPTp-SP)' helps to curtail these problems. However, the frequent taking of these drugs is likely to create SP drug pressure which may lead to the emergence of parasite strains that are not readily killed by the drugs. In order to ascertain this phenomenon and advice stakeholders, this study determined the prevalence of certain 'materials' certified as markers of parasite resistance to SP. Alarmingly, more than 5% of all the category of women recruited to participate in this study were found to harbour the parasites that causes malaria. The outcome, also suggest the existence of high levels of strains of the malaria parasite, carrying the materials that make them to become resistant to SP. Policy makers must pay attention to these observations and institute measures to avoid escalation of the situation.

A proteomic glimpse into the effect of antimalarial drugs on Plasmodium falciparum proteome towards highlighting possible therapeutic targets

There is no effective vaccine against malaria; therefore, chemotherapy is to date the only choice to fight against this infectious disease. However, there is growing evidences of drug-resistance mechanisms in malaria treatments. Therefore, the identification of new drug targets is an urgent need for the clinical management of the disease. Proteomic approaches offer the chance of determining the effects of antimalarial drugs on the proteome of Plasmodium parasites. Accordingly, we reviewed the effects of antimalarial drugs on the Plasmodium falciparum proteome pointing out the relevance of several proteins as possible drug targets in malaria treatment. In addition, some of the P. falciparum stage-specific altered proteins and parasite-host interactions might play important roles in pathogenicity, survival, invasion and metabolic pathways and thus serve as potential sources of drug targets. In this review, we have identified several proteins, including thioredoxin reductase, helicases, peptidyl-prolyl cis-trans isomerase, endoplasmic reticulum-resident calcium-binding protein, choline/ethanolamine phosphotransferase, purine nucleoside phosphorylase, apical membrane antigen 1, glutamate dehydrogenase, hypoxanthine guanine phosphoribosyl transferase, heat shock protein 70x, knob-associated histidine-rich protein and erythrocyte membrane protein 1, as promising antimalarial drugs targets. Overall, proteomic approaches are able to partially facilitate finding possible drug targets. However, the integration of other 'omics' and specific pharmaceutical techniques with proteomics may increase the therapeutic properties of the critical proteins identified in the P. falciparum proteome.

Evolution of Antimalarial Drug Resistance Markers in the Reservoir of Plasmodium falciparum Infections in the Upper East Region of Ghana

BACKGROUND

The majority of Plasmodium falciparum infections, constituting the reservoir in all ages, are asymptomatic in high-transmission settings in Africa. The role of this reservoir in the evolution and spread of drug resistance was explored.

METHODS

Population genetic analyses of the key drug resistance-mediating polymorphisms were analyzed in a cross-sectional survey of asymptomatic P. falciparum infections across all ages in Bongo District, Ghana.

RESULTS

Seven years after the policy change to artemisinin-based combination therapies in 2005, the pfcrt K76 and pfmdr1 N86 wild-type alleles have nearly reached fixation and have expanded via soft selective sweeps on multiple genetic backgrounds. By constructing the pfcrt-pfmdr1-pfdhfr-pfdhps multilocus haplotypes, we found that the alleles at these loci were in linkage equilibrium and that multidrug-resistant parasites have not expanded in this reservoir. For pfk13, 32 nonsynonymous mutations were identified; however, none were associated with artemisinin-based combination therapy resistance.

CONCLUSIONS

The prevalence and selection of alleles/haplotypes by antimalarials were similar to that observed among clinical cases in Ghana, indicating that they do not represent 2 subpopulations with respect to these markers. Thus, the P. falciparum reservoir in all ages can contribute to the maintenance and spread of antimalarial resistance.

Delayed clinical presentation of Plasmodium falciparum malaria after leaving an endemic area: a tale of two patients

Malaria due to Plasmodium falciparum (Pf) may be missed if patients present with symptoms outside of the expected incubation period. We describe two patients who developed Pf malaria more than one year after visiting malaria-endemic countries. Both worked at an international airport, but no source of infection was identified.