Low Prevalence of Deletions of the pfhrp2 and pfhrp3 Genes in Plasmodium falciparum Parasites in Freetown, Sierra Leone in 2015.

Sierra Leone relies heavily on histidine-rich protein 2-based diagnostics for malaria because of the high transmission of Plasmodium falciparum. During the 2015 recombinant vesicular stomatitis virus (VSV)-Zaire Ebola virus envelope glycoprotein (GP) vaccine trial, 77 participants with asymptomatic Plasmodium infection were enrolled, with all but four having P. falciparum malaria. Of the 73 participants with P. falciparum malaria, one infection (1 of 73, 1.4%; 95% CI, 0.03-7.4) showed P. falciparum with a pfhrp3 single deletion, and two P. falciparum infections (2 of 73, 2.7%; 95% CI, 0.03-9.6) showed pfhrp2/pfhrp3 dual deletions. This study shows evidence of pfhrp2- and pfhrp3-deleted parasites in Freetown, Sierra Leone. Additional studies for more precise estimates of prevalence are warranted.

Nationwide Monitoring for Plasmodium falciparum Drug-Resistance Alleles to Chloroquine, Sulfadoxine, and Pyrimethamine, Haiti, 2016-2017.

Haiti is striving for zero local malaria transmission by the year 2025. Chloroquine remains the first-line treatment, and sulfadoxine/pyrimethamine (SP) has been used for mass drug-administration pilot programs. In March 2016, nationwide molecular surveillance was initiated to assess molecular resistance signatures for chloroquine and SP. For 778 samples collected through December 2017, we used Sanger sequencing to investigate putative resistance markers to chloroquine (Pfcrt codons 72, 74, 75, and 76), sulfadoxine (Pfdhps codons 436, 437, 540, 581, 613), and pyrimethamine (Pfdhfr codons 50, 51, 59, 108, 164). No parasites harbored Pfcrt point mutations. Prevalence of the Pfdhfr S108N single mutation was 47%, and we found the triple mutant Pfdhfr haplotype (108N, 51I, and 59R) in a single isolate. We observed no Pfdhps variants except in 1 isolate (A437G mutation). These data confirm the lack of highly resistant chloroquine and SP alleles in Haiti and support the continued use of chloroquine and SP.

Multiplex malaria antigen detection by bead-based assay and molecular confirmation by PCR shows no evidence of Pfhrp2 and Pfhrp3 deletion in Haiti.

BACKGROUND: The Plasmodium falciparum parasite is the only human malaria that produces the histidine-rich protein 2 and 3 (HRP2/3) antigens. Currently, HRP2/3 are widely used in malaria rapid diagnostic tests (RDTs), but several global reports have recently emerged showing genetic deletion of one or both of these antigens in parasites. Deletion of these antigens could pose a major concern for P. falciparum diagnosis in Haiti which currently uses RDTs based solely on the detection of the HRP2/3 antigens. METHODS: From September 2012 through February 2014, dried blood spots (DBS) were collected in Haiti from 9317 febrile patients presenting to 17 health facilities in 5 departments throughout the country as part of a bed net intervention study. All DBS from RDT positive persons and a random sampling of DBS from RDT negative persons were assayed for P. falciparum DNA by nested and PET-PCR (n = 2695 total). All PCR positive samples (n = 331) and a subset of PCR negative samples (n = 95) were assayed for three malaria antigens by a multiplex bead assay: pan-Plasmodium aldolase (pAldo), pan-Plasmodium lactate dehydrogenase (pLDH), and HRP2/3. Any samples positive for P. falciparum DNA, but negative for HRP2/3 antigens were tested by nested PCR for Pfhrp2 and Pfhrp3 gene deletions. RESULTS: Of 2695 DBS tested for Plasmodium DNA, 345 (12.8%) were originally found to be positive for P. falciparum DNA; 331 of these had DBS available for antigen detection. Of these, 266 (80.4%) were positive for pAldo, 221 (66.8%) positive for pLDH, and 324 (97.9%) were positive for HRP2/3 antigens. Seven samples (2.1%) positive for P. falciparum DNA were not positive for any of the three antigens by the bead assay, and were investigated for potential Pfhrp2/3 gene deletion by PCR. These samples either successfully amplified Pfhrp2/3 genes or were at an estimated parasite density too low for sufficient DNA to perform successful genotyping. CONCLUSIONS: Malaria positive samples in multiple Haitian sites were found to contain the HRP2/3 antigens, and no evidence was found of Pfhrp2/3 deletions. Malaria RDTs based on the detection of the HRP2/3 antigens remain a reliable P. falciparum diagnostic tool as Haiti works towards malaria elimination.

Specificity of the IgG antibody response to Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale MSP119 subunit proteins in multiplexed serologic assays.

BACKGROUND: Multiplex bead assays (MBA) that measure IgG antibodies to the carboxy-terminal 19-kDa sub-unit of the merozoite surface protein 1 (MSP119) are currently used to determine malaria seroprevalence in human populations living in areas with both stable and unstable transmission. However, the species specificities of the IgG antibody responses to the malaria MSP119 antigens have not been extensively characterized using MBA. METHODS: Recombinant Plasmodium falciparum (3D7), Plasmodium malariae (China I), Plasmodium ovale (Nigeria I), and Plasmodium vivax (Belem) MSP119 proteins were covalently coupled to beads for MBA. Threshold cut-off values for the assays were estimated using sera from US citizens with no history of foreign travel and by receiver operator characteristic curve analysis using diagnostic samples. Banked sera from experimentally infected chimpanzees, sera from humans from low transmission regions of Haiti and Cambodia (N = 12), and elutions from blood spots from humans selected from a high transmission region of Mozambique (N = 20) were used to develop an antigen competition MBA for antibody cross-reactivity studies. A sub-set of samples was further characterized using antibody capture/elution MBA, IgG subclass determination, and antibody avidity measurement. RESULTS: Total IgG antibody responses in experimentally infected chimpanzees were species specific and could be completely suppressed by homologous competitor protein at a concentration of 10 µg/ml. Eleven of 12 samples from the low transmission regions and 12 of 20 samples from the high transmission area had antibody responses that were completely species specific. For 7 additional samples, the P. falciparum MSP119 responses were species specific, but various levels of incomplete heterologous competition were observed for the non-P. falciparum assays. A pan-malaria MSP119 cross-reactive antibody response was observed in elutions of blood spots from two 20-30 years old Mozambique donors. The antibody response from one of these two donors had low avidity and skewed almost entirely to the IgG3 subclass. CONCLUSIONS: Even when P. falciparum, P. malariae, P. ovale, and P. vivax are co-endemic in a high transmission setting, most antibody responses to MSP119 antigens are species-specific and are likely indicative of previous infection history. True pan-malaria cross-reactive responses were found to occur rarely.

Molecular Profile of Malaria Drug Resistance Markers of Plasmodium falciparum in Suriname.

In Suriname, an artesunate monotherapy therapeutic efficacy trial was recently conducted to evaluate partial artemisinin resistance emerging in Plasmodium falciparum We genotyped the PfK13 propeller domain of P. falciparum in 40 samples as well as other mutations proposed to be associated with artemisinin-resistant mutants. We did not find any mutations previously associated with artemisinin resistance in Southeast Asia, but we found fixed resistance mutations for chloroquine (CQ) and sulfadoxine-pyrimethamine. Additionally, the PfCRT C350R mutation, associated with reversal of CQ resistance and piperaquine-selective pressure, was present in 62% of the samples. Our results from neutral microsatellite data also confirmed a high parasite gene flow in the Guiana Shield. Although recruiting participants for therapeutic efficacy studies is challenging in areas where malaria endemicity is very low due to the low number of malaria cases reported, conducting these studies along with molecular surveillance remains essential for the monitoring of artemisinin-resistant alleles and for the characterization of the population structure of P. falciparum in areas targeted for malaria elimination.

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

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

Effectiveness of insecticide-treated bednets in malaria prevention in Haiti: a case-control study.

BACKGROUND: Insecticide-treated bednets (ITNs) are effective in preventing malaria where vectors primarily bite indoors and late at night, but their effectiveness is uncertain where vectors bite outdoors and earlier in the evening. We studied the effectiveness of ITNs following a mass distribution in Haiti from May to September, 2012, where the Anopheles albimanus vector bites primarily outdoors and often when people are awake. METHODS: In this case-control study, we enrolled febrile patients presenting to outpatient departments at 17 health facilities throughout Haiti from Sept 4, 2012, to Feb 27, 2014, who were tested with malaria rapid diagnostic tests (RDTs), and administered questionnaires on ITN use and other risk factors. Cases were defined by positive RDT and controls were febrile patients from the same clinic with a negative RDT. Our primary analysis retrospectively matched cases and controls by age, sex, location, and date, and used conditional logistic regression on the matched sample. A sensitivity analysis used propensity scores to match patients on ITN use propensity and analyse malaria among ITN users and non-users. Additional ITN bioefficacy and entomological data were collected. FINDINGS: We enrolled 9317 patients, including 378 (4%) RDT-positive cases. 1202 (13%) patients reported ITN use. Post-hoc matching of cases and controls yielded 362 cases and 1201 matched controls, 19% (333) of whom reported consistent campaign net use. After using propensity scores to match on consistent campaign ITN use, 2298 patients, including 138 (7%) RDT-positive cases, were included: 1149 consistent campaign ITN users and 1149 non-consistent campaign ITN users. Both analyses revealed that ITNs did not significantly protect against clinical malaria (odds ratio [OR]=0·95, 95% CI 0·68-1·32, p=0·745 for case-control analysis; OR=0·95, 95% CI 0·45-1·97, p=0·884 for propensity score analysis). ITN and entomological data indicated good ITN physical integrity and bioefficacy, and no permethrin resistance among local mosquitoes. INTERPRETATION: We found no evidence that mass ITN campaigns reduce clinical malaria in this observational study in Haiti; alternative malaria control strategies should be prioritised. FUNDING: The Global Fund to Fight AIDS, Tuberculosis, and Malaria, and the US-based Centers for Disease Control and Prevention (CDC).

Plasmodium falciparum Drug-Resistant Haplotypes and Population Structure in Postearthquake Haiti, 2010.

Chloroquine (CQ) remains the first-line treatment of malaria in Haiti. Given the challenges of conducting in vivo drug efficacy trials in low-endemic settings like Haiti, molecular surveillance for drug resistance markers is a reasonable approach for detecting resistant parasites. In this study, 349 blood spots were collected from suspected malaria cases in areas in and around Port-au-Prince from March to July 2010. Among them, 121 samples that were Plasmodium falciparum positive by polymerase chain reaction were genotyped for drug-resistant pfcrt, pfdhfr, pfdhps, and pfmdr1 alleles. Among the 108 samples that were successfully sequenced for CQ resistant markers in pfcrt, 107 were wild type (CVMNK), whereas one sample carried a CQ-resistant allele (CVIET). Neutral microsatellite genotyping revealed that the CQ-resistant isolate was distinct from all other samples in this study. Furthermore, the remaining parasite specimens appeared to be genetically distinct from other reported Central and South American populations.

Independent Emergence of the Plasmodium falciparum Kelch Propeller Domain Mutant Allele C580Y in Guyana.

Suspected artemisinin resistance in Plasmodium falciparum can be explored by examining polymorphisms in the Kelch (PfK13) propeller domain. Sequencing of PfK13 and other gene resistance markers was performed on 98 samples from Guyana. Five of these samples carried the C580Y allele in the PfK13 propeller domain, with flanking microsatellite profiles different from those observed in Southeast Asia. These molecular data demonstrate independent emergence of the C580Y K13 mutant allele in Guyana, where resistance alleles to previously used drugs are fixed. Therefore, in Guyana and neighboring countries, continued molecular surveillance and periodic assessment of the therapeutic efficacy of artemisinin-based combination therapy are warranted.

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

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

Variation in Plasmodium falciparum Histidine-Rich Protein 2 (Pfhrp2) and Plasmodium falciparum Histidine-Rich Protein 3 (Pfhrp3) Gene Deletions in Guyana and Suriname.

Guyana and Suriname have made important progress in reducing the burden of malaria. While both countries use microscopy as the primary tool for clinical diagnosis, malaria rapid diagnostic tests (RDTs) are useful in remote areas of the interior where laboratory support may be limited or unavailable. Recent reports indicate that histidine-rich protein 2 (PfHRP2)-based diagnostic tests specific for detection of P. falciparum may provide false negative results in some parts of South America due to the emergence of P. falciparum parasites that lack the pfhrp2 gene, and thus produce no PfHRP2 antigen. Pfhrp2 and pfhrp3 genes were amplified in parasite isolates collected from Guyana and Suriname to determine if there were circulating isolates with deletions in these genes. Pfhrp3 deletions were monitored because some monoclonal antibodies utilized in PfHRP2-based RDTs cross-react with the PfHRP3 protein. We found that all 97 isolates from Guyana that met the inclusion criteria were both pfhrp2- and pfhrp3-positive. In Suriname (N = 78), 14% of the samples tested were pfhrp2-negative while 4% were pfhrp3-negative. Furthermore, analysis of the genomic region proximal to pfhrp2 and pfhrp3 revealed that genomic deletions extended to the flanking genes. We also investigated the population substructure of the isolates collected to determine if the parasites that had deletions of pfhrp2 and pfhrp3 belonged to any genetic subtypes. Cluster analysis revealed that there was no predominant P. falciparum population substructure among the isolates from either country, an indication of genetic admixture among the parasite populations. Furthermore, the pfhrp2-deleted parasites from Suriname did not appear to share a single, unique genetic background.

PET-PCR method for the molecular detection of malaria parasites in a national malaria surveillance study in Haiti, 2011.

BACKGROUND: Recently, a real-time PCR assay known as photo-induced electron transfer (PET)-PCR which relies on self-quenching primers for the detection of Plasmodium spp. and Plasmodium falciparum was described. PET-PCR assay was found to be robust, and easier to use when compared to currently available real-time PCR methods. The potential of PET-PCR for molecular detection of malaria parasites in a nationwide malaria community survey in Haiti was investigated. METHODS: DNA from the dried blood spots was extracted using QIAGEN methodology. All 2,989 samples were screened using the PET-PCR assay in duplicate. Samples with a cycle threshold (CT) of 40 or less were scored as positive. A subset of the total samples (534) was retested using a nested PCR assay for confirmation. In addition, these same samples were also tested using a TaqMan-based real-time PCR assay. RESULTS: A total of 12 out of the 2,989 samples screened (0.4%) were found to be positive by PET-PCR (mean CT value of 35.7). These same samples were also found to be positive by the nested and TaqMan-based methods. The nested PCR detected an additional positive sample in a subset of 534 samples that was not detected by either PET-PCR or TaqMan-based PCR method. CONCLUSION: While the nested PCR was found to be slightly more sensitive than the PET-PCR, it is not ideal for high throughput screening of samples. Given the ease of use and lower cost than the nested PCR, the PET-PCR provides an alternative assay for the rapid screening of a large number of samples in laboratory settings.

Novel Mutation in Cytochrome B of Plasmodium falciparum in One of Two Atovaquone-Proguanil Treatment Failures in Travelers Returning From Same Site in Nigeria.

BACKGROUND: Atovaquone-proguanil (AP) is the most commonly used treatment for uncomplicated Plasmodium falciparum malaria in the United States. Apparent AP treatment failures were reported 7 months apart in 2 American travelers who stayed in the same compound for foreign workers in Rivers State, Nigeria. METHODS: We analyzed pretreatment (day 0) and day of failure samples from both travelers for mutations in the P falciparum cytochrome B (pfcytb) and dihydrofolate reductase (pfdhfr) genes associated with resistance to atovaquone and cycloguanil, the active metabolite of proguanil, respectively. We genotyped the parasites and sequenced their mitochondrial genomes. RESULTS: On day 0, both travelers had proguanil-resistant genotypes but atovaquone-sensitive cytb sequences. Day of failure samples exhibited mutations in cytb for both travelers. One traveler had the common Y268S mutation, whereas the other traveler had a previously unreported mutation, I258M. The travelers had unrelated parasite genotypes and different mitochondrial genomes. CONCLUSIONS: Despite the infections likely having been contracted in the same site, there is no evidence that the cases were related. The mutations likely arose independently during the acute infection or treatment. Our results highlight the importance of genotyping parasites and sequencing the full cytb and dhfr genes in AP failures to rule out transmission of AP-resistant strains and identify novel mechanisms of AP resistance.

Adaptation of a multi-drug resistant strain of Plasmodium falciparum from Peru to Aotus lemurinus griseimembra, A. nancymaae, and A. vociferans monkeys.

A strain of Plasmodium falciparum from Peru was adapted to splenectomized Aotus nancymaae and Aotus vociferans monkeys. The Peru 134/CDC strain of P. falciparum was shown to be resistant to treatment with chloroquine in monkeys and partially resistant to mefloquine and malarone. Genetic mutations in crt, dhfr, dhps, and cytochrome b genes conferring drug resistance were also determined for this Peruvian strain of P. falciparum.

Antifolate resistance in Plasmodium falciparum: multiple origins and identification of novel dhfr alleles.

BACKGROUND: Sulfadoxine-pyrimethamine has been widely used as first-line therapy for uncomplicated malaria throughout sub-Saharan Africa. Recent studies conducted in Asia and Africa suggest the triple-mutant dhfr genotype (51I/59R/108N) may have been generated as a single event in Southeast Asia, with subsequent spread of the single lineage to the African continent, but this hypothesis needs further validation. METHODS: Direct sequencing of polymerase chain reaction (PCR) products, pyrosequencing, and cloning of PCR products were utilized to identify mutations in dhfr. To investigate the evolutionary history of dhfr alleles, we assayed microsatellite loci flanking dhfr along chromosome 4. RESULTS: A total of 15 of 479 samples from western Kenya showed the presence of I164L, in 5 different genotypes. We document C50R in 2 of our samples. Using microsatellite markers, we show 2 haplotypes for both the 51I/108N/164L and 51I/59R/108N/164L genotypes. Our results also show multiple lineages for the triple-mutant dhfr genotype in Africa. CONCLUSIONS: These findings highlight the importance of local characterization of alleles before molecular surveillance of drug-resistant alleles is considered in different endemic settings and populations.

Dramatic difference in diversity between Plasmodium falciparum and Plasmodium vivax reticulocyte binding-like genes.

Malaria parasite proteins involved in erythrocyte invasion are considered important vaccine targets. Members of the reticulocyte binding-like (RBL) family of Plasmodium merozoite proteins are found in human, simian, and rodent malaria parasites and function in the initial steps of erythrocyte selection and invasion. The RBL genes are large, ranging in size from 7.7 to 10 kb, and the extent of any sequence diversity in parasite populations is unknown. We present the first assessment of sequence diversity within RBL genes from the two major human malaria parasites: Plasmodium falciparum and P. vivax. Polymorphism within the RBL genes is generally limited, except for P. vivax reticulocyte binding protein 2 (PvRBP2), which has nucleotide diversity levels 25-fold higher than the other RBL genes. The PvRBP2 haplotypes appear to fall into two distinct classes of alleles, suggesting large-scale dimorphism in this gene. Polymorphisms were frequently clustered, suggesting that different RBL domains may be evolving under different selection and functional pressures.

A SICAvar switching event in Plasmodium knowlesi is associated with the DNA rearrangement of conserved 3' non-coding sequences.

Plasmodium knowlesi variant antigens are expressed at the surface of infected erythrocytes and are encoded by the Schizont Infected Cell Agglutination variant antigen (SICAvar) multigene family. The 3' region of the SICAvar gene locus encoding the 205 kDa variant antigen expressed in the Pk1(B+)1+ parasites was found to be altered compared to the Pk1(A+) parental clone. Here we report that this alteration is the result of a DNA rearrangement and that the original and altered 205 SICAvar alleles appear to encode bona fide variant antigens. Importantly, 205A and 205B SICAvar RNA sequences are detectable in similar apparent quantities as determined by quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR) amplification experiments. However, expression of the 205 kDa SICA protein at the surface of the infected erythrocyte is not characteristic of the Pk1(A+) parasites and the 205 SICAvar transcript has not been detected in Pk1(A+) parasites by northern blot analysis. Furthermore, we report that many distinct SICAvar transcripts were detected in P. knowlesi Pk1(B+)1+ cDNA library hybridization screens. Of special interest, in light of these data, distinctive differences at the 3' end of the 205A and 205B alleles are observed, which may be of functional importance. An analysis of the 3' untranslated region (UTR) of SICAvar genes in more than 100 sequences revealed a surprising common sequence pattern characterized by blocks of imperfect, GT-rich, heptad repeated motifs (Block I), followed by A and T rich homopolymers (Block II) and in a large number of genes, GC-rich segments (Block III). We show that this region undergoes extensive recombination and that the preferential stability of the 205 SICAvar transcript in Pk1(B+)1+ parasites may be associated with the presence of its specific Block III sequences. We speculate that the conserved yet polymorphic SICAvar 3'UTR sequences, and comparable regions in P. falciparum var genes, function in the stage-specific and developmentally regulated post-transcriptional gene silencing (PTGS) of variant antigen transcripts.

Plasmodium vivax merozoite surface protein PvMSP-3 beta is radically polymorphic through mutation and large insertions and deletions.

Plasmodium vivax causes the majority of malaria outside of sub-Saharan Africa and is an important burden for affected countries. The recent spread of drug-resistant P. vivax strains in these countries has led to renewed pressure for the development of a P. vivax vaccine. The complex life cycle of P. vivax presents many potential vaccine targets, but among the most promising candidates are subunits of the surface coat that surrounds the merozoite, the parasite stage that infects erythrocytes and initiates much of the pathology of malaria. Although the genes for several constituents of the P. vivax surface coat have now been cloned and sequenced, little is known about the extent to which these proteins vary between populations, an important consideration in vaccine development. The merozoite surface protein MSP-3beta is a member of a family of related merozoite surface proteins, all of which contain a central alanine-rich domain that is predicted to form a coiled-coil tertiary structure. We have sequenced the PvMSP-3 beta gene from P. vivax isolates originating in Central and South America, Asia and the Pacific. In this first assessment of PvMSP-3 beta variation between populations, we discovered widespread and significant diversity, mostly within the alanine-rich central region. We observed frequent differences in PvMSP-3 beta gene size, caused by the insertion and/or deletion of several large sequence blocks, as well as numerous single nucleotide polymorphisms and smaller scale insertions and deletions. Despite this high level of sequence diversity, certain physical properties of the encoded protein are maintained, particularly the ability to form coiled-coil tertiary structures, suggesting that although PvMSP-3 beta varies widely, it is under functional constraints. The implications for PvMSP-3 beta function and vaccine development are discussed.

Evaluation of a malaria rapid diagnostic test for assessing the burden of malaria during pregnancy.

Plasmodium falciparum infection during pregnancy may cause placental malaria and subsequently low birth weight, primarily through the placental sequestration of infected red blood cells. Measuring the burden of malaria during pregnancy usually involves determining the prevalence of placental malaria infection through microscopic examination of placental blood films, a difficult and error-prone process. A number of rapid diagnostic tests (RDTs) for malaria have been developed, most of them immunochromatographic dipstick assays. However, none have been tested for the direct determination of malaria antigen in placental blood. We undertook an evaluation of the Malaria Rapid Test (MAKROmed in determining placental malaria infection. The prevalence of placental parasitemia was 22.6% by microscopy, 51.0% by a polymerase chain reaction (PCR), and 43.1% by RDT. When the PCR was used as the gold standard, RDTs had a sensitivity of 89% and a specificity of 76%. The MAKROmed RDT was highly sensitive in the detection of placental malaria, but had lower than expected specificity.