Merozoite surface protein-3 alpha as a genetic marker for epidemiologic studies in Plasmodium vivax: a cautionary note

Genetic diversity of Plasmodium vivax populations from the China-Myanmar border identified by genotyping merozoite surface protein markers

BACKGROUND

Parasite diversity and population structure influence malaria control measures. Malaria transmission at international borders affects indigenous residents and migrants, defying management efforts and resulting in malaria re-introduction. Here we aimed to determine the extent and distribution of genetic variations in Plasmodium vivax populations and the complexity of infections along the China-Myanmar border.

METHODS

We collected clinical P. vivax samples from local and migrant malaria patients from Laiza and Myitsone, Kachin State, Myanmar, respectively. We characterized the polymorphisms in two P. vivax merozoite surface protein markers, Pvmsp-3α and Pvmsp-3β, by PCR-restriction fragment length polymorphism (PCR-RFLP) analysis. We sought to determine whether these genetic markers could differentiate these two neighboring parasite populations.

RESULTS

PCR revealed three major size variants for Pvmsp-3α and four for Pvmsp-3β among the 370 and 378 samples, respectively. PCR-RFLP resolved 26 fragment-size alleles by digesting Pvmsp-3α with Alu I and Hha I and 28 alleles by digesting Pvmsp-3β with Pst I. PCR-RFLP analysis of Pvmsp-3α found that infections in migrant laborers from Myitsone bore more alleles than did infections in residents of Laiza, while such difference was not evident from genotyping Pvmsp-3β. Infections originating from these two places contained distinct but overlapping subpopulations of P. vivax. Infections from Myitsone had a higher multiplicity of infection as judged by the size of the Pvmsp-3α amplicons and alleles after Alu I/Hha I digestion.

CONCLUSIONS

Migrant laborers from Myitsone and indigenous residents from Laiza harbored overlapping but genetically distinct P. vivax parasite populations. The results suggested a more diverse P. vivax population in Myitsone than in the border town of Laiza. PCR-RFLP of Pvmsp-3α offers a convenient method to determine the complexity of P. vivax infections and differentiate parasite populations.

PvMSP-3α and PvMSP-3β genotyping reveals higher genetic diversity in Plasmodium vivax parasites from migrant workers than residents at the China-Myanmar border

BACKGROUND

The genetic diversity of malaria parasites traces the origin and spread of new variants and can be used to evaluate the effectiveness of malaria control measures. Therefore, this study aims to improve the understanding of the molecular epidemiology of Plasmodium vivax malaria at the China-Myanmar border by genotyping the PvMSP-3α and PvMSP-3β genes.

METHODS

Blood samples were collected from P. vivax malaria patients along the China-Myanmar border. The PvMSP-3α and PvMSP-3β genes were amplified by polymerase chain reaction (PCR) and the genetic polymorphism and haplotype of the two genes were analyzed.

RESULTS

A total of 422 blood samples were used for this study, of which 224 were analyzed at PvMSP-3α and 126 at PvMSP-3β. Samples mainly were from young adults aged 18-45 years, although local patients were significantly younger than migrant laborers crossing the border at Tengchong (P < 0.0001). Molecular evolutionary analysis revealed that PvMSP-3α and PvMSP-3β underwent diversifying natural selection, and intragenic recombination contributed to the diversity of the isolates. Based on the length of the genes, we identified three types of PvMSP-3α [1.9-2.0 kb (Type-A), 1.4-1.5 kb (Type-B), and 1.1-1.3 kb (Type-C)] and two types of PvMSP-3β [1.7-2.2 kb (Type-A) and 1.4-1.5 kb (Type-B)]. Migrant laborers returning to China through Tengchong bore P. vivax infections displaying significantly higher genetic diversity than local residents.

CONCLUSIONS

Both PvMSP-3 paralogs were subjected to diversifying selection in each sample population. Clustering of alleles supports ephemeral endemic differentiation of alleles, but the broader phylogeny suggests that alleles transit the globe, perhaps accelerated by movements of migrants such as those transiting Tengchong.

Allelic variation of msp-3α gene in Plasmodium vivax isolates and its correlation with the severity of disease in vivax malaria

Malaria is a global socio-economic burden of which Plasmodium vivax contributes for about 70-80 million cases on an annual basis worldwide and 60-65% cases in India. Diversity observed in highly polymorphic Merozoite Surface Protein-3α (msp-3α) encoded by MSP-3 gene family, has been used efficiently for genotyping of P. vivax infection. This study aims to correlate the severity of clinical symptoms with parasite load, genotype of P. vivax and multiplicity of infection. Based on clinical symptoms classification, 31 (67.9%) out of 46 cases were found to be severe while 15 (32.6%) were non-severe and correlation of the severity of vivax infection with parasite load was not observed. Analysis of msp3-α allele genotype showed that out of 31 severe cases, 19 (61.2%) were single-clone infection cases whereas 12 (38.7%) were multi-clone infections. Similarly, out of 15 non-severe cases, 9 (60%) were single clone and 6 (40%) were multi-clone infections indicating the absence of a correlation between the multiplicity of infection and disease severity. Allele frequency observed was 65.9%, 23.4%, 23.4%, and 28.2% for allele A, B, C and D, respectively. An important finding was the greater distribution of allele D than alleles B and C, which has been reported as a rare allele otherwise. Further, of 13 cases with allele D, 76.9% (10/13) cases were severe. This study showed the absence of a correlation between the severity of clinical symptoms with parasite load and multiplicity of infection but at the same time drives a possibility of severe vivax malarial symptoms to have an association with the persistence of allele D in the population. This upon exploration can lead to the development of a target in detection of severe cases of malaria.

Malaria Molecular Epidemiology: An Evolutionary Genetics Perspective

Malaria is a vector-borne disease that involves multiple parasite species in a variety of ecological settings. However, the parasite species causing the disease, the prevalence of subclinical infections, the emergence of drug resistance, the scale-up of interventions, and the ecological factors affecting malaria transmission, among others, are aspects that vary across areas where malaria is endemic. Such complexities have propelled the study of parasite genetic diversity patterns in the context of epidemiologic investigations. Importantly, molecular studies indicate that the time and spatial distribution of malaria cases reflect epidemiologic processes that cannot be fully understood without characterizing the evolutionary forces shaping parasite population genetic patterns. Although broad in scope, this review in the Microbiology Spectrum Curated Collection: Advances in Molecular Epidemiology highlights the need for understanding population genetic concepts when interpreting parasite molecular data. First, we discuss malaria complexity in terms of the parasite species involved. Second, we describe how molecular data are changing our understanding of malaria incidence and infectiousness. Third, we compare different approaches to generate parasite genetic information in the context of epidemiologically relevant questions related to malaria control. Finally, we describe a few Plasmodium genomic studies as evidence of how these approaches will provide new insights into the malaria disease dynamics. *This article is part of a curated collection.

Polymorphism and natural selection in the merozoite surface protein 3F2 (PVX_97710) locus of Plasmodium vivax among field isolates

Plasmodium vivax, the chronic relapsing human malaria parasite with the most widespread distribution, possesses proteins associated with the merozoite surface that could be targets for host immune responses and potential vaccine candidates. Of these, the merozoite surface protein 3 of P. vivax (PvMSP3) is an attractive vaccine target as well as a genetic marker for epidemiological surveillance. PvMSP3 comprises a group of protein members encoded by a multigene family. Although some protein members, i.e. PvMSP3α and PvMSP3β, have been targets for molecular and immunological investigations, the most abundantly expressed protein member during late asexual erythrocytic stages, PvMSP3F2 (PVX_97710), remains unexplored. To address domain organization and evolution of this locus, the complete coding sequences of 31 P. vivax isolates from diverse malaria endemic areas of Thailand were analyzed and compared with 10 previously reported sequences. Results revealed that all PvMSP3F2 sequences differed but could be divided into 5 repeat-containing domains flanked by 6 non-repeat domains. Repeat domains II and IV at the 5' portion and domain X at the 3' portion exhibited extensive sequence and length variation whereas repeat domains VI and VIII located at the central region were relatively conserved. Despite a repertoire of PvMSP3F2 variants, predicted coiled-coil tertiary structure and predicted B-cell epitopes seem to be maintained. Evidence of intragenic recombination has been detected among field isolates in Thailand that could enhance sequence diversity at this locus. Non-repeat domains I and IX located at the 5' end and at the 3' portion, respectively, seem to have evolved under purifying selection. Evidence of positive selection was found in non-repeat domains III, V and VII where a number of predicted HLA class I epitopes were identified. Amino acid substitutions in these predicted epitopes could alter predicted peptide binding affinity or abolish peptide epitope property, suggesting that polymorphism in these epitopes conferred host immune evasion. Further studies on PvMSP3F2 are warranted, particularly on interaction with host immune system and the potential role of this PvMSP3 protein member as a vaccine target.

Effect of low complexity regions within the PvMSP3α block II on the tertiary structure of the protein and implications to immune escape mechanisms

Background

Plasmodium vivax merozoite surface protein 3α (PvMSP3α) is a promising vaccine candidate which has shown strong association with immunogenicity and protectiveness. Its use is however complicated by evolutionary plasticity features which enhance immune evasion. Low complexity regions (LCRs) provide plasticity in surface proteins of Plasmodium species, but its implication in vaccine design remain unexplored. Here population genetic, comparative phylogenetic and structural biology analysis was performed on the gene encoding PvMSP3α.

Results

Three LCRs were found in PvMSP3α block II. Both the predicted tertiary structure of the protein and the phylogenetic trees based on this region were influenced by the presence of the LCRs. The LCRs were mainly B cell epitopes within or adjacent. In addition a repeat motif mimicking one of the B cell epitopes was found within the PvMSP3a block II low complexity region. This particular B cell epitope also featured rampant alanine substitutions which might impair antibody binding.

Conclusion

The findings indicate that PvMSP3α block II possesses LCRs which might confer a strong phenotypic plasticity. The phenomenon of phenotypic plasticity and implication of LCRs in malaria immunology in general and vaccine candidate genes in particular merits further exploration.

Electronic supplementary material

The online version of this article (10.1186/s12900-019-0104-0) contains supplementary material, which is available to authorized users.

Microgeographically diverse Plasmodium vivax populations at the Thai-Myanmar border

Malaria transmission along international borders of the Greater Mekong Subregion is a big challenge for regional malaria elimination. At the Thai-Myanmar border, Plasmodium falciparum cases have dropped dramatically; however, increasing P. vivax prevalence and the emerging reports on hidden malaria burden due to asymptomatic infections demand attention. We conducted cross-sectional surveys to detect asymptomatic malaria infections in a small village located at Thai-Myanmar border and genotyped P. vivax infections in order to understand the level of genetic diversity on such a microgeographic scale. PCR/RFLP and DNA sequencing identified high levels of genetic polymorphisms at both Pvmsp3α and Pvmsp3β loci among P. vivax infections. Combining the PCR/RFLP patterns of Pvmsp3α and Pvmsp3β, a total of 10 genotypes were observed among 17 samples, while concatenated DNA sequences of Pvmsp3α and 3β generated 14 haplotypes with haplotype diversity of 0.97. These markedly diverse parasites on a microgeographic scale suggest the circulation of a considerably large parasite population at the international border.

Colorimetric Detection of Plasmodium vivax in Urine Using MSP10 Oligonucleotides and Gold Nanoparticles

Plasmodium vivax is the most prevalent cause of human malaria in the world and can lead to severe disease with high potential for relapse. Its genetic and geographic diversities make it challenging to control. P. vivax is understudied and to achieve control of malaria in endemic areas, a rapid, accurate, and simple diagnostic tool is necessary. In this pilot study, we found that a colorimetric system using AuNPs and MSP10 DNA detection in urine can provide fast, easy, and inexpensive identification of P. vivax. The test exhibited promising sensitivity (84%), high specificity (97%), and only mild cross-reactivity with P. falciparum (21%). It is simple to use, with a visible color change that negates the need for a spectrometer, making it suitable for use in austere conditions. Using urine eliminates the need for finger-prick, increasing both the safety profile and patient acceptance of this model.

Clinical manifestations and molecular mechanisms in the changing paradigm of vivax malaria in India

BACKGROUND

Plasmodium vivax once considered benign is now being increasingly associated with complicated malaria where the spectrum of complications is vast and like Plasmodium falciparum. The clinical data is important with respect to the immunopathological status of the patient. Several genes like the vir genes and pvcrt-o are speculated to be attributing to the severity of P. vivax malaria.

METHODS

In the present study we carried out the transcription analysis of five vir genes (vir 14-related, vir 12, vir 17-like, putative vir 14 and vir 10-related) and pvcrt-o gene in severe (n=12) and non-severe (n=7) P. vivax clinical infections and studied the correlation of these genes with clinical disease severity.

RESULTS

This study revealed multiorgan involvement in severe vivax cases with severe thrombocytopenia and anemia, the predominantly occurring symptoms. Four out of five vir genes and pvcrt-o showed a significant increase in expression levels of severe infections compared to the non-severe infections indicating their possible role in the changing pathogenesis of P. vivax.

CONCLUSIONS

The increased virulence in vivax malaria seems to be the result of multifactorial parameters changing it phenotypically as well as genotypically. However more studies are needed to understand the still nascent severity of P. vivax malaria.

Genetic diversity among Plasmodium vivax isolates along the Thai-Myanmar border of Thailand

Background

Knowledge of the population genetics and transmission dynamics of Plasmodium vivax is crucial in predicting the emergence of drug resistance, relapse pattern and novel parasite phenotypes, all of which are relevant to the control of vivax infections. The aim of this study was to analyse changes in the genetic diversity of P. vivax genes from field isolates collected at different times along the Thai–Myanmar border.

Methods

Two hundred and fifty-four P. vivax isolates collected during two periods 10 years apart along the Thai–Myanmar border were analysed. The parasites were genotyped by nested-PCR and PCR–RFLP targeting selected polymorphic loci of Pvmsp1, Pvmsp3α and Pvcsp genes.

Results

The total number of distinguishable allelic variants observed for Pvcsp, Pvmsp1, and Pvmsp3α was 17, 7 and 3, respectively. High genetic diversity was observed for Pvcsp (H_E = 0.846) and Pvmsp1 (H_E = 0.709). Of the 254 isolates, 4.3 and 14.6 % harboured mixed Pvmsp1 and Pvcsp genotypes with a mean multiplicity of infection (MOI) of 1.06 and 1.15, respectively. The overall frequency of multiple genotypes was 16.9 %. When the frequencies of allelic variants of each gene during the two distinct periods were analysed, significant differences were noted for Pvmsp1 (P = 0.018) and the Pvcsp (P = 0.033) allelic variants.

Conclusion

Despite the low malaria transmission levels in Thailand, P. vivax population exhibit a relatively high degree of genetic diversity along the Thai–Myanmar border of Thailand, in particular for Pvmsp1 and Pvcsp, with indication of geographic and temporal variation in frequencies for some variants. These results are of relevance to monitoring the emergence of drug resistance and to the elaboration of measures to control vivax malaria.

Multiplicity of Infection and Disease Severity in Plasmodium vivax

Background

Multiplicity of infection (MOI) refers to the average number of distinct parasite genotypes concurrently infecting a patient. Although several studies have reported on MOI and the frequency of multiclonal infections in Plasmodium falciparum, there is limited data on Plasmodium vivax. Here, MOI and the frequency of multiclonal infections were studied in areas from South America where P. vivax and P. falciparum can be compared.

Methodology/Principal Findings

As part of a passive surveillance study, 1,328 positive malaria patients were recruited between 2011 and 2013 in low transmission areas from Colombia. Of those, there were only 38 P. vivax and 24 P. falciparum clinically complicated cases scattered throughout the time of the study. Samples from uncomplicated cases were matched in time and location with the complicated cases in order to compare the circulating genotypes for these two categories. A total of 92 P. vivax and 57 P. falciparum uncomplicated cases were randomly subsampled. All samples were genotyped by using neutral microsatellites. Plasmodium vivax showed more multiclonal infections (47.7%) than P. falciparum (14.8%). Population genetics and haplotype network analyses did not detect differences in the circulating genotypes between complicated and uncomplicated cases in each parasite. However, a Fisher exact test yielded a significant association between having multiclonal P. vivax infections and complicated malaria. No association was found for P. falciparum infections.

Conclusion

The association between multiclonal infections and disease severity in P. vivax is consistent with previous observations made in rodent malaria. The contrasting pattern between P. vivax and P. falciparum could be explained, at least in part, by the fact that P. vivax infections have lineages that were more distantly related among them than in the case of the P. falciparum multiclonal infections. Future research should address the possible role that acquired immunity and exposure may have on multiclonal infections and their association with disease severity.

Previous studies on rodent malarias and mathematical models have postulated a link between multiclonal infections and disease severity. This association has been tested in Plasmodium falciparum mostly in Africa with limited information on P. vivax. Furthermore, there is a paucity of information from areas with low transmission. Here, we used samples available from a passive surveillance carried out in Colombia, South America. We found an association between multiclonal infections and disease severity in P. vivax but not in P. falciparum. Although the number of complicated malaria cases is low, the contrasting pattern between these two species emphasizes their epidemiological differences. We discuss how this pattern could be the result of a higher divergence among the P. vivax lineages co-infecting a patient. We hypothesize that low levels of acquired immunity may play a role in the association between multiclonal infections and disease severity.

Clinical Variation of Plasmodium falciparum eba-175, ama-1, and msp-3 Genotypes in Young Children Living in a Seasonally High Malaria Transmission Setting in Burkina Faso

The association between P. falciparum eba-175, ama-1, and msp-3 polymorphism in the pathogenicity of malaria disease was investigated. We therefore compared the prevalence of different alleles between symptomatic and asymptomatic malarial children under five years of age living in Burkina Faso. Blood filter papers were collected during the 2008 malaria transmission season from 228 symptomatic and 199 asymptomatic children under five years of age. All patients were living in the rural area of Saponé at about 50 km from Ouagadougou, the capital city of Burkina Faso. P. falciparum parasite DNA was extracted using QIAGEN kits and the alleles diversity was assessed by a nested PCR. PCR products were then digested by restriction enzymes based on already described polymorphic regions of the eba-175, ama-1, and msp-3 genes. The individual alleles eba-175_FCR3 and msp-3_K1 frequencies were statistically higher (p < 0.0001) in the asymptomatic group compared to the symptomatic ones. No statistically significant difference was noted in the prevalence of ama-1-3D7, ama-1-K1, and ama-1-HB3 genotypes between the two groups (p > 0.05). The comparative analysis of P. falciparum genotypes indicated that the polymorphism in eba-175 and msp-3 genotypes varied between asymptomatic and symptomatic clinical groups and may contribute to the pathogenesis of malaria.

Assessment of an automated capillary system for Plasmodium vivax microsatellite genotyping

BACKGROUND

Several platforms have been used to generate the primary data for microsatellite analysis of malaria parasite genotypes. Each has relative advantages but share a limitation of being time- and cost-intensive. A commercially available automated capillary gel cartridge system was assessed in the microsatellite analysis of Plasmodium vivax diversity in the Peruvian Amazon.

METHODS

The reproducibility and accuracy of a commercially-available automated capillary system, QIAxcel, was assessed using a sequenced PCR product of 227 base pairs. This product was measured 42 times, then 27 P. vivax samples from Peruvian Amazon subjects were analyzed with this instrument using five informative microsatellites. Results from the QIAxcel system were compared with a Sanger-type sequencing machine, the ABI PRISM(®) 3100 Genetic Analyzer.

RESULTS

Significant differences were seen between the sequenced amplicons and the results from the QIAxcel instrument. Different runs, plates and cartridges yielded significantly different results. Additionally, allele size decreased with each run by 0.045, or 1 bp, every three plates. QIAxcel and ABI PRISM systems differed in giving different values than those obtained by ABI PRISM, and too many (i.e. inaccurate) alleles per locus were also seen with the automated instrument.

CONCLUSIONS

While P. vivax diversity could generally be estimated using an automated capillary gel cartridge system, the data demonstrate that this system is not sufficiently precise for reliably identifying parasite strains via microsatellite analysis. This conclusion reached after systematic analysis was due both to inadequate precision and poor reproducibility in measuring PCR product size.

Molecular Evolution of PvMSP3α Block II in Plasmodium vivax from Diverse Geographic Origins

Block II of Plasmodium vivax merozoite surface protein 3α (PvMSP3α) is conserved and has been proposed as a potential candidate for a malaria vaccine. The present study aimed to compare sequence diversity in PvMSP3a block II at a local microgeographic scale in a village as well as from larger geographic regions (countries and worldwide). Blood samples were collected from asymptomatic carriers of P. vivax in a village at the western border of Thailand and PvMSP3α was amplified and sequenced. For population genetic analysis, 237 PvMSP3α block II sequences from eleven P. vivax endemic countries were analyzed. PvMSP3α sequences from 20 village-level samples revealed two length variant types with one type containing a large deletion in block I. In contrast, block II was relatively conserved; especially, some non-synonymous mutations were extensively shared among 11 parasite populations. However, the majority of the low-frequency synonymous variations were population specific. The conserved pattern of nucleotide diversity in block II sequences was probably due to functional/structural constraints, which were further supported by the tests of neutrality. Notably, a small region in block II that encodes a predicted B cell epitope was highly polymorphic and showed signs of balancing selection, signifying that this region might be influenced by the immune selection and may serve as a starting point for designing multi-antigen/stage epitope based vaccines against this parasite.

Malaria Molecular Epidemiology: Lessons from the International Centers of Excellence for Malaria Research Network

Molecular epidemiology leverages genetic information to study the risk factors that affect the frequency and distribution of malaria cases. This article describes molecular epidemiologic investigations currently being carried out by the International Centers of Excellence for Malaria Research (ICEMR) network in a variety of malaria-endemic settings. First, we discuss various novel approaches to understand malaria incidence and gametocytemia, focusing on Plasmodium falciparum and Plasmodium vivax. Second, we describe and compare different parasite genotyping methods commonly used in malaria epidemiology and population genetics. Finally, we discuss potential applications of molecular epidemiological tools and methods toward malaria control and elimination efforts.

The utility of genomic data for Plasmodium vivax population surveillance

Genetic polymorphisms identified from genomic sequencing can be used to track changes in parasite populations through time. Such tracking is particularly informative when applying control strategies and evaluating their effectiveness. Using genomic approaches may also enable improved ability to categorise populations and to stratify them according to the likely effectiveness of intervention. Clinical applications of genomic approaches also allow relapses to be classified according to reinfection or recrudescence. These tools can be used not only to assess the effectiveness of malaria interventions but also to appraise the strategies for malaria elimination.

Uncovering the transmission dynamics of Plasmodium vivax using population genetics

Population genetic analysis of malaria parasites has the power to reveal key insights into malaria epidemiology and transmission dynamics with the potential to deliver tools to support control and elimination efforts. Analyses of parasite genetic diversity have suggested that Plasmodium vivax populations are more genetically diverse and less structured than those of Plasmodium falciparum indicating that P. vivax may be a more ancient parasite of humans and/or less susceptible to population bottlenecks, as well as more efficient at disseminating its genes. These population genetic insights into P. vivax transmission dynamics provide an explanation for its relative resilience to control efforts. Here, we describe current knowledge on P. vivax population genetic structure, its relevance to understanding transmission patterns and relapse and how this information can inform malaria control and elimination programmes.

Genetic diversity of Plasmodium vivax population before elimination of malaria in Hainan Province, China

BACKGROUND

Hainan Province is one of the most severe endemic regions with high transmission of Plasmodium falciparum and Plasmodium vivax in China. However, the incidence of P. falciparum and P. vivax has dropped dramatically since 2007 and a national elimination malaria programme (NEMP) was launched after 2010. To better understand the genetic information on P. vivax population before elimination of malaria in Hainan Province, the extent of genetic diversity of P. vivax isolates in Hainan Province was investigated using four polymorphic genetic markers, including P. vivax merozoite surface proteins 1, 3α, and 3β (pvmsp-1, pvmsp-3α, and pvmsp-3β) and circumsporozoite protein (pvcsp).

METHODS

Isolates of P. vivax (n = 27) from Hainan Province were collected from 2009 to 2010 and pvmsp-1 and pvcsp were analysed by DNA sequencing, respectively. Using polymerase chain reaction-restriction fragment length polymorphism were analysed in pvmsp-3α, and pvmsp-3β.

RESULTS

The DNA sequencing analysis on pvmsp1 revealed that there were three allele types: Salvador-1 (Sal-1), Belem and recombinant (R) types. Among them, Sal-1 type was a dominant strain with eight variant subtypes (88.9%), whereas R- (3.7%) and Belem-type strains (7.4%) had one variant subtypes, respectively. All the isolates carried pvcsp with VK210 type accounting for 85.2% (23/27 isolates) and VK247 type accounting for 14.8% (4/27). Only type A and type B alleles were successfully amplified in pvmsp-3α gene, and a high level of polymorphism was observed in pvmsp-3α. Considering pvmsp-3β gene, type A was the predominant type in 17 isolates (63%), whereas type B was dominant in only ten isolates (37%).

CONCLUSION

The present data indicate that there was high degree of genetic diversity among P. vivax population in Hainan Province of China during the pre-elimination stage of malaria, with 26 unique haplotypes observed among 27 samples.

The Plasmodium vivax merozoite surface protein 3β sequence reveals contrasting parasite populations in southern and northwestern Thailand

BACKGROUND

Malaria control efforts have a significant impact on the epidemiology and parasite population dynamics. In countries aiming for malaria elimination, malaria transmission may be restricted to limited transmission hot spots, where parasite populations may be isolated from each other and experience different selection forces. Here we aim to examine the Plasmodium vivax population divergence in geographically isolated transmission zones in Thailand.

METHODOLOGY

We employed the P. vivax merozoite surface protein 3β (PvMSP3β) as a molecular marker for characterizing P. vivax populations based on the extensive diversity of this gene in Southeast Asian parasite populations. To examine two parasite populations with different transmission levels in Thailand, we obtained 45 P. vivax isolates from Tak Province, northwestern Thailand, where the annual parasite incidence (API) was more than 2%, and 28 isolates from Yala and Narathiwat Provinces, southern Thailand, where the API was less than 0.02%. We sequenced the PvMSP3β gene and examined its genetic diversity and molecular evolution between the parasite populations.

PRINCIPAL FINDINGS

Of 58 isolates containing single PvMSP3β alleles, 31 sequence types were identified. The overall haplotype diversity was 0.77 ± 0.06 and nucleotide diversity 0.0877±0.0054. The northwestern vivax malaria population exhibited extensive haplotype diversity (HD) of PvMSP3β (HD=1.0). In contrast, the southern parasite population displayed a single PvMSP3β allele (HD=0), suggesting a clonal population expansion. This result revealed that the extent of allelic diversity in P. vivax populations in Thailand varies among endemic areas.

CONCLUSION

Malaria parasite populations in a given region may vary significantly in genetic diversity, which may be the result of control and influenced by the magnitude of malaria transmission intensity. This is an issue that should be taken into account for the implementation of P. vivax control measures such as drug policy and vaccine development.

Antirelapse Efficacy of Various Primaquine Regimens for Plasmodium vivax

Background. Efficacy of standard dose of primaquine (PQ) as antirelapse for P. vivax has decreased. We aimed to assess efficacy of different PQ regimens. Methods. It was an open label, randomized, controlled, parallel group, assessor blind study comparing antirelapse efficacy of 3 PQ regimens (B = 15 mg/day × 14 days, C = 30 mg/day × 7 days, and D = 30 mg/day × 14 days) with no PQ group (A) in P. vivax patients. Paired primary and recurrence samples were subjected to 3 methods: (i) month of recurrence and genotyping, (ii) by PCR-RFLP, and (iii) PCR sequencing, to differentiate relapse and reinfection. The rates of recurrence relapse and reinfection were compared. Methods were compared for concordance between them. Results. The recurrence rate was 16.39%, 8.07%, 10.07%, and 6.62% in groups A, B, C, and D, respectively (P = 0.004). The relapse rate was 6.89%, 1.55%, 4%, and 3.85% as per the month of recurrence; 8.2%, 2%, 4.58%, and 3.68% (P = 0.007) as per PCR-RFLP; and 2.73%, 1.47%, 1.55%, and 1.53% as per PCR sequencing for groups A, B, C, and D, respectively. The concordance between methods was low, 45%. Conclusion. The higher recurrence rate in no PQ as compared to PQ groups documents PQ antirelapse activity. Regimens tested were safe. However, probable resistance to PQ warrants continuous monitoring and low concordance and limitations in the methods warrant caution in interpreting.

The origin and diversification of the merozoite surface protein 3 (msp3) multi-gene family in Plasmodium vivax and related parasites

The genus Plasmodium is a diversified group of parasites with more than 200 known species that includes those causing malaria in humans. These parasites use numerous proteins in a complex process that allows them to invade the red blood cells of their vertebrate hosts. Many of those proteins are part of multi-gene families; one of which is the merozoite surface protein-3 (msp3) family. The msp3 multi-gene family is considered important in the two main human parasites, Plasmodium vivax and Plasmodium falciparum, as its paralogs are simultaneously expressed in the blood stage (merozoite) and are immunogenic. There are large differences among Plasmodium species in the number of paralogs in this family. Such differences have been previously explained, in part, as adaptations that allow the different Plasmodium species to invade their hosts. To investigate this, we characterized the array containing msp3 genes among several Plasmodium species, including P. falciparum and P. vivax. We first found no evidence indicating that the msp3 family of P. falciparum was homologous to that of P. vivax. Subsequently, by focusing on the diverse clade of nonhuman primate parasites to which P. vivax is closely related, where homology was evident, we found no evidence indicating that the interspecies variation in the number of paralogs was an adaptation related to changes in host range or host switches. Overall, we hypothesize that the evolution of the msp3 family in P. vivax is consistent with a model of multi-allelic diversifying selection where the paralogs may have functionally redundant roles in terms of increasing antigenic diversity. Thus, we suggest that the expressed MSP3 proteins could serve as "decoys", via antigenic diversity, during the critical process of invading the host red blood cells.

Polymorphic patterns of the merozoite surface protein-3β in Korean isolates of Plasmodium vivax

BACKGROUND

The merozoite surface protein-3β of Plasmodium vivax (PvMSP-3β) is one of the candidate antigens for blood stage malaria vaccine development. The polymorphisms in PvMSP-3β have been reported in certain P. vivax isolates. However, the diversity of PvMSP-3β throughout its global distribution has not been well understood. In this study, the genetic diversity and the effects of natural selection in PvMSP-3β among P. vivax Korean isolates were analysed.

METHODS

Blood samples were collected from 95 patients with vivax malaria in Korea. The region flanking full-length PvMSP-3β was amplified by polymerase chain reaction and cloned into a TA cloning vector. The PvMSP-3β sequence of each isolate was determined and the polymorphic characteristics and effects of natural selection were analysed using the DNASTAR, MEGA4, and DnaSP programs.

RESULTS

Five different subtypes of PvMSP-3β were identified based on single nucleotide polymorphisms (SNPs), insertions, and deletions. Although a high level of sequence diversity was observed in the PvMSP-3β gene, the coiled-coil tertiary structure of the PvMSP-3β protein was well conserved in all of the sequences. The PvMSP-3β of Korean isolates is under natural selection. DNA polymerase slippage and intragenic recombination likely contributed to PvMSP-3β diversity in Korean P. vivax isolates.

CONCLUSIONS

The PvMSP-3β of Korean P. vivax isolates displayed polymorphisms, with SNPs, insertions and deletions scattered throughout of the gene. These results of parasite heterogeneity are relevant to the development of a PvMSP-3β based vaccine against P. vivax and the implementation of malaria control programmes in Korea.