The population dynamics in mosquitoes and humans of two Plasmodium vivax polymorphs distinguished by different circumsporozoite protein repeat regions

Genetic diversity of Plasmodium vivax population in Anhui province of China

Background

Although the numbers of malaria cases in China have been declining in recent years, outbreaks of Plasmodium vivax malaria were still being reported in rural areas south of the Yellow River. To better understand the transmission dynamics of P. vivax parasites in China, the extent of genetic diversity of P. vivax populations circulating in Bozhou of Anhui province of China were investigated using three polymorphic genetic markers: merozoite surface proteins 1 and 3α (pvmsp-1 and pvmsp-3α) and circumsporozoite protein (pvcsp).

Methods

Forty-five P. vivax clinical isolates from Bouzhou of Anhui province were collected from 2009 to 2010 and were analysed using PCR/RFLP or DNA sequencing.

Results

Seven and six distinct allelic variants were identified using PCR/RFLP analysis of pvmsp-3α with HhaI and AluI, respectively. DNA sequence analysis of pvmsp-1 (variable block 5) revealed that there were Sal-I and recombinant types but not Belem type, and seven distinct allelic variants in pvmsp-1 were detected, with recombinant subtype 2 (R2) being predominant (66.7%). All the isolates carried pvcsp with VK210 type but not VK247 or P. vivax-like types in the samples. Sequence analysis of pvcsp gene revealed 12 distinct allelic variants, with VK210-1 being predominant (41.5%).

Conclusions

The present data indicate that there is some degree of genetic diversity among P. vivax populations in Anhui province of China. The genetic data obtained may assist in the surveillance of P. vivax infection in endemic areas or in tracking potential future disease outbreak.

A study on relapse/re-infection rate of Plasmodium vivax malaria and identification of the predominant genotypes of P. vivax in two endemic districts of Nepal

Background

Malaria is a major public health problem in Nepal inflicted primarily by the parasite Plasmodium vivax, - the only species responsible for relapse cases in Nepal. Knowledge on its relapse rate is important for successful malaria control, but is lacking in Nepal. The information on circulating predominant genotypes of P. vivax is equally relevant for high endemic districts of Nepal to understand the transmission dynamics of the parasite and to uncover the coverage and efficacy of potential vaccine beforehand.

Methods

A prospective observational study with a six months follow-up period was conducted from August 2010 to May 2011 in four health centres of Kailali and Kanchanpur districts of Nepal to access the relapse/re-infection rate of P. vivax. The prevalence and heterogeneity of its genotypes were identified by PCR-RFLP assay targeting central repeat region of circumsporozoite protein (Pvcsp).

Results

In total, 137 cases microscopically suspected to have P. vivax infection were enrolled in the study. Of these, 23 cases (17%) were detected for the relapse/ re-infection-during a six-month period, with a high proportion being male cases of age group 11–20 years. For genotyping, 100 whole blood samples were analysed, of which 95% of the parasite isolates were found to be of VK210 genotype. The minor genotype VK247 existed either in isolation or as mixed infection with VK210 in rest of the samples.

Conclusions

The relapse/re-infection rate of 17% was determined for P. vivax in Kailali and Kanchanpur districts of Nepal. A heterogeneous Pvcsp genotypic distribution of P. vivax was detected with VK210 being a predominant type, suggesting a complex transmission dynamics of the parasite. Expanding such study in other endemic regions of Nepal would help provide a complete picture on relapse/re-infection rate and parasite genotypic variability that can help in effective control and management of malaria in Nepal.

A review of mixed malaria species infections in anopheline mosquitoes

Background

In patients with malaria mixed species infections are common and under reported. In PCR studies conducted in Asia mixed infection rates often exceed 20%. In South-East Asia, approximately one third of patients treated for falciparum malaria experience a subsequent Plasmodium vivax infection with a time interval suggesting relapse. It is uncertain whether the two infections are acquired simultaneously or separately. To determine whether mixed species infections in humans are derived from mainly from simultaneous or separate mosquito inoculations the literature on malaria species infection in wild captured anopheline mosquitoes was reviewed.

Methods

The biomedical literature was searched for studies of malaria infection and species identification in trapped wild mosquitoes and artificially infected mosquitoes. The study location and year, collection methods, mosquito species, number of specimens, parasite stage examined (oocysts or sporozoites), and the methods of parasite detection and speciation were tabulated. The entomological results in South East Asia were compared with mixed infection rates documented in patients in clinical studies.

Results

In total 63 studies were identified. Individual anopheline mosquitoes were examined for different malaria species in 28 of these. There were 14 studies from Africa; four with species evaluations in individual captured mosquitoes (SEICM). One study, from Ghana, identified a single mixed infection. No mixed infections were identified in Central and South America (seven studies, two SEICM). 42 studies were conducted in Asia and Oceania (11 from Thailand; 27 SEICM). The proportion of anophelines infected with Plasmodium falciparum parasites only was 0.51% (95% CI: 0.44 to 0.57%), for P. vivax only was 0.26% (95% CI: 0.21 to 0.30%), and for mixed P. falciparum and P. vivax infections was 0.036% (95% CI: 0.016 to 0.056%). The proportion of mixed infections in mosquitoes was significantly higher than expected by chance (P < 0.001), but was one fifth of that sufficient to explain the high rates of clinical mixed infections by simultaneous inoculation.

Conclusions

There are relatively few data on mixed infection rates in mosquitoes from Africa. Mixed species malaria infections may be acquired by simultaneous inoculation of sporozoites from multiply infected anopheline mosquitoes but this is relatively unusual. In South East Asia, where P. vivax infection follows P. falciparum malaria in one third of cases, the available entomological information suggests that the majority of these mixed species malaria infections are acquired from separate inoculations.

High-throughput molecular diagnosis of circumsporozoite variants VK210 and VK247 detects complex Plasmodium vivax infections in malaria endemic populations in Papua New Guinea

Malaria is endemic in lowland and coastal regions of Papua New Guinea (PNG), and is caused by Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae and Plasmodium ovale. Infection by P. vivax is attributed to distinct strains, VK210 and VK247, which differ in the sequence of the circumsporozoite protein (pvcsp). Here, based upon sequence polymorphisms in pvcsp, we developed a post-PCR ligation detection reaction-fluorescent microsphere assay (LDR-FMA) to distinguish these P. vivax strains. This diagnostic assay was designed to detect the presence of both VK210 and VK247 P. vivax strains simultaneously in a high-throughput 96-well format. Using this assay, we analyzed human blood samples from the Wosera (n=703) and Mugil (n=986) regions to evaluate the prevalence of these P. vivax strains. VK210 and VK247 strains were found in both study sites. In the Wosera, single infections with VK210 strain were observed to be most common (41.7%), followed by mixed-strain (36.8%) and VK247 single-strain infections (21.5%). Similarly, in Mugil, VK210 single-strain infections were most common (51.6%), followed by mixed-strain (34.4%) and VK247 single-strain infections (14%). These results suggest that the distribution of P. vivax infections was similar between the two study sites. Interestingly, we observed a non-random distribution of these two P. vivax strains, as mixed-strain infections were significantly more prevalent than expected in both study sites (Wosera and Mugil χ(2)p-value<0.001). Additionally, DNA sequence analysis of a subset of P. vivax infections showed that no individual pvcsp alleles were shared between the two study sites. Overall, our results illustrate that PNG malaria-endemic regions harbor a complex mixture of P. vivax strains, and emphasize the importance of malaria control strategies that would be effective against a highly diverse parasite population.

Prevalence of Plasmodium vivax VK210 and VK247 subtype in Myanmar

Background

Plasmodium vivax is divided into two subtypes, a dominant form, VK210 and a variant form, VK247. This division is dependent on the amino acid composition of the circumsporozoite (CS) protein. In this study, the prevalence of the VK247 variant form of P. vivax was investigated in Myanmar.

Methods

The existence of malaria parasites in blood samples was determined by microscopic examination, polymerase chain reaction (PCR) and DNA hybridization assays. To test for antibodies against P. vivax and Plasmodium falciparum in blood samples, an indirect immunofluorescence antibody test (IFAT) was performed using asexual blood antigens. An enzyme-linked immunosorbent assay with synthetic VK210 and VK247 antigens was carried out to discriminate between the P. vivax subtypes.

Results

By thick smear examination, 73 (n = 100) patients were single infected with P. vivax, one with P. falciparum and 13 with both species. By thin smear, 53 patients were single infected with P. vivax, eight with only P. falciparum and 16 with both. Most of the collected blood samples were shown to be P. vivax positive (n = 95) by PCR. All cases that were positive for P. falciparum by PCR (n = 43) were also positive for P. vivax. However, 52 cases were single infected with P. vivax. IFAT showed antibody titres from 1:32 to 1:4,096. Additionally, using specific antibodies for VK210 and VK247, ELISA showed that 12 patients had antibodies for only the VK210 subtype, 4 patients had only VK247 subtype antibodies and 21 patients had antibodies for both subtypes. Using a DNA hybridization test, 47 patients were infected with the VK210 type, one patient was infected with VK247 and 23 patients were infected with both subtypes.

Conclusions

The proportion of the VK247 subtype in Myanmar was 43.1% (n = 25) among 58 positive cases by serodiagnosis and 25.6% (n = 24) among 94 positive cases by genetic diagnosis. In both diagnostic methods, the infection status of malaria patients is highly diverse with respect to malaria species, and multiple clonal infections are prevalent in Myanmar. Therefore, the complexity of the infection should be considered carefully when diagnosing malaria in Myanmar.

Prevalence of anopheline species and their Plasmodium infection status in epidemic-prone border areas of Bangladesh

Background

Information related to malaria vectors is very limited in Bangladesh. In the changing environment and various Anopheles species may be incriminated and play role in the transmission cycle. This study was designed with an intention to identify anopheline species and possible malaria vectors in the border belt areas, where the malaria is endemic in Bangladesh.

Methods

Anopheles mosquitoes were collected from three border belt areas (Lengura, Deorgachh and Matiranga) during the peak malaria transmission season (May to August). Three different methods were used: human landing catches, resting collecting by mouth aspirator and CDC light traps. Enzyme-linked immunosorbent assay (ELISA) was done to detect Plasmodium falciparum, Plasmodium vivax-210 and Plasmodium vivax-247 circumsporozoite proteins (CSP) from the collected female species.

Results

A total of 634 female Anopheles mosquitoes belonging to 17 species were collected. Anopheles vagus (was the dominant species (18.6%) followed by Anopheles nigerrimus (14.5%) and Anopheles philippinensis (11.0%). Infection rate was found 2.6% within 622 mosquitoes tested with CSP-ELISA. Eight (1.3%) mosquitoes belonging to five species were positive for P. falciparum, seven (1.1%) mosquitoes belonging to five species were positive for P. vivax -210 and a single mosquito (0.2%) identified as Anopheles maculatus was positive for P. vivax-247. No mixed infection was found. Highest infection rate was found in Anopheles karwari (22.2%) followed by An. maculatus (14.3%) and Anopheles barbirostris (9.5%). Other positive species were An. nigerrimus (4.4%), An. vagus (4.3%), Anopheles subpictus (1.5%) and An. philippinensis (1.4%). Anopheles vagus and An. philippinensis were previously incriminated as malaria vector in Bangladesh. In contrast, An. karwari, An. maculatus, An. barbirostris, An. nigerrimus and An. subpictus had never previously been incriminated in Bangladesh.

Conclusion

Findings of this study suggested that in absence of major malaria vectors there is a possibility that other Anopheles species may have been playing role in malaria transmission in Bangladesh. Therefore, further studies are required with the positive mosquito species found in this study to investigate their possible role in malaria transmission in Bangladesh.

Molecular characterization of Plasmodium vivax clinical isolates in Pakistan and Iran using pvmsp-1, pvmsp-3alpha and pvcsp genes as molecular markers

In this study, the diversity of Plasmodium vivax populations circulating in Pakistan and Iran has been investigated by using circumsporozoite protein (csp) and merozoite surface proteins 1 and 3alpha (msp-1 and msp-3alpha) genes as genetic markers. Infected P. vivax blood samples were collected from Pakistan (n=187) and Iran (n=150) during April to October 2008, and were analyzed using nested-PCR/RFLP and sequencing methods. Genotyping pvmsp-1 (variable block 5) revealed the presence of type 1, type 2 and recombinant type 3 allelic variants, with type 1 predominant, in both study areas. The sequence analysis of 33 P. vivax isolates from Pakistan and 30 from Iran identified 16 distinct alleles each, with one allele (R-8) from Iran which was not reported previously. Genotyping pvcsp gene also showed that VK210 type is predominant in both countries. Moreover, based on the size of amplified fragment of pvmsp-3alpha, three major types: type A (1800bp), type B (1500bp) and type C (1200bp), were distinguished among the examined isolates that type A was predominant among Pakistani (72.7%) and Iranian (77.3%) parasites. PCR/RFLP products of pvmsp-3alpha with HhaI and AluI have detected 40 and 39 distinct variants among Pakistani and Iranian examined isolates, respectively. Based on these three studied genes, the rate of combined multiple genotypes were 30% and 24.6% for Pakistani and Iranian P. vivax isolates, respectively. These results indicate an extensive diversity in the P. vivax populations in both studies.

Genetic diversity and multiple infections of Plasmodium vivax malaria in Western Thailand

Using two polymorphic genetic markers, the merozoite surface protein-3alpha (MSP-3alpha) and the circumsporozoite protein (CSP), we investigated the population diversity of Plasmodium vivax in Mae Sod, Thailand from April 2000 through June 2001. Genotyping the parasites isolated from 90 malaria patients attending two local clinics for the dimorphic CSP gene revealed that the majority of the parasites (77%) were the VK210 type. Genotyping the MSP3-alpha gene indicated that P. vivax populations exhibited an equally high level of polymorphism as those from Papua New Guinea, a hyperendemic region. Based on the length of polymerase chain reaction products, three major types of the MSP-3alpha locus were distinguished, with frequencies of 74.8%, 18.7%, and 6.5%, respectively. The 13 alleles distinguished by restriction fragment length polymorphism analysis did not show a significant seasonal variation in frequency. Genotyping the MSP-3alpha and CSP genes showed that 19.3% and 25.6% of the patients had multiple infections, respectively, and the combined rate was 35.6%. Comparisons of MSP-3alpha sequences from nine clones further confirmed the high level of genetic diversity of the parasite and also suggested that geographic isolation may exist. These results strongly indicate that P. vivax populations are highly diverse and multiple clonal infections are common in this malaria-hypoendemic region of Thailand.

The genetic diversity of Plasmodium vivax populations

Little is known of the genetic diversity and population structure of Plasmodium vivax, a debilitating and highly prevalent malaria parasite of humans. This article reviews the known polymorphic genetic markers, summarizes current data on the population structure of this parasite and discusses future prospects for using knowledge of the genetic diversity to improve control measures.

Plasmodium vivax blood-stage dynamics

We examine the dynamics of parasitemia and gametocytemia reflected in the preintervention charts of 221 malaria-naive U.S. neurosyphilis patients infected with the St. Elizabeth strain of Plasmodium vivax, for malariatherapy, focusing on the 109 charts for which 15 or more days of patency preceded intervention and daily records encompassed an average 98% of the duration of each infection. Our approximations of merogony cycles (via "local peaks" in parasitemia) seldom fit patterns that correspond to "textbook" tertian brood structures. Peak parasitemia was higher in trophozoite-induced infections than in sporozoite-induced ones. Relative densities of male and female gametocytes appeared to alternate, though without a discernably regular period. Successful transmission to mosquitoes did not depend on detectable gametocytemia or on absence of fever. When gametocytes were detected, transmission success depended on densities of only male gametocytes. Successful feeds occurred on average 4.7 days later in an infection than did failures. Parasitemia was lower in homologous reinfection, gametocytemia lower or absent.

Naturally occurring mixed infection of Plasmodium vivax VK210 and P. vivax VK247 in anopheles mosquitoes (Diptera: Culicidae) in western Thailand

We report the natural co-infection of a single Anopheles mosquito with Plasmodium vivax Grassi & Feletti phenotypes VK210 and VK247. In total, 8,452 anopheline mosquitoes collected between June 1999 and July 2001 were tested by ELISA for the presence of circumsporozoite (CS) protein to VK210, VK247, and P. falciparum (Welch) (PF). A total of 29 species was represented; however, the predominant species tested were A. minimus Theobald (4,632), A. sawadwongporni Rattanarithikul & Green (1,248), A. maculatus Theobald (1,201), A. campestris Reid (478), and A. barbirostris Van der Wulp (391). A total of 17 positive mosquitoes was identified by ELISA, and included the following: A. minimus infected with VK210 (5), PF (3), and both VK210 and VK247 (1), A. maculatus infected with VK210 (1), VK247 (1), and both VK210 and VK247 (1), A. campestris infected with VK210 (2), A. sawadwongporni infected with VK247 (1) and PF (1), and A. hodgkini Reid infected with VK247 (1). This is the first report of a single mosquito naturally infected with both VK210 and VK247.

Plasmodium vivax malaria vaccine development

Plasmodium vivax represents the most widespread malaria parasite worldwide. Although it does not result in as high a mortality rate as P. falciparum, it inflicts debilitating morbidity and consequent economic impact in endemic communities. In addition, the relapsing behavior of this malaria parasite and the recent resistance to anti-malarials contribute to making its control more difficult. Although the biology of P. vivax is different from that of P. falciparum and the human immune response to this parasite species has been rather poorly studied, significant progress is being made to develop a P. vivax-specific vaccine based on the information and experience gained in the search for a P. falciparum vaccine. We have devoted great effort to antigenically characterize the P. vivax CS protein and to test its immunogenicity using the Aotus monkey model. Together with other groups we are also assessing the immunogenicity and protective efficacy of the asexual blood stage vaccine candidates MSP-1 and DBP in the monkey model, as well as the immunogenicity of Pvs25 and Pvs28 ookinete surface proteins. The transmission-blocking efficacy of the responses induced by these latter antigens is being assessed using Anopheles albimanus mosquitoes. The current status of these vaccine candidates and other antigens currently being studied is described.

Malaria transmission by Anopheles punctulatus in the highlands of Irian Jaya, Indonesia

A 21-month (1991-1992) assessment of malaria and the relative contribution of Anopheles punctulatus and An. koliensis to its transmission was conducted in Oksibil, a highland community in Irian Jaya, Indonesia. Nearly all (98%) of the 2577 Anopheles from indoor or outdoor human-landing collections were An. punctulatus. Nineteen (0.75%) of the 2518 An. punctulatus tested were positive when tested for malaria circumsporozoite protein (CSP) by ELISA: nine (45% of positives) for Plasmodium vivax-210, six (30%) for Plasmodium falciparum, three (15%) for P. vivax-247 and two (10%) for P. malariae. One mosquito was positive for both P. vivax 210 and P. falciparum. All 59 An. koliensis tested were negative. All mosquitoes positive for CSP were from indoor collections, 89% from the main houses and the remaining 11% from the traditional dwellings (bokams). These findings are discussed with respect to highland malaria epidemiology and transmission in the Oksibil Valley.