Blood-stage malaria parasites in the vertebrate host can develop either into the asexual, multiplying forms, called schizonts, or into gametocytes, the sexual stages of the parasite. In the present work we studied the differentiation into asexual parasites or gametocytes of the progeny of single, isolated schizonts of the clone 3D7A of Plasmodium falciparum, using monoclonal antibodies specific for the sexual or asexual stages of the parasite. We observed that schizonts obtained from a continuous culture undergoing serial cycles of growth and dilution with fresh red blood cells produced either only gametocytes or only asexual parasites, showing a high degree of commitment to one or the other developmental pathway. The relative proportion of schizonts which produced gametocytes was very low at low parasite densities in culture, while at high parasite densities a much greater proportion of schizonts produced gametocytes. Nevertheless, at both low and high parasite densities individual schizonts were almost always fully committed to producing only gametocytes or only asexual parasites.
The dynamics of multiple Plasmodium infections in asymptomatic children living under intense malaria transmission pressure provide evidence for a density-dependent regulation that transcends species as well as genotype. This regulation, in combination with species- and genotype-specific immune responses, results in nonindependent, sequential episodes of infection with each species.
Abstract. Allelic diversity at the Plasmodium vivax merozoite surface protein-3␣ (PvMsp-3␣) locus was investigated using a combined polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) protocol. Symptomatic patient isolates from global geographic origins showed a high level of polymorphism at the nucleotide level. These samples were used to validate the sensitivity, specificity, and reproducibility of the PCR/RFLP method. It was then used to investigate PvMsp3␣ diversity in field samples from children living in a single village in a malariaendemic region of Papua New Guinea, with the aim of assessing the usefulness of this locus as an epidemiologic marker of P. vivax infections. Eleven PvMsp-3␣ alleles were distinguishable in 16 samples with single infections, revealing extensive parasite polymorphism within this restricted area. Multiple infections were easily detected and accounted for 5 (23%) of 22 positive samples. Pairs of samples from individual children provided preliminary evidence for high turnover of P. vivax populations.Epidemiologic analyses of the population structure of Plasmodium parasites within and between endemic areas is essential for understanding the role of parasite diversity in the transmission of malaria as well as for designing and evaluating malaria vaccines. 1 Several large-scale studies have been conducted for P. falciparum, where the presence and dynamics of either single or multiple polymorphic antigenencoding genes have been investigated. 2-6 Comparable studies using highly polymorphic markers have yet to be reported for P. vivax. Here we present a P. vivax polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) protocol that will facilitate such analyses. Using this protocol, we demonstrate that multiple genotypes of P. vivax are present in an endemic area of Papua New Guinea and provide preliminary evidence for a rapid turnover of P. vivax genotypes within individuals. The analysis is based on the evaluation of the presence and number of P. vivax merozoite surface protein-3␣ (PvMsp-3␣) alleles. 7
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