These observations prompt a modification of the current paradigms of the pathogenesis of malaria and clear the way to investigate the pathophysiology of P. vivax infections.
Abstract. In an earlier study, we found that individuals with patent infection had significantly higher IgG antibody titers to the 19-kD C-terminal region of Plasmodium vivax merozoite surface protein 1 (PvMSP1) than individuals treated for malaria 1-4 months earlier. These results suggested that the antibody levels decreased rapidly following treatment. The present study was designed to determine the persistence of antibody response to the N-and C-terminal regions of PvMSP1 after infection with P. vivax in individuals from the city of Bélem in northern Brazil. Our results demonstrated that the vast majority of individuals had a significant decrease in antibody titers to the C-terminal region of PvMSP1 in a period of two months following treatment. Among responders to the C-terminal region, 44.4% became serologically negative and 44.4% had their antibody titers reduced by an average of 13-fold. Only 11.2% of the individuals had their antibody titers maintained or slightly increased during that period. A decrease in the antibody response to the recombinant protein representing the N-terminal region of PvMSP1 was also noted; however, it was not as dramatic. The rapid decrease in the antibody levels to the C-terminal region of PvMSP1 might contribute to the high risk of reinfection in these individuals.Plasmodium vivax is the second most prevalent species that causes malaria in humans and accounts for approximately 35 millions cases of the disease every year.1 In many countries, e.g., Brazil, P. vivax is the most common species, where it caused 76.8% of the 405,051 cases reported in 1997. 2 In spite of being highly prevalent in many parts of the world, the immunologic mechanisms operating in individuals exposed to P. vivax are poorly understood. We have characterized serum antibody and T cell reactivity of individuals from northern Brazil recently exposed to P. vivax malaria with 11 recombinant proteins representing the N-and Cterminal regions of the merozoite surface protein 1 of P. vivax (PvMSP1). 3 We found that a high frequency of individuals had IgG antibodies and T cell reactivity to at least one recombinant protein derived from PvMSP1. 3 The recombinant protein, which is based on the 19-kD C-terminal region of PvMSP1 (PvMSP1 19 ) containing the two epidermal growth factor (EGF)-like regions, was the most immunogenic during natural infection in humans. Antibodies or T cells of 83.8% of the individuals recognized this recombinant protein. 3 Furthermore, the antibody titers to the C-terminal region of PvMSP1 were higher than the titers to the N-terminal region. This high frequency of responders was also described in an independent survey performed in Papua New Guinea, where the sera of more than 80% of the individuals reacted with a recombinant protein representing PvMSP1 19 . 4 These immunoepidemiologic studies on naturally acquired immunity to the C-terminal of PvMSP1 are of particular importance since this region of MSP1 is being intensively studied as a candidate for development of a vaccine against malaria...
BackgroundWith low and markedly seasonal malaria transmission, increasingly sensitive tools for better stratifying the risk of infection and targeting control interventions are needed. A cross-sectional survey to characterize the current malaria transmission patterns, identify hotspots, and detect recent changes using parasitological and serological measures was conducted in three sites of the Peruvian Amazon.Material and MethodsAfter full census of the study population, 651 participants were interviewed, clinically examined and had a blood sample taken for the detection of malaria parasites (microscopy and PCR) and antibodies against P. vivax (PvMSP119, PvAMA1) and P. falciparum (PfGLURP, PfAMA1) antigens by ELISA. Risk factors for malaria infection (positive PCR) and malaria exposure (seropositivity) were assessed by multivariate survey logistic regression models. Age-specific seroprevalence was analyzed using a reversible catalytic conversion model based on maximum likelihood for generating seroconversion rates (SCR, λ). SaTScan was used to detect spatial clusters of serology-positive individuals within each site.ResultsThe overall parasite prevalence by PCR was low, i.e. 3.9% for P. vivax and 6.7% for P. falciparum, while the seroprevalence was substantially higher, 33.6% for P. vivax and 22.0% for P. falciparum, with major differences between study sites. Age and location (site) were significantly associated with P. vivax exposure; while location, age and outdoor occupation were associated with P. falciparum exposure. P. falciparum seroprevalence curves showed a stable transmission throughout time, while for P. vivax transmission was better described by a model with two SCRs. The spatial analysis identified well-defined clusters of P. falciparum seropositive individuals in two sites, while it detected only a very small cluster of P. vivax exposure.ConclusionThe use of a single parasitological and serological malaria survey has proven to be an efficient and accurate method to characterize the species specific heterogeneity in malaria transmission at micro-geographical level as well as to identify recent changes in transmission.
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