Article

Editorial: Asymptomatic Plasmodium parasitemia and the ecology of malaria transmission

The American journal of tropical medicine and hygiene (Impact Factor: 2.74). 07/2002; 66(6):639-40.
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Available from: Joseph M Vinetz, Sep 04, 2015
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    • "In the low transmission setting of Amazonia, asymptomatic malaria parasitaemia is surprisingly common [3,6,11,13,14]. This paradoxical pattern stands in contrast to high transmission regions where acquired immunity is manifested by asymptomatic parasitaemia that typically takes years and intense seasonal or continuous transmission to develop. "
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    ABSTRACT: Previous data have suggested that regulatory T cells (Tregs) balance protective immune responses with immune mediated pathology in malaria. This study aimed to determine to test the hypothesis that Treg proportions or absolute levels are associated with parasitaemia and malaria symptoms. Treg cells were quantified by flow cytometry as CD4+ CD25+, Foxp3+, CD127low T cells. Three patient groups were assessed: patients with symptomatic Plasmodium falciparum malaria (S), subjects with asymptomatic P. falciparum parasitaemia (AS) and uninfected control individuals (C). S, AS and C groups had similar absolute numbers and percentage of Tregs (3.9%, 3.5% and 3.5% respectively). Levels of parasitaemia were not associated with Treg percentage (p = 0.47). Neither relative nor absolute regulatory T cell numbers were found to be associated with malaria-related symptomatology in this study. Immune mechanisms other than Tregs are likely to be responsible for the state of asymptomatic P. falciparum parasitaemia in the Peruvian Amazon; but further study to explore these mechanisms is needed.
    Malaria Journal 03/2014; 13(1):108. DOI:10.1186/1475-2875-13-108 · 3.49 Impact Factor
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    • "Despite having been conducted at the end of rainy season, when the health information system usually records the highest malaria incidence [7], few malaria-infected individuals were identified and most of them had not had malaria-related symptoms within the two days prior to the survey. These results are in line with findings from other areas in the northern coast [39] and the Peruvian Amazon region [40,41] and support the hypothesis that this asymptomatic and submicroscopic reservoir of infections contributes to malaria transmission [42]. "
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    ABSTRACT: Where malaria endemicity is low, control programmes need increasingly sensitive tools for monitoring malaria transmission intensity (MTI) and to better define health priorities. A cross-sectional survey was conducted in a low endemicity area of the Peruvian north-western coast to assess the MTI using both molecular and serological tools. Epidemiological, parasitological and serological data were collected from 2,667 individuals in three settlements of Bellavista district, in May 2010. Parasite infection was detected using microscopy and polymerase chain reaction (PCR). Antibodies to Plasmodium vivax merozoite surface protein-119 (PvMSP119) and to Plasmodium falciparum glutamate-rich protein (PfGLURP) were detected by ELISA. Risk factors for exposure to malaria (seropositivity) were assessed by multivariate survey logistic regression models. Age-specific antibody prevalence of both P. falciparum and P. vivax were analysed using a previously published catalytic conversion model based on maximum likelihood for generating seroconversion rates (SCR). The overall parasite prevalence by microscopy and PCR were extremely low: 0.3 and 0.9%, respectively for P. vivax, and 0 and 0.04%, respectively for P. falciparum, while seroprevalence was much higher, 13.6% for P. vivax and 9.8% for P. falciparum. Settlement, age and occupation as moto-taxi driver during previous year were significantly associated with P. falciparum exposure, while age and distance to the water drain were associated with P. vivax exposure. Likelihood ratio tests supported age seroprevalence curves with two SCR rather for both P. vivax and P. falciparum indicating significant changes in the MTI over time. The SCR for PfGLURP was 19-fold lower after 2002 as compared to before (lamda1 = 0.022 versus lamda2 = 0.431), and the SCR for PvMSP119 was four-fold higher after 2006 as compared to before (lamda1 = 0.024 versus lamda2 = 0.006) . Combining molecular and serological tools considerably enhanced the capacity of detecting current and past exposure to malaria infections and related risks factors in this very low endemicity area. This allowed for an improved characterization of the current human reservoir of infections, largely hidden and heterogeneous, as well as providing insights into recent changes in species specific MTIs. This approach will be of key importance for evaluating and monitoring future malaria elimination strategies.
    Malaria Journal 09/2013; 12(1):339. DOI:10.1186/1475-2875-12-339 · 3.49 Impact Factor
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    • "Memory T and B cells to both P. vivax pre-erythrocytic stage antigens have also been shown to persist for years in areas of low transmission (e.g. Brazil, Thailand) (Bilsborough et al., 1993; Wipasa et al., 2010; Zevering et al., 1994) where individuals are frequently observed with asymptomatic parasitaemia (Alves et al., 2002; Branch et al., 2005; Vinetz and Gilman, 2002) indicating development of NAI. Thus, it appears that long-term immunological memory frequently develops in areas of low P. vivax transmission a ssociated with NAI. "
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    ABSTRACT: Population studies show that individuals acquire immunity to Plasmodium vivax more quickly than Plasmodium falciparum irrespective of overall transmission intensity, resulting in the peak burden of P. vivax malaria in younger age groups. Similarly, actively induced P. vivax infections in malaria therapy patients resulted in faster and generally more strain-transcending acquisition of immunity than P. falciparum infections. The mechanisms behind the more rapid acquisition of immunity to P. vivax are poorly understood. Natural acquired immune responses to P. vivax target both pre-erythrocytic and blood-stage antigens and include humoral and cellular components. To date, only a few studies have investigated the association of these immune responses with protection, with most studies focussing on a few merozoite antigens (such as the Pv Duffy binding protein (PvDBP), the Pv reticulocyte binding proteins (PvRBPs), or the Pv merozoite surface proteins (PvMSP1, 3 & 9)) or the circumsporozoite protein (PvCSP). Naturally acquired transmission-blocking (TB) immunity (TBI) was also found in several populations. Although limited, these data support the premise that developing a multi-stage P. vivax vaccine may be feasible and is worth pursuing.
    Advances in Parasitology 01/2013; 81:77-131. DOI:10.1016/B978-0-12-407826-0.00003-5 · 4.36 Impact Factor
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