Abstract
BackgroundSeroepidemiology can provide evidence for temporal changes in malaria transmission and is an important tool to evaluate the effectiveness of control interventions. During the early 2000s, Vanuatu experienced an acute increase in malaria incidence due to a lapse in funding for vector control. After the distribution of subsidised insecticide-treated nets (ITNs) resumed in 2003, malaria incidence decreased in the subsequent years. This study was conducted to find the serological evidence supporting the impact of ITN on exposure to Anopheles vector bites and parasite prevalence.MethodsOn Ambae Island, blood samples were collected from 231 and 282 individuals in 2003 and 2007, respectively. Parasite prevalence was determined by microscopy. Antibodies to three Plasmodium falciparum (PfSE, PfMSP-119, and PfAMA-1) and three Plasmodium vivax (PvSE, PvMSP-119, and PvAMA-1) antigens, as well as the Anopheles-specific salivary antigen gSG6, were detected by ELISA. Age-specific seroprevalence was analysed using a reverse catalytic modelling approach to estimate seroconversion rates (SCRs).ResultsParasite rate decreased significantly (P < 0.001) from 19.0% in 2003 to 3.2% in 2007, with a shift from P. falciparum predominance to P. falciparum-P. vivax co-dominance. Significant (P < 0.001) decreases were observed in seroprevalence to all three P. falciparum antigens but only two of three P. vivax antigens (except PvAMA-1; P = 0.153), consistent with the more pronounced decrease in P. falciparum prevalence. Seroprevalence to gSG6 also decreased significantly (P < 0.001), suggesting that reduced exposure to vector bites was important to the decrease in parasite prevalence between 2003 and 2007. Analyses of age-specific seroprevalence showed a three-fold decrease in P. falciparum transmission, but the evidence for the decrease in P. vivax transmission was less clear.ConclusionsSerological markers pointed to the effectiveness of ITNs in reducing malaria prevalence on Ambae Island between 2003 and 2007. The recombinant gSG6 antigen originally developed to indicate exposure to the Afrotropical vector An. gambiae may be used in the Pacific to complement the traditional measure of entomological inoculation rate (EIR).
Highlights
Seroepidemiology can provide evidence for temporal changes in malaria transmission and is an important tool to evaluate the effectiveness of control interventions
Characteristics of the study population and parasite prevalence In total, 231 and 282 people were sampled from Ambae in 2003 and 2007, respectively; 392 people were recruited from Futuna in 2011
A significant decrease in parasite prevalence was accompanied by significant decreases in seropositivity to most parasite antigens and the vector salivary antigen, suggesting that reinforced vector control played an important role in the reduction of malaria transmission on Ambae
Summary
Seroepidemiology can provide evidence for temporal changes in malaria transmission and is an important tool to evaluate the effectiveness of control interventions. During the early 2000s, Vanuatu experienced an acute increase in malaria incidence due to a lapse in funding for vector control. In areas of low transmission, longlasting antibody responses may be easier to detect than parasite prevalence in human populations or infected mosquitoes [2], making serological measures useful adjunct metrics to measure malaria transmission. Serological evidence of exposure to malaria has demonstrated successful interventions in some lowtransmission settings [7,8,9]. Malaria incidence in Vanuatu shows seasonal fluctuations, which are more pronounced for P. falciparum than P. vivax [10, 12]. The only known malaria vector is Anopheles farauti (s.s.), a member of the Anopheles punctulatus complex [13]
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