Immunity and immunological surveillance for malaria elimination in tropical islands

Abstract: Malaria remains one of the most significant global public health challenges. Nearly half of the world’s population remains at risk, largely in African Region. In the past decade, considerable progress has been made in the global fight to control and eliminate malaria. In some endemic countries, aggressive malaria control has reduced the malaria burden to a point where malaria elimination is becoming feasible. Nevertheless, sustained malaria control is crucial to prolong this downward trend for endemic countries. Understanding the contribution of local transmission, parasites movement, asymptomatic and sub-microscopic reservoirs can shape how active surveillances are used to pursue malaria elimination. Furthermore, a better understanding of the epidemiological effects of naturally acquired immunity against malaria is warranted to guide efforts to control or potentially eliminate the disease. In five cross-sectional surveys in Kenya conducted between 2012 and 2014 (N = 10,430), malaria prevalence (i.e. microscopy and PCR) and clinical assessments were evaluated to investigate the distribution and extent of malaria infections on islands (Mfangano, Takawiri, Kibuogi, and Ngodhe) and a mainland area (Ungoye) in Lake Victoria. Malaria prevalence varied significantly among setting; highest in the mainland, moderate in the large island, and lowest in small islands. More than 90% of infected populations were asymptomatic, and 50% of them were sub-microscopic with age-dependency for both proportions. These observations provide support for the inclusion of MDA in the area. Using the two surveys in 2012 (N = 4,112), antibody responses to P. falciparum PfAMA-1, PfMSP-119 and PfCSP were measured in order to describe transmission patterns and heterogeneity in Lake Victoria. The overall seroprevalence in Lake Victoria was 64% for PfAMA-1, 40% for PfMSP-119 and 13% for PfCSP. A clear relation between serological outcomes of PfAMA-1 and PfMSP-119 was observed with parasite prevalence and serology-derived EIR in heterogeneity in transmission. These observations collectively suggest that malaria serological measure could be an effective adjunct tool for assessing differences in transmission as well as for monitoring control and elimination in the high endemic area. Using msp1 and csp data from samples collected from 1996 to 2002, patterns of gene flow and population genetic structure of P. falciparum (N = 316) and P. vivax (N = 314) from seven sites on five islands (Gaua, Santo, Pentecost, Malakula, and Tanna) were analysed in order to understand the transmission and movement of Plasmodium parasites in Vanuatu. In general, genetic diversity was higher in P. vivax than P. falciparum from the same site. In P. vivax, high genetic diversity was likely maintained by a greater extent of gene flow among sites and islands. The results suggest that the current malaria control strategy in Vanuatu might need to be bolstered in order to control P. vivax movements across islands. To understand the impact of vector control interventions (i.e. ITNs) in Vanuatu, samples collected in 2003 (N = 231) and 2007 (N = 282) on Ambae Island were assessed for parasite infection (i.e. microscopy) and measured for antibody responses against three P. falciparum, three P. vivax and Anopheles-specific salivary gSG6 antigens. Decreases in seroprevalence were observed to all P. falciparum antigens but two of three P. vivax antigens, consistent with the pronounced decrease in parasite prevalence from 19% in 2003 to 3% in 2007. Seroprevalence to gSG6 also reduced significantly, suggesting that reduced exposure to vector bites was important to decrease in parasite prevalence. Together, decrease in both parasitological and seroepidemiological malaria metrics from 2003, and 2007 implied that reinforced vector control played a major role in the reduction of malaria transmission on Ambae Island.