The molecular epidemiology of malaria in Solomon Islands

Abstract

© 2016 Dr. Andreea WaltmannHistorically, Solomon Islands in the Southwest Pacific has endured considerable P. falciparum and P. vivax burden. In the last 20 years, it has achieved 90% reduction in malaria cases through sustained, intensified malaria interventions (long lasting insecticide nets, indoor residual sprays and artemisinin-combination therapy) and is aiming for elimination by 2030. In 2012 and 2013, we conducted two cross-sectional surveys (study 1, all age, n=3501; study 2, age 0.5-12 years, n=1078) in Ngella, an area of low to moderate transmission. We aimed to investigate the natural reservoir and local epidemiology of P. vivax and P. falciparum. The contrast was striking. In the 2012 survey, only five clonal P. falciparum infections were identified from a single village and had the same msp2 genotpye. P. vivax prevalence was found to be moderately high (PCR, 13.4%), with predominantly afebrile, submicroscopic infections. The P. vivax infections displayed high genetic complexity (by genotyping with msp1F3 and MS16) and considerable spatial heterogeneity among and within different Ngella regions, and even at sub-village level with some households disproportionately harboring more infected co-inhabitants than others. In the 2013 study, a further seven P. falciparum infections were found in multiple locations, indicating that transmission of this species is continuing but at very low levels and infections are predominantly asymptomatic. To investigate the transmission scenario of the two species in more detail, we undertook population genetics analyses. We typed the five 2012 P. falciparum infections at 10 polymorphic microsatellite loci and 323 P. vivax infections at nine microsatellite loci. The five P. falciparum infections also clonal by this panel of 10 markers. Subsequent analyses of diversity (FST, GST, Jost’s D) and structure (Bayesian clustering) for P. vivax, revealed a genetically diverse population, but spatially fragmented, even among villages 6-15km apart. This indicates that whilst P. vivax may be more difficult to eliminate than P. falciparum, local parasite populations of both species have been affected by control interventions. A noteworthy epidemiological result from the 2012 survey was that living in a household with at least one other P. vivax carrier increased the risk of P. vivax infection, suggesting possible intra-household transmission. Subsequent analysis of genetic relatedness of P. vivax infections within households vs. among households indicated supported this hypothesis. Isolates from the same household were more genetically related than isolates from different households, and a high level of genetic kinship was retained among households located up to 100 meters of each other. Associations of P. vivax infection with human genetic factors known to confer protection against infection (α-thalassemia and Southeast Asian ovalocytosis, SAO) have been investigated in a second cross-sectional study conducted in 2013 in children aged 6 months to 12 years of age. SAO was not found in Ngella, whereas approximately a third of 1078 subjects were found to harbor the α-thalassemia alleles. The findings presented in this thesis will be discussed in the context of factors which may impact on follow-up elimination strategies in Solomon Islands, the Southwest Pacific and elsewhere in the endemic world where both P. falciparum and P. vivax are co-endemic.