Insights into the local epidemiology of Plasmodium vivax in Solomon Islands

Background: Intensified malaria control programs over the last two decades have reduced malaria transmission in Solomon Islands by 90%. While malaria incidence has declined dramatically, there is an increase in the proportion of infections caused by Plasmodium vivax, the current predominant species in Solomon Islands. This study was designed to understand the P. vivax epidemiology in an island setting and determine the associated infection risks. Materials and methods: A longitudinal cohort study of children (6 months to 12 years old) was established in Ngella (Central Islands Province, Solomon Islands) at 20 villages in 2013-2014. Monthly active case detection (ACD) and passive case detection (PCD) were conducted for a period of 11 months. Plasmodium spp. infections were detected by light microscopy, rapid diagnostics test (RDT) and qPCR. Results: Of 1,111 children enrolled, 860 who attended ≥6 ACDs were included in the analyses. Prevalence of P. vivax infections was 11.9% (989 episodes) while P. falciparum infections was 0.3% (27 episodes). Most infections detected were submicroscopic (68%) and/or asymptomatic (92.8%). Of children who had at least one P. vivax infection, two thirds had subsequent infections, indicating that relapses from hypnozoites contributed a majority of P. vivax infections. The average incidence of new P. vivax blood stage infection (molFOB) was 1.1/child/year (CI95[0.9, 1.2]). Strong spatial heterogeneity within Ngella was observed, with prevalence ranging from 1.2% to 47.4% (p-value<0.001) while the molFOB ranged from 0.05/year to 4.6/year (p-value < 0.001). P. vivax prevalence risks were observed to increase with age (adjOR: 1.18 [1.13,1.23]), having a previous antimalarial record (adjOR: 2.03 [1.45,2.83]), or individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency (adjOR: 1.77 [1.16,2.70]). Bed net usage conferred significant protection against P. vivax infections (adjOR: 0.32 [0.14,0.72]), indicating that the distribution of bed nets in Solomon Islands malaria control programs is useful. Conclusion: Our findings suggest that P. vivaxremains endemic in Ngella and infections are geographically heterogeneous. Associated risks observed from this study should provide better guidance for the Solomon Islands National Malaria Control Program to develop a more comprehensive surveillance and case detection strategy that targets the hot spots of persisting asymptomatic P. vivax infections