Measures of malaria transmission, infection, and disease in an area bordering two districts with and without sustained indoor residual spraying of insecticide in Uganda

Abstract

Abstract Introduction: Tororo District, in Eastern Uganda, experienced a dramatic decline in malaria burden following the implementation of indoor residual spraying of insecticide (IRS) in 2014; the adjacent Busia District did not receive IRS. Data from areas near the margins of IRS coverage are limited. In this study, measures of malaria transmission, infection, and disease were compared between two parishes in Tororo District (Kayoro and Osukuru) and one in Busia District (Buteba), where IRS was not implemented.Methods: A cohort of 483 residents from 80 randomly selected households were followed from August 2020 to January 2021. Mosquitoes were collected every 2 weeks using CDC light traps in rooms where participants slept; parasitemia and gametoctyemia measured every 4 weeks by microscopy and PCR, and symptomatic malaria measured by passive surveillance. Results: The annual entomological inoculation rate was significantly higher in Buteba (108.2 infective bites/person/year), compared to Osukuru (59.0, p=0.001) and Kayoro (27.4, p<0.001). Overall, parasite prevalence was 19.5% by microscopy and 50.7% by PCR, with no significant differences between the three parishes. Among infected individuals, gametocyte prevalence by PCR was 45.5% and similar between sites. The incidence of malaria was significantly higher in Osukuru (2.46 episodes PPY) compared to Buteba (1.47, p=0.005) and Kayoro (1.09, p<0.001). For participants over 15 years of age, the risk of symptomatic malaria if microscopic parasitemia was present was higher in Osukuru (relative risk [RR]=2.99, p=0.03) compared to Buteba. Conclusions: Transmission intensity was modestly lower in two border parishes of Tororo District, where IRS was sustained for 6 years, compared to that in Busia District, where IRS was not implemented. Although parasite prevalence was similar between the three parishes, incidence of malaria was higher in one parish in Tororo. These observations highlight the complexity of relationships between malaria transmission intensity, parasite prevalence and disease incidence, and the potential for differences in immunity and clinical presentation resulting from differing transmission dynamics over time.