Differential impact of environment on malaria due to control interventions in Uganda, 2010-2018

Abstract

Background and aim Studies have estimated the impact of environment on malaria incidence although few have explored the differential impact due to vector control interventions. We aimed to evaluate the influence of temperature, rainfall, humidity, and vegetation, in presence or absence of long-lasting insecticide treated bednets (LLIN) and indoor residual spraying (IRS). Methods This study used weekly malaria cases from 2010 to 2018 from six health facility-based malaria surveillance in Uganda. Environmental variables were extracted from remote sensing sources and include enhanced vegetation index (MODIS), cumulative rainfall (ARC2), minimum and maximum temperature (ERA5), specific humidity (ERA5), averaged over different time periods (one to four months). Non-linearity of environmental variables was investigated, and general linear models based on a negative binomial distribution was used to explore the influence of ITN and LLIN on the malaria-environment relationship. Results A total of 204,252 malaria cases were laboratory confirmed and the median (range) weekly cases was 58.0 (0-597), rainfall 18.6 mm (0-129), minimum temperature 17.6˚C (12.3-24.2), maximum temperature 26.7˚C (20.1-34.8), and humidity 0.014 kg.kg (0.006-0.018). The best fit model was with the meteorological measures averaged over 3 months. All environmental variables showed a relatively linear pattern. Both IRS and LLIN were significantly associated with risk reduction (IRR: 0.39, 95% CI: 0.36–0.42 ; IRR: 0.71, 95% CI: 0.67–0.75, respectively). Marginal effects of environmental variables showed that joint effect of IRS and LLIN reduced the weekly predicted counts of malaria by 72.5% compared to no intervention. Conclusion LLIN and IRS both reduced the influence of environmental drivers of malaria and therefore morbidity in various transmission setting in Uganda. The benefits appeared to be greatest when the two interventions are used in combination.