Geospatial modeling of pre-intervention prevalence of Onchocerca volvulus infection in Ethiopia as an aid to onchocerciasis elimination

Abstract BackgroundOnchocerciasis is a neglected tropical and filarial disease transmitted by the bites of blackflies, causing blindness and severe skin lesions. The change in focus for onchocerciasis management from control to elimination requires thorough mapping of pre-control endemicity to identify areas requiring interventions and to monitor progress. Onchocerca volvulus infection prevalence in sub-Saharan Africa is spatially continuous and heterogeneous, and highly endemic areas may contribute to transmission in areas of low endemicity or vice-versa. Ethiopia is one such onchocerciasis-endemic country with heterogeneous O. volvulus infection prevalence, and many districts are still unmapped despite their potential for O. volvulus infection transmission. Methodology/Principle findingsA Bayesian geostatistical model was fitted for retrospective pre-intervention nodule prevalence data collected from 916 unique sites and 35,077 people across Ethiopia. We used multiple environmental, socio-demographic, and climate variables to estimate the pre-intervention prevalence of O. volvulus infection across Ethiopia and to explore their relationship with prevalence. Prevalence was high in southern and northwestern Ethiopia and low in Ethiopia’s central and eastern parts. Distance to the nearest river (-0.015, 95% BCI: -0.025 – -0.005), precipitation seasonality (-0.017, 95% BCI: -0.032 – -0.001), and flow accumulation (-0.042, 95% BCI: -0.07 – -0.019) were negatively associated with O. volvulus infection prevalence, while soil moisture (0.0216, 95% BCI: 0.014 – 0.03) was positively associated. Conclusions/SignificanceInfection distribution was correlated with habitat suitability for vector breeding and associated biting behavior. The modeled pre-intervention prevalence can be used as a guide for determining priority for intervention in regions of Ethiopia that are currently unmapped, most of which have comparatively low infection prevalence. Author’s summaryAreas with unknown onchocerciasis endemicity may pose a threat to the goal of eliminating transmission: they may re-introduce onchocerciasis to areas where interventions have been successful. Additionally, because the vectors (and thus Onchocerca volvulus transmission) have specific ecological requirements for growth and development, changes in these ecological factors due to human activities (deforestation, modification of river flows by dam construction, climate change) might change patterns of parasite transmission and endemicity. To estimate the impact of these environmental changes, we must first identify ecological factors that determine transmission and prevalence. We have employed Bayesian geostatistical modeling to create a nationwide O. volvulus infection prevalence map for Ethiopia based on pre-intervention nodule prevalence and have explored the effect of environmental variables on O. volvulus infection prevalence. We have also identified areas that need additional data to increase the prediction accuracy of the map. We found that hydrological variables such as distance to the nearest river, precipitation seasonality, soil moisture, and flow accumulation are associated significantly with O. volvulus infection prevalence. We show that the pre-intervention prevalence can be estimated based on the ecological data and that predicted prevalence can be used as a guide to prioritize pre-intervention mapping.