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

Abstract

Background Onchocerciasis is a neglected tropical 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 nodule 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 nodule prevalence, and many districts are still unmapped despite their potential for onchocerciasis transmission. Methodology/Principle findings A 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 nodules 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 (RR: 0.9850, 95% BCI: 0.9751–0.995), precipitation seasonality (RR: 0.9837, 95% BCI: 0.9681–0.9995), and flow accumulation (RR: 0.9586, 95% BCI: 0.9321–0.9816) were negatively associated with O. volvulus nodule prevalence, while soil moisture (RR: 1.0218, 95% BCI: 1.0135–1.0302) was positively associated. The model estimated the number of pre-intervention cases of O. volvulus nodules in Ethiopia to be around 6.48 million (95% BCI: 3.53–13.04 million). Conclusions/Significance Nodule prevalence 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 priorities for elimination mapping in regions of Ethiopia that are currently unmapped, most of which have comparatively low infection prevalence.

Synthesising environmental, epidemiological, and genetic data to assist decision making for onchocerciasis elimination

“…A likely explanation for this correlation is that greater topological slope results in faster river flow essential for vector breeding. Similarly, soil moisture was also identified to be significant in an analysis of Ethiopian O. volvulus nodule prevalence data, where areas with high soil moisture occur in arable land that are usually inhabited by people who are exposed more to vector bites [2,64,97]. In contrast, temperature seasonality was negatively associated with mf prevalence (Additional file Table S4).…”

“…Finally, we fitted the model and assessed the relationship of environmental variables with the prevalence data. The details of fitting a spatial model to the prevalence data for geospatial mapping are available in [2]. The prediction of the posterior prevalence was made at a 2 km resolution considering the high computational cost of prediction on a lower resolution.…”

“…We compiled different continuous environmental rasters which might be ecologically relevant to the onchocerciasis distribution based on the published literature, field experiments on blackflies [63,64] and ecological factors identified with previous geospatial modelling studies [2,17,[65][66][67]. These environmental variables included distance to the nearest river, soil moisture, elevation, slope, temperature, and precipitation [2,65,67]. In addition, the dispersal capacity of the Simulium vector is dependent on the vegetation type and time of the year [68].…”

Geospatial distribution and predictive modeling of onchocerciasis in Ogun State, Nigeria

“…In view of this, we used our model to predict the hotspots for onchocerciasis in Ogun State. It is important to note that predicted prevalence in locations that are not currently occupied by humans can increase risk of transmission when people with infection establish communities in such places [ 42 ].…”

“…In Nigeria, the focus of onchocerciasis programs has shifted from control to elimination of O . volvulus transmission, prompting the need to employ newer tools to aid in efficient prioritization of decisions regarding elimination mapping and interventions [ 42 ] especially in areas that were not previously considered for intervention. As a result, we have generated a baseline prevalence map for Ogun State using geospatial modeling technique.…”