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

Abstract

Background: Population genetics is crucial for understanding the transmission dynamics of diseases like onchocerciasis. Landscape genetics identifies the ecological features that impact genetic variation between sampling sites. Here, we have used a landscape genetics framework to understand the relationship between environmental features and gene flow of the filarial parasiteOnchocerca volvulusand of its intermediate host and vector, blackflies in the genusSimulium. We analysed samples from the ecological transition region separating the savannah and forest ecological regions of Ghana, where the transmission ofO. volvulushas persisted despite almost half a century of onchocerciasis control efforts. Methods: We generated a baseline microfilarial prevalence map from the point estimates of pre-ivermectin microfilarial prevalence from 47 locations in the study area. We analysed mitochondrial data from 164 parasites and 93 blackflies collected from 15 communities and four breeding sites, respectively. We estimated population genetic diversity and identified correlations with environmental variables. Finally, we compared baseline prevalence maps to movement suitability maps that were based on significant environmental variables. Results: We found that the resistance surfaces derived from elevation (r = 0.793, p = 0.005) and soil moisture (r = 0.507, p = 0.002) were significantly associated with genetic distance between parasite sampling locations. Similarly, for the vector populations, the resistance surfaces derived from soil moisture (r = 0.788, p = 0.0417) and precipitation (r = 0.835, p = 0.0417) were significant. The correlation between the baseline parasite prevalence map and the parasite resistance surface map was stronger than the correlation between baseline prevalence and the vector resistance surface map. The central parts of the transition region which were conducive for both the parasite and the vector gene flow were most strongly associated with high baseline onchocerciasis prevalence. Conclusions: We present a framework for incorporating environmental, genetic, and prevalence data for identifying when ecological conditions are favourable for onchocerciasis transmission between communities. We identified areas with higher suitability for parasite and vector gene flow, which ultimately might help us gain deeper insights into defining transmission zones for onchocerciasis. Furthermore, this framework is translatable to other onchocerciasis endemic areas and to other vector-borne diseases.

Procedure for Handling and Storage of Onchocerca volvulus Microfilariae Obtained from Skin Snips for Downstream Genetic Work

“…Our goal is to develop tools for national program managers that will help them address some of these challenges, including tools for discrimination of parasites from different areas and delineation of transmission zones [19][20][21], assessment of the risks for recrudescence if interventions are stopped in some geographic areas while transmission is ongoing in other parts of the transmission zone [19], monitoring the decline in the number of reproductively active parasites [22], and monitoring the prevalence of parasites responding sub-optimally to ivermectin [23]. To support development of these tools, we are exploiting the information available in the parasite and vector genomes [24].…”