Kuesel, A. C. | bioRxiv Science

bioRxiv2019

Utility of the Onchocerca volvulus mitochondrial genome for delineation of parasite transmission zones

In 2012, the reduction in Onchocerca volvulus infection prevalence through long-term mass ivermectin distribution in African meso- and hyperendemic areas motivated expanding control of onchocerciasis (river blindness) as a public health problem to elimination of parasite transmission. Given the large contiguous hypo-, meso- and hyperendemic areas with an estimated population of 204 million, sustainable elimination requires an understanding of the geographic, and in turn genetic, boundaries of different parasite populations to ensure interventions are only stopped where the risk of re-introduction of the parasite through vector or human migration from areas with ongoing transmission is acceptable. These boundaries, which define the transmission zones of the parasite, may be delineated by characterising the parasite genetic population structure within and between potential zones. We analysed whole mitochondrial genome sequences of 189 O. volvulus adults to determine the pattern of genetic similarity across three West African countries: Ghana, Mali, and Cote dIvoire. Population structure measures indicate that parasites from the Pru, Daka and Black Volta/Tombe river basins in central Ghana belong to one parasite population, showing that different river basins cannot be assumed to constitute independent transmission zones. This research forms the basis for developing tools for elimination programs to delineate transmission zones, to estimate the risk of parasite re-introduction via vector or human movement when mass ivermectin administration is stopped in one area while transmission is ongoing in others, to identify the origin of infections detected post-treatment cessation, and to investigate whether migration contributes to persisting prevalence levels during interventions.

evolutionary biology↗

bioRxiv2018

Reduced variation in Wolbachia density of larval stages in comparison with adults of Onchocerca volvulus: implications for clinical outcome of infection?

BackgroundWolbachia are important endosymbionts of filarial parasites. The Wolbachia of Onchocerca volvulus, the filarial pathogen responsible for the human disease onchocerciasis, is implicated in the immunopathology of the disease and may be associated with disease severity dependent on the density of Wolbachia. However, little is known in regards to the density and heterogeneity of Wolbachia in microfilariae, the life stage that is thought to be responsible for the pathology.\n\nResultsWe used a real-time qPCR relative copy number assay to estimate the number of Wolbachia genome(s) per nuclear genome of skin microfilariae (Mf), vector L1 and iL3, and nodulectomy adult male and female O. volvulus worms sampled in Ghana and the Democratic Republic of the Congo. Relatively low median Wolbachia copy numbers and variation was observed in the Mf and vector stages, in contrast to significantly higher median and more variable Wolbachia copy number from the iL3 stage to the adult worm stages.\n\nConclusionsThis study provides the first insight into variation in Wolbachia density between the major life stages of the parasite. The relatively invariant ratios observed for Mf and vector stages is in strong contrast to the high degree of variability of Wolbachia to nuclear ratios in adults and may indicate that the mutualistic relationship between the nematode and Wolbachia in these earlier stages is regulated differently, and certainly more stringently, than the relationship in adults.

molecular biology↗

bioRxiv2016

Genome-wide analysis of ivermectin response by Onchocerca volvulus reveals that genetic drift and soft selective sweeps contribute to loss of drug sensitivity

BackgroundTreatment of onchocerciasis using mass ivermectin administration has reduced morbidity and transmission throughout Africa and Central/South America. Mass drug administration is likely to exert selection pressure on parasites, and phenotypic and genetic changes in several Onchocerca volvulus populations from Cameroon and Ghana - exposed to more than a decade of regular ivermectin treatment - have raised concern that sub-optimal responses to ivermectins anti-fecundity effect are becoming more frequent and may spread.\n\nMethodology/Principal FindingsPooled next generation sequencing (Pool-seq) was used to characterise genetic diversity within and between 108 adult female worms differing in ivermectin treatment history and response. Genome-wide analyses revealed genetic variation that significantly differentiated good responder (GR) and sub-optimal responder (SOR) parasites. These variants were not randomly distributed but clustered in ~31 quantitative trait loci (QTLs), with little overlap in putative QTL position and gene content between countries. Published candidate ivermectin SOR genes were largely absent in these regions; QTLs differentiating GR and SOR worms were enriched for genes in molecular pathways associated with neurotransmission, development, and stress responses. Finally, single worm genotyping demonstrated that geographic isolation and genetic change over time (in the presence of drug exposure) had a significantly greater role in shaping genetic diversity than the evolution of SOR.\n\nConclusions/SignificanceThis study is one of the first genome-wide association analyses in a parasitic nematode, and provides insight into the genomics of ivermectin response and population structure of O. volvulus. We argue that ivermectin response is a polygenically-determined quantitative trait in which identical or related molecular pathways but not necessarily individual genes likely determine the extent of ivermectin response in different parasite populations. Furthermore, we propose that genetic drift rather than genetic selection of SOR is the underlying driver of population differentiation, which has significant implications for the emergence and potential spread of SOR within and between these parasite populations.\n\nAuthor summaryOnchocerciasis is a human parasitic disease endemic across large areas of Sub-Saharan Africa, where more that 99% of the estimated 100 million people globally at-risk live. The microfilarial stage of Onchocerca volvulus causes pathologies ranging from mild itching to visual impairment and ultimately, irreversible blindness. Mass administration of ivermectin kills microfilariae and has an anti-fecundity effect on adult worms by temporarily inhibiting the development in utero and/or release into the skin of new microfilariae, thereby reducing morbidity and transmission. Phenotypic and genetic changes in some parasite populations that have undergone multiple ivermectin treatments in Cameroon and Ghana have raised concern that sub-optimal response to ivermectins anti-fecundity effect may increase in frequency, reducing the impact of ivermectin-based control measures. We used next generation sequencing of small pools of parasites to define genome-wide genetic differences between phenotypically characterised good and sub-optimal responder parasites from Cameroon and Ghana, and identified multiple genomic regions differentiating the response types. These regions were largely different between parasites from both countries but revealed common molecular pathways that might be involved in determining the extent of response to ivermectins anti-fecundity effect. These data reveal a more complex than previously described pattern of genetic diversity among O. volvulus populations that differ in their geography and response to ivermectin treatment.

genomics↗