OV16 antibody seropositivity reveals persistent onchocerciasis transmission and cross-border risk in Magwi County, South Sudan: A population-based survey
Introduction:
Onchocerciasis, or river blindness, is the second leading infectious cause of blindness after trachoma. It is a parasitic disease caused by Onchocerca volvulus, a filarial worm transmitted by black flies. In 34 countries, 249.5 million people are at risk, of which 14.6 million are infected, including 1.15 million people who are visually impaired or have gone blind. Onchocerciasis remains endemic in many parts of South Sudan. Magwi is an onchocerciasis-endemic county in South Sudan bordering northern Uganda districts where onchocerciasis transmission has been suppressed. The study was conducted in Magwi County to determine the prevalence of onchocerciasis by testing for Onchocerca volvulus-16 (OV16) antibodies to estimate the risk of cross-border reintroduction of onchocerciasis into northern Uganda.
Method:
The study adapted a community-based cross-sectional serological survey in Magwi County of South Sudan. The study recruited children aged 6-9 years. The study used a stratified cluster sampling approach, informed by data on population distribution and the proximity of communities to rivers infested with blackflies. Blood samples were collected using filter paper as dried blood spots. ELISA testing was done in the Uganda Onchocerciasis Reference Laboratory to confirm OV16 antibodies. Analysis software was Stata v17. The ethical standards were adhered to.
Results:
3,290 children participated in the study. 1,746 (53.1%) were males, and 6-7 years were 1,723 (52.4%). The overall OV16 seropositivity was 0.9%. Among the children, those aged 8-9 years had the highest seropositivity prevalence of 17 (1.1%). Across the clusters, transmission varied statistically (χ² = 15.23, p < 0.001), with a small effect size (Cramer’s V = 0.068). Whereas most clusters had no positive cases, the highest seroprevalence was in Garamu (9.6%) which followed by Malakia Central (7.6%), Olikwi (4.2%), and Kamuli (2.7%).
Conclusion:
Persistent onchocerciasis transmission in Magwi County poses a risk of cross-border transmission into Northern Uganda.