Onchocerciasis Elimination Mapping in Chad: Absence of <i>Simulium damnosum</i> confirms No Local Transmission in formerly Hypoendemic Regions.

Abstract

The World Health Organisation guidelines recommend the process of onchocerciasis elimination mapping in areas previously considered hypoendemic to verify the elimination of disease transmission. Different areas of Chad were initially considered non-endemic or hypoendemic after the WHO REMO surveys were conducted in the country in 1998. Since then, no investigation has been conducted in these regions, classified as hypoendemic, to determine the onchocerciasis transmission status. In the era of elimination, it is crucial to verify the onchocerciasis situation in these regions to confirm or deny their hypoendemic status. This article presents the results of onchocerciasis elimination mapping (OEM) conducted since 2021 in Chad to verify the actual onchocerciasis transmission status in regions formerly considered hypoendemic. Following WHO recommendations, activities consisted of a desk review to assess potential disease transmission zones as a function of available literature on vectors, parasites, geographical data, and so on. The review excluded areas unsuitable for the vector Simulium damnosum, survival, and prioritised suspected
hypoendemic areas for further investigation. Potential hypoendemic areas were then investigated in the field for six months through breeding site assessments. These activities aimed to verify the presence or absence of the disease vector S. damnosum, which is the main indicator of potential local transmission. The Desk review mapping excluded all northern Chad, which is a desert zone and retains four provinces, Sila, Salamat, Guera, and Chari Baguirmi, as potential onchocerciasis transmission zones, given the presence of several rivers. Field breeding site assessments were conducted in a total of 19 S. damnosum s.l. known breeding rivers, located in the four hypoendemic areas. No S. damnosum larvae or pupae were found after the assessment of the 19 breeding sites. Our results indicate that Chad regions, considered previously hypoendemic, have no risk of local onchocerciasis transmission, and the country is close to interrupting onchocerciasis transmission at the national level.