Onchocerciasis in the Cameroon-Chad border area after more than twenty years of annual mass ivermectin distribution

Abstract

Background: The main vectors of onchocerciasis in Africa are Simulium damnosum sensu lato, which transmit the causative agent Onchocerca volvulus. The force of transmission is driven by the vector density, hence influencing the disease prevalence and intensity. Onchocerciasis is currently targeted for elimination using Mass Drug Administration (MDA) of ivermectin, a potent microfilaricide. MDA in Cameroon began in 1987 in the Vina valley, an endemic cross-border area with Chad, known for high vector densities and pre-control endemicity. Evaluations in 2008-2010 in this area showed ongoing transmission, meanwhile, border areas in Chad were close to interrupting transmission. This study aimed to evaluate transmission in this area after several rounds of MDA since the last evaluation surveys. Methods: Blackflies were collected by Human Landing Catches at seven border sites in Cameroon, twice a week, from August 2021 to March 2022. A fraction of the flies was dissected for parity assessment and identification of Onchocerca larval stages. The transmission indices were estimated. Blackfly larvae were also collected from the breeding sites at the fly catching sites and identified to species level by cytotaxonomy. Results: A total of 14,322 female flies were collected and 6,901 were dissected. Of these 4421 (64.1%) were parous. The total biting rates were high, reaching up to 16,407 bites/man, and transmission potential (3rd stage larvae (L3) from head/All L3) were 367/702, 146/506, 51/55, 20/32, 0/3, 0/0, and 0/0 infective larvae/man, respectively, for Mbere-Tchad, Babidan, Hajam/V5, Gor, Djeing, Touoro and Koinderi. Infectivity rates (L3 from head) were 16.00, 12.75, 5.15 and 3.00 infective females (L3H)/1000 parous flies for Haijam, Mbere-Tchad, Babidan and Gor, respectively. These values exceed the WHO thresholds of <20 ATP or <1 infective female/1000 parous females. The major vectors identified were Simulium damnosum sensu stricto, S. squamosum, and for the first time in the area, S. yahense. Conclusions: More than twenty years of MDA has not eliminated onchocerciasis in the study area; hence this area is a potential source of re-introduction of onchocerciasis in Chad and would require alternative treatment strategies. Many factors such as MDA efficiencies, effectiveness of ivermectin and cytospecies composition may be contributing to transmission persistence.