Individual exposure to Simulium bites and intensity of Onchocerca volvulus infection

Abstract

BackgroundOnchocerca volvulus, the causative agent of river blindness, is transmitted through the black fly Simulium damnosum s.l., which breeds in turbulent river waters. To date, the number of flies attacking humans has only been determined by standard fly collectors near the river or the village. In our study, we counted the actual number of attacking and successfully feeding S. damnosum s.l. flies landing on individual villagers during their routine day-time activities in two villages of the Sudan-savannah and rainforest of Cameroon. We compared these numbers to the number of flies caught by a standard vector-collector, one positioned near the particular villager during his/her daily activity and the other sitting at the nearest Simulium breeding site.ResultsUsing these data obtained by the two vector-collectors, we were able to calculate the Actual Index of Exposure (AIE). While the AIE in the savannah was on average 6,3%, it was 34% in the rainforest. The Effective Annual Transmission Potential (EATP) for individual villagers was about 20 fold higher in the rainforest compared to the savannah.ConclusionsHere we show for the first time that it is possible to determine the EATP. Further studies with more subjects are needed in the future. These data are important for the development of future treatment strategies.

Development of a Novel Trap for the Collection of Black Flies of the Simulium ochraceum Complex

“…This method involves stationing adult humans in areas of high Simulium densities and collecting black flies that attempt to land and blood-feed. Apart from the fact that this method has been a subject of criticism due to the potential risk of human collector’s exposure to infection during the early stages of a control program [19], it may be difficult for fly collectors to capture the large number flies needed to document that transmission has been interrupted where biting rates are low or highly seasonal. Thus, the development of trap(s) to replace human landing collections is becoming increasingly important as the focus in onchocerciasis shifts from control to elimination, and in some cases post-treatment surveillance [20].…”

Transmission of Onchocerca volvulus Continues in Nyagak-Bondo Focus of Northwestern Uganda after 18 Years of a Single Dose of Annual Treatment with Ivermectin

“…rates 21,22. Examination of the larval stages of O. volvulus involved dissection of the abdomens, thoraces, and heads of the collected Simulium flies.…”

Modelling exposure heterogeneity and density dependence in onchocerciasis using a novel individual-based transmission model, EPIONCHO-IBM: Implications for elimination and data needs

“…Since direct estimation of density-dependent parasite establishment within humans is not feasible experimentally (with the only available observational study on parasite establishment rates being that of [9]), data collection on heterogeneity in exposure to fly bites will be an important step in better resolving density dependence. This has been discussed previously [26] but available data remain limited. Since blackflies have very specific environmental requirements, with breeding sites varying in distance to human settlements, the estimation of individual-level variation in exposure to bites poses a substantial challenge.…”

“…Individuals may differ in their exposure to blackfly bites due to their attractiveness to flies [24], occupation [25,26] or age and sex [21,27]. Variation in exposure to vector bites has received theoretical and empirical attention in the context of lymphatic filariasis [28,29], schistosomiasis [30] and malaria [31,32] but is less well studied for onchocerciasis (although see [33]).…”