Uptake ofOnchocerca volvulus(Nematoda: Onchocercidae) bySimulium(Diptera: Simuliidae) Is Not Strongly Dependent on the Density of Skin Microfilariae in the Human Host

Abstract

The relation between the number of microfilariae (mf) ingested by host-seeking vectors of human onchocerciasis and skin mf load is an important component of the population biology of Onchocerca volvulus, with implications for disease control and evaluation of the risk of transmission recrudescence. The microsimulation model ONCHOSIM has been used to assess such risk in the area of the Onchocerciasis Control Program (OCP) in West Africa, based on a strongly nonlinear relation between vector mf uptake and human mf skin density previously published. However, observed levels of recrudescence have exceeded predictions, warranting a recalibration of the model. To this end, we present the results of a series of fly-feeding experiments carried out in savanna and forest localities of West Africa. Flies belonging to Simulium damnosum s.s., S. sirbanum, S. soubrense, and S. leonense were fed on mf carriers and dissected to assess the number of ingested mf escaping imprisonment by the peritrophic matrix (the number of exo-peritrophic mf), a predictor of infective larval output. The method of instrumental variables was used to obtain (nearly) unbiased estimates of the parameters of interest, taking into account error in the measurement of skin mf density. This error is often neglected in these types of studies, making it difficult to ascertain the degree of density-dependence truly present in the relation between mf uptake and skin load. We conclude that this relation is weakly (yet significantly) nonlinear in savanna settings but indistinguishable from linearity in forest vectors. Exo-peritrophic mf uptake does not account for most of the density dependence in the transmission dynamics of the parasite as previously thought. The number of exo-mf in forest simuliids is at least five times higher than in the savanna vectors. Parasite abundance in human onchocerciasis is regulated by poorly known mechanisms operating mainly on other stages of the lifecycle.

Feasibility of Onchocerciasis Elimination with Ivermectin Treatment in Endemic Foci in Africa: First Evidence from Studies in Mali and Senegal

“…In the wet savanna the distribution of S. sirbanum overlaps with that of S. damnosum s.s. These two savanna species cannot be differentiated morphologically and there exist only few studies that have analyzed elements of vector competence for these two species separately, showing no consistent difference between S. sirbanum and S. damnosum s.s. [52],[53] …”

Onchocerciasis transmission in Ghana: biting and parous rates of host-seeking sibling species of the Simulium damnosum complex

“…Models of O. volvulus transmission dynamics that investigate intervention outcomes are mostly parameterised using S. damnosum s.s./S. sirbanum [18][19][20][21][22][23][24][25][26], with the exception of the model by Davies (1993) [27], based on transmission of forest onchocerciasis by S. soubrense B sensu Post, some quantitative analyses on other S. damnosum complex species including S. leonense and S. squamosum B [28,29], and a recent modelling study of the effect of climate change on O. volvulus transmission in Ghana and Liberia, including S. soubrense [30]. This research gap will need addressing in areas where different species compositions exist in the human-biting blackfly population.…”

Field-based evidence of single and few doses of annual ivermectin treatment efficacy in eliminating skin microfilaria load after a decade of intervention

“…The precarious public health situation of river blindness in these villages within the Gurara river basin was being mitigated. Moreover, black fly uptake of mf has been shown not to be associated with the level of skin mf density (8). Therefore, the surrounding villages need to be enlisted for treatment to avert few pockets of few infected individuals serving as reservoir of infection.Yet, a panacea for effective control based on ComDTI is to attain 100% geographic coverage and at least 50% individual treatment compliance.…”

“…Various epidemiological mathematical models have been designed to project the duration for treatment that may likely achieve the eradication of onchocerciasis. Among them are the projection by ONCHOSIM (onchocerciasis simulation model), a microsimulation mathematical model of the dynamics of onchocerciasis transmission, to explore the potentials of a hypothetical macrofilaricidal drug for the elimination of onchocerciasis under different epidemiological conditions (8). In all cases, a 10-year programme of annual macrofilariciding was considered.…”