The biology of Onchocerca ochengi, a filarial nematode from African cattle, and the implications on the epidemiology of the causative agent of river blindness, Onchocerca volvulus

Abstract

Onchocerca ochengi is a common filarial parasite in African zebu cattle, which is non-pathogenic, in contrast to its closest known relative Onchocerca volvulus, which causes onchocerciasis (river blindness) in humans. In Cameroon it serves as an animal model to study the vector-borne disease, transmitted by the same black fly species of the Simulium damnosum complex. The present study focuses on the biology of O. ochengi, including its population dynamics in a longitudinal follow-up study of a cattle herd exposed to natural transmission since their birth. Another research focus lies on the decline in biting rates and transmission potentials of the local vector in formerly meso- and hyperendemic onchocerciasis foci in North Cameroon, in order to determine the actual risk for the population and prospects for elimination of river blindness. In the dry savannah focus at the Vina du Nord river basin the annual transmission potential dropped to 3.5 % of pre-control levels after 25 years of annual treatment of the human population with the microfilaricide ivermectin, whereas at the Vina du Sud focus on a highland plateau no transmission could be confirmed by molecular detection after 15 years of consecutive treatment intervention. The high cattle stock density on the Adamawa highlands has contributed to the regional interruption of transmission, and demonstrates that annual mass drug administration alone may not be sufficient to prevent onchocerciasis in hyperendemic foci. Moreover, a high genetic diversity of O. ochengi was shown in cattle and vector, including the discovery of a new mitochondrial haplotype with phylogenetic similarities to O. volvulus, and a putative hybrid of O. ochengi with O. dukei. Other results in studying the bovine onchocercosis model were insights into the reproductive biology of O. ochengi, time-scaled dynamics of immunological reactivity to recombinant O. volvulus antigens in zebu cattle naturally exposed to O. ochengi, the production and biochemical analysis of excretory-secretory products of O. ochengi adults, including their putative function, and the optimization of separating viable microfilariae of O. ochengi and O. gutturosa from adult uteri and cattle skin for subsequent culturing.