Is elimination of human onchocerciasis possible? Epidemiological, entomological and parasitological studies in a bovine model in Cameroon

点击翻译按钮获取中文摘要

Onchocerca volvulus, the causative agent of human onchocerciasis, which we study in Cameroon since 1976, has been brought close to elimination by the mass-distribution of the microfilaricidal drug ivermectin. Whether the few infective larvae that we still find in the man-biting vectors are sufficient to maintain the parasite endemic, shall be studied in three different geographic regions: The Adamawa highland, where transmission seems to be interrupted already due to zooprophylaxix, in the Sudan savanna in the North, where transmission still goes on at a rate close to the threshold level, and in the Metchum valley near Bamenda, from where there are no data yet. Simulium-fly catches on man and cattle shall provide data on the ongoing transmission and the proportion of bovine and animal filariae in the vectors, which are indicators of zooprophylaxis.Whether human onchocerciasis can be finaly eradicated shall basically depend from the population dynamics and reproductive biology of the Onchocerca worms, about which we know very little, because it cannot be studied in the human host. Instead we focus on a bovine homologue, Onchocerca ochengi, which we study in a herd of zebu cattle, exposed to natural transmission:- How do these worms regulate their population density in the vertebrate host?- Which factors govern the establishment of new-coming larvae and the mating and reproduction behaviour of adult worms?- How does the evolution of Onchocerca-species and strains go on, and how quickly resistance may be distributed amongst the worms?The bovine filaria O. ochengi, which is highly prevalent in our study area, provides an excellent research tool. In our herd of Zebu cattle, exposed to natural transmission from birth since almost three years, we can study the worm population dynamics in an ideal system. We expect their parasite population to reach its equilibrium during the period of this project, i.e. the rate of acquisition of new worms should equal the natural death-rate of old parasites. This has never been followed-up by a longitudinal study under well-controlled conditions!At the Max-Planck-Institute in Tübingen, molecular genetic techniques were developed that allow assigning single microfilariae to their mother and father worm. So we can study the reproductive behaviour and population dynamics in great detail. At the same time, we focus on the evolution of Onchocerca strains and species that are present in this hot-spot of Onchocerca speciation in the savanna of Central Africa.This project is based on over 20 years of successful collaboration with the Cameroonian Institute for Research for Rural Development (IRAD), which aims to study and control cattle parasites. Though bovine filariae are not of much veterinary importance, their research forms a bridge to human diseases and the participation of the Ngaoundere and Bamenda universities strengthens the research and teaching capacities of these institutions.