Population biology of human onchocerciasis

Human onchocerciasis (river blindness) is the filarial infection caused by Onchocerca volvulus and transmitted among people through the bites of the S imulium vector. Some 86 million people around the world are at risk of acqu iring the nematode, with 18 million people infected and 600 000 visually im paired, half of them partially or totally blind. 99% of cases occur in trop ical Africa; scattered foci exist in Latin America. Until recently control programmes, in operation since 1975, have consisted of antivectorial measur es. With the introduction of ivermectin in 1988, safe and effective chemoth erapy is now available. With the original Onchocerciasis Control Programme of West Africa coming to an end, both the new African Programme for Onchoce rciasis Control and the Onchocerciasis Elimination Programme for the Americ as, rely heavily on ivermectin self-sustained mass delivery In consequence, the need for understanding the processes regulating parasite abundance in human and simuliid populations is of utmost importance. We present a simple mathematical framework built around recent analyses of expsuree- and densi ty-dependent processes operating, respectively, within the human and vector hosts. An expression for the basic reproductive ratio, R-0, is derived and related to the minimum vector density required for parasite persistence in localities of West Africa in general and northern Cameroon in particular. Model outputs suggest that constraints acting against parasite establishmen t in both humans and vectors are necessary to reproduce field observations, but those in humans may not fully protect against reinfection. Analyses of host age-profiles of infection prevalence, intensity, and aggregation for increasing levels of endemicity and intensity of transmission in the Vina v alley of northern Cameroon are in agreement with these results and discusse d in light of novel work on onchocerciasis immunology.