The Health Impact of Onchocerciasis Control in Africa

Abstract

Onchocerciasis is a tropical disease endemic to Sub-Saharan Africa, Yemen, and parts of Latin America, and is caused by the filarial nematode Onchocerca volvulus, which is found exclusively in humans. Adult specimens of this roundworm reside in subcutaneous and deep-tissue nodules, and have an estimated average reproductive life span of about ten years (Duke 1968; Plaisier et al. 1991b). While female worms grow up to 50 cm long and remain sedentary in the nodules, male worms are somewhat smaller (up to 42 cm) and migrate from nodule to nodule, inseminating female worms (Schulz- Key and Albiez 1977; Duke 1993). As long as they are regularly inseminated, female worms produce larvae, so-called microfilariae (mf), which measure up to 360 μm and migrate through the host’s tissues and live up to two years (Duke 1968). Mf may be picked up from the host’s skin by the bite of a blackfly (genus Simulium). In Africa, species of the S. damnosum complex are mainly responsible for transmission of infection, whereas in Latin America, several other Simulium species drive transmission (Basáñez, Churcher, and Grillet 2009; Adler, Cheke, and Post 2010). All Simulium species breed in oxygen-rich water (e.g. river rapids). During their passage through a blackfly, the mf—now called stage L1 larvae—develop and moult into infective stage L3 larvae, which may be transmitted to another human during a fly’s next bloodmeal (Duke, Moore, and De León 1967). A small fraction of the transmitted stage L3 larvae eventually develop into adult worms (male or female). In turn, these may start reproducing after a prepatent period of 12 to 16 months (Duke 1968, 1980), completing the cycle of transmission (Figure 1.1).