Impact of annual ivermectin distribution on the morbidity associated with onchocerciasis in savannah areas of Africa. (a) Prevalence of blindness due to onchocerciasis (across all ages). (b) Prevalence of visual impairment due to onchocerciasis (across all ages). (c) Prevalence of troublesome itch due to onchocerciasis (across all ages). Red, blue and green lines correspond, respectively, to a baseline endemicity of 80%, 60% and 40% microfilarial prevalence. Results shown assume a therapeutic coverage of 80%, 0.1% of systematic non-compliance, perennial transmission, and a 7% cumulative reduction in microfilarial production by female adult worms per ivermectin dose. The commencement of the intervention at year 1 is represented by the vertical dashed lines. Delays in the decrease of blindness and visual impairment are due to a two-year lag between vision loss in the present and microfilarial infection in the past. The initial sharp decline in troublesome itch is due to the assumed therapeutic effect of ivermectin followed by a more gradual decrease as adult worm prevalence declines.