Description of a computer model of forest onchocerciasis transmission and its application to field scenarios of vector control and chemotherapy

Abstract

This paper describes a computer simulation model for onchocerciasis (SIMON). Using epidemiological and entomological data from a specific hyperendemic village in the forest area of Sierra Leone, the model is used to examine the effect of vector and chemotherapeutic control strategies, both separately and in combination, as well as the risk to an uninfected population caused by immigrant, infected Simulium damnosum and humans. The model suggests that, in this village, the human population of about 420 requires an average annual input of about 200 mature fecund, female Onchocerca volvulus per year to maintain a skin-snip prevalence of just under 70%. SIMON also predicts that 99% effective vector control would lead to eradication of all adult worms in 18 years, and that abandoning control before 14 years could lead to recrudescence. Chemotherapy with ivermectin at six-month intervals reaching 90% of eligible persons (effective 66%) might take 29 years to achieve eradication because of continuing transmission, particularly in the early years, but it would probably be possible to abandon treatments after 18 years because the residual worm population would no longer be self-sustaining. Combined ivermectin and vector control, both at reduced levels, could be as effective as 99% vector control. Immigrant infected flies are likely to pose a greater threat to an uninfected human population than small numbers of infected persons.

The model suggests that, at levels of infection undetectable by skin-snip, the parasite could linger in the human population for 30 or more years sustained by sporadic transmission. It is considered that, at these low levels, the parasite is more influenced by extrinsic factors, such as changes in human behaviour and ecologically-induced changes in vector density, than by the main parameters of its transmission dynamics.