Prospective evaluation of onchocerciasis control strategies.

After more than 13 years of aerial larviciding, the Onchocerciasis Control Programme in West-Africa (OCP) is considering the future of its control activities. In the original OCP-area the prevalence of infection has been brought down to very low levels and cessation of larviciding is being contemplated. The introduction of ivermectin has broadened the spectrum of available control measures. Countries outside the OCP are also planning chemotherapy-based onchocerciasis control. The prediction of the epidemiological impact of the different control options is difficult in view of the many factors which affect the dynamics of a chronic infectious disease like onchocerciasis. We developed the computer simulation model ONCHOSIM, which takes these many factors and their interrelationships into account. The program has a broad spectrum of uses of which some examples are given. It is explained how the model is used to simulate endemic situations and epidemiological trends during vector control. These trends can be explained in terms of longevity of the parasite and other model parameters. Secondly, predictions are made about the risk of recrudescence after stopping larviciding. Finally, the possible role of ivermectin to control recrudescence after interruption of vector control and to reduce disease symptoms in an endemic situation is explored. The latter application is also important outside the OCP-area. This shows the usefulness of ONCHOSIM as a means for transferring the OCP-knowledge about onchocerciasis control to other countries. From our experience with ONCHOSIM, it is concluded that model based prospective evaluation of control measures is useful in synthesizing existing knowledge, in steering applied research, in interpreting the results of experimental studies, and last but not least, in aiding the policy-making process.