Onchocerciasis distribution and severity in five West African countries

The Onchocerciasis Control Programme in West Africa recently extended its operation to Guinea, Guinea-Bissau, the western part of Mali, Senegal and Sierra Leone. To estimate the number of people infected and blinded by the disease and to determine its distribution and severity in the extension area, 215 villages were selected, using a stratified random sampling procedure, and surveyed. All the relevant entomological information available at the time was used in the sampling procedure and in the selection of 92 non-representative villages that were surveyed to confirm the findings. In addition, the populations of 608 villages were examined to map out in detail the distribution of onchocerciasis in the areas at a high risk of onchocercal blindness. The study estimated that 1,475,367 people were infected and 23,728 were blinded from onchocerciasis out of a rural population of 4,464,183. The northern and western part of the study area and the lower Niger basin presented a low or no risk of onchocercal blindness. The upper Niger basin, the south-central part of Sierra Leone, and three small foci in the Gambia, Bakoye, and lower Niger river basins were areas with a high risk of onchocercal blindness. The other parts of the study area presented a medium risk of onchocercal blindness. By detecting the communities at risk of onchocercal disease this study permits the selection of populations for disease control based on mass distribution of ivermectin, a microfilaricide

The predicted and observed decline in onchocerciasis infection during 14 years of successful control of Simulium spp. in West Africa

In 55 villages from the well-protected central area of the Onchocerciasis Control Programme in West Africa (OCP), skin snip surveys have been carried out at regular intervals since the programme started, and the latest round of surveys was undertaken after 12-14 years of successful vector control. The observed trends in the prevalence and intensity of onchocerciasis infection in cohorts of adults were compared with the trends predicted using a host-parasite model. After 12-14 years of control the community microfilarial load (CMFL) was close to zero in all villages. During the last few years of control, the prevalence of infection declined at an accelerated rate, and this was predicted by the model. There was generally good agreement between observed and predicted trends. The predictions were based on an estimated average duration of infection of 10.4 years, which corresponds to a mean reproductive lifespan for Onchocerca volvulus of 9-9.5 years, and an upper limit of 15 years for 95% of the infections. Differences between the observed and predicted data included the trend in CMFL between the first and second surveys, which in 18 villages did not show the predicted decline. Furthermore, the observed final decline in prevalence was faster than predicted in the north-eastern part of the central OCP area. After 14 years of vector control, the level of onchocerciasis has fallen to such a low level that consideration is being given to ending larviciding