Estimating the distribution and abundance of Rhipicephalus appendiculatus in Africa

The brown ear tick Rhipicephalus appendiculatus is responsible for transmitting the parasite Theileria parva in eastern, central and southern Africa, where it causes East Coast fever, Corridor disease and January disease in cattle. In an effort to assess the impact of these diseases and their control on livestock production in the region, studies are underway to model the factors controlling the distribution of the vector tick. Three recent studies have attempted to quantify and evaluate the variables influencing the distribution of R. appendiculatus, and these are reviewed and discussed in this paper.

The relationship between the distribution of an ecoclimatic index of suitability for the tick, calculated by the model CLIMEX on a 25 km² interpolated climate database for Africa, and of recorded observations of the tick itself, are evaluated. The sensitivity and specificity of the ecoclimatic index were calculated as 70.2–82.6% and 69.4–84.7%, respectively. In addition, addition, satellite-derived mean maximum normalized difference vegetation index (NDVI) for 1987 was assessed as a predictor of R. appendiculatus habitat in eastern Africa, by comparing the distribution of NDVI values in areas in which the presence of the tick has and has not been recorded. Some visual correlation was observed between mean maximum NDVI values of 0.15 or greater and known R. appendiculatus distribution in Kenya. In addition, this range of NDVI values corresponded closely to areas in Ethiopia where the tick does not occur but for which climatic suitability was predicted by CLIMEX. However, these NDVI values should not be considered universal predictors of habitat suitable for R. appendiculatus, and further study of the NDVI with ground truthing of habitat is considered necessary.