THE SPREAD OF TICK-BORNE BORRELIOSIS IN WEST-AFRICA AND ITS RELATIONSHIP TO SUB-SAHARAN DROUGHT

In West Africa, tick-borne relapsing fever is due to the spirochete Bo rrelia crocidurae and its geographic distribution is classically limit ed to the Sahel and Saharan regions where the vector tick Alectorobius sonrai is distributed. We report results of epidemiologic investigati ons carried out in the Sudan savanna of Senegal where the existence of the disease was unknown. A two-year prospective investigation of a ru ral community indicated that 10% of the study population developed an infection during the study period. Transmission patterns of B. crocidu rae to humans and the small wild mammals who act as reservoirs for inf ection were similar to those previously described in the Sahel region. Examination of 1,197 burrows and blood samples from 2,531 small mamma ls indicated a considerable spread of the known areas of distribution of A. sonrai and B. crocidurae. The actual spread of the vector and th e disease has affected those regions where the average rainfall, befor e the start of the extended drought in West Africa, reached up to 1,00 0 mm and corresponds to the movement of the 750-mm isohyet toward the south from 1970 to 1992. Our findings suggest that the persistence of sub-Saharan drought, allowing the vector to colonize new areas in the Sudan savanna of West Africa, is probably responsible for a considerab le spread of tick-borne borreliosis in this part of Africa.