Tick-borne borreliosis in West Africa is classically considered a rare disease whose geographic distribution is limited to Saharan and Sahelian regions. We report results of epidemiological investigations which indicate that tick-borne borreliosis is endemic in all regions of Senegal north to the 13 degrees 30'N latitude and is a major cause of morbidity in these areas. Our findings indicate a considerable range extension for the vector tick Alectorobius sonrai and suggest that the persistence of Subsaharan drought is responsible for a large spread of tick-borne borreliosis in West Africa.