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Summary: Background: The ongoing drought in sub-Saharan countries has led to the colonisation of west African Savanna by Ornithodoros sonrai; this tick acts as a vector for Borrelia crocidurae, which causes tick-borne relapsing fever (TBRF). Our aim was to ascertain the incidence of TBRF in west Africa. Methods: From 1990 to 2003, we monitored the incidence of TBRF in Dielmo, Senegal, by daily clinical surveillance and by blood testing of individuals with a fever. From 2002 to 2005, we investigated the presence of O sonrai in 30 villages in Senegal, Mauritania, and Mali, and measured by PCR the prevalence of B crocidurae. Findings: The average incidence of TBRF over 14 years was 11 per 100 person-years (range from 4 in 1990 to 25 in 1997). All age-groups presented a high incidence of the disease. In addition to relapses, repeated infections in the same individuals were common, with some affected by up to six distinct infections during the study period. Epidemiological studies indicated that 26 of the 30 studied villages (87%) were colonised by the vector tick O sonrai and that the average B crocidurae infection rate of the vector was 31%. Interpretation: The incidence of TBRF at the community level is the highest described in Africa for any bacterial disease. The presence of the vector tick in most villages investigated and its high infection rate suggest that TBRF is a common cause of fever in most rural areas of Senegal, Mauritania, and Mali. [ABSTRACT FROM AUTHOR]