In West Africa, tick-borne relapsing fever is due to the spirochete Borrelia crocidurae and its geographic distribution is classically limited to the Sahel and Saharan regions where the vector tick Alectorobirrs sonrai is distributed. We report results of epidemiologic investigations carried out in the Sudan savanna of Senegal where the existence of the disease was unknown. A two-year prospective investigation of a rural community indicated that 10% of the study population developed an infection during the study period. Transmission patterns of B. crocidurae to humans and the small wild mammals who act as reservoirs for infection were similar to those previously described in the Sahel region. Examination of 1,197 burrows and blood samples from 2,531 small mammals indicated a considerable spread of the known areas of distribution of A. sonrai and B. crocidurae. The actual spread of the vector and the disease has affected those regions where the average rainfall, before the start of the extended drought in West Africa, reached up to 1,000 mm and corresponds to the movement of the 750-mm isohyet toward the south fiom 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 considerable spread of tick-borne borreliosis in this part of Africa.

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We report results of a longitudinal survey designed to determine the importance and the dynamics of Borrelia crocidurae, the spirochete responsible for tick-borne relapsing fever in West Africa in rodents and insectivores in a rural area of northern Senegal. A total of 954 animals were caught during bimonthly capture sessions over a two-year period. Positive thick blood smears were recorded in 17.6% of the 740 rodents and 7.3% of the 55 musk shrews tested. Variations of prevalence were analyzed in Arvicanthis niloticus and Mastomys huberti, which represented 62.7% and 28.3%, respectively. of the animals captured, and 65.7% and 27.6%, respectively, of the animals found infected. Borrelia crocidurae prevalence was significantly different between captures and fluctuated separately for each species. Age-specific prevalence of B. crocidurae showed distinct patterns, decreasing with age from 50% in younger juveniles to 3% in older adults for A. niloticus, while increasing with age from 8% to 23% for M. huberti. No relationship was observed with animal abundance or with the season of the year for either species. These findings suggest that the diversity of the population dynamics of host-vector-parasite associations in the Sahel region of Senegal may be a key factor for the relative stability of the borreliosis reservoir.