The Spread of Tick-Borne Borreliosis in West Africa and Its Relationship to Sub-Saharan Drought

Abstract

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.

From the recent lessons of the Malagasy foci towards a global understanding of the factors involved in plague reemergence

“…In West Africa for example, as a consequence of the drought observed since 1970, the distribution area of gerbils increased southwards by around 200 km [20] 3 . The same increase has been observed for the tick, Alectorobius sonrai, vector of borreliosis and human cases have appeared in this area [59]. But global warming does not necessarily have the same consequences for plague: even though it is clearly a positive factor for the expansion of plague reservoirs, it may be the opposite for plague vectors.…”

Identification ofBartonellaein the Soft Tick SpeciesOrnithodoros sonraiin Senegal

“…The Ornithodoros sonrai soft tick is the only vector transmitting Borrelia crocidurae, the causative agent of relapsing fever (Hoogstraal 1985, Trape et al 1996, Vial et al 2006, Parola et al 2011. The average incidence of tick-borne relapsing fever in West Africa may be as high as 25 out of every 100 people per year, and the average infection rate of the vector with B. crocidurae is 31% (Vial et al 2006).…”

Interactions between landscape changes and host communities can regulateEchinococcus multilocularistransmission

“…(Rioux, Dereure & Perières, 1990), for Rickettsia spp. (Trape et al 1996), plague (Golvan & Rioux 1961), Schistosoma spp. (Combes, Leger & Golvan 1975), and more recently hantavirus (Weigler, 1995 ;Barclay & Rubinstein, 1997 ;Lundkvist et al 1997).…”