Patterns of infection of haemoparasites in the fat sand rat, Psammomys obesus, in Tunisia, and effect on the host.

Two bacterial and one protozoan blood parasite, belonging to the genera Bartonella, Borrelia and Babesia, were studied in a Tunisian population of Psammomys obesus. Seasonal changes in the abundance of the parasites and host were monitored in a longitudinal field survey lasting 17 months. Blood samples collected during eight rodent-trapping sessions, between September 1995 and January 1997, were examined microscopically. Bartonella sp. showed a seasonal pattern, with most transmission occurring in summer and autumn; most rodents (90%) were infected in August-September, when they were at low density and adult. Borrelia sp. showed low prevalences, with few seasonal fluctuations, and Babesia sp. showed an intermediate pattern, differing from one year to another. In the cohort of adult rats, infections with Bartonella sp. and Babesia sp. were less prevalent in winter than in the previous summer. Single and mixed infections were equally prevalent in females and males, and in sexually active and inactive adults. In addition, infection had no apparent effect on the weight of adult P. obesus. The observation that the proportion of erythrocytes infected with Bartonella sp. decreased with increasing host age is probably indicative of some acquired immunity to this micro-organism. The absence of detectable infections with Borrelia sp. in old rats indicates that the prevalence and/or intensity of infection declines with host age or that infected animals die selectively. However, there was no indication that any of these parasites combined sufficient pathogenicity and abundance to have any measurable effect on the rodent population.