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 Bar tonella, Borrelia and Babesia, were studied in a Tunisian population of Psa mmomys obesus. Seasonal changes in the abundance of the parasites and host were monitored in a longitudinal field survey lasting 17 months. Blood samp les collected during eight rodent-trapping sessions, between September 1995 and January 1997, were examined microscopically. Bartonella sp. showed a s easonal pattern, with most transmission occurring in summer and autumn; mos t rodents (90%) were infected in August-September, when they were at low de nsity and adult. Borrelia sp. showed low prevalences, with few seasonal flu ctuations, and Babesia sp. showed an intermediate pattern, differing from o ne 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 sum mer. Single and mixed infections were equally prevalent in females and male s, and in sexually active and inactive adults. In addition, infection had n o apparent effect on the weight of adult P. obesus. The observation that th e proportion of erythrocytes infected with Bartonella sp. decreased with in creasing host age is probably indicative of some acquired immunity to this micro-organism. The absence of detectable infections with Borrelia sp. in o ld rats indicates that the prevalence and/or intensity of infection decline s with host age or that infected animals die selectively. However, there wa s no indication that any of these parasites combined sufficient pathogenici ty and abundance to have any measurable effect on the rodent population.