Seasonal trends in intestinal nematode infection and medicinal plant use among chimpanzees in the Mahale Mountains, Tanzania

A longitudinal study of nematode infection in chimpanzees was conducted between 1989 and 1994 on the M group chimpanzees of the Mahale Mountains National Park, Tanzania during two annual dry and rainy season periods and a third rainy season. Chemical and physical antiparasite properties of medicinal plant use against the strongyle nematode Oesophagostomum stephanostomum have recently been reported at Mahale. Here, the incidence of nematode infections were analyzed for seasonal trends to elucidate the possible influence of parasite infection on previously reported seasonality of medicinal plant use and to test the hypothesis that the use of these plants is stimulated by O. stephanostomum. The number of chimpanzees infected by O. stephanostomum was significantly higher in the rainy season than in the dry season of both 1989-1990 and 1991-1992. However, the incidence of Trichuris trichiura and Strongyloides fuelleborni showed no seasonality. Reinfection of individuals by O. stephanostomum occurred in synchrony with annual variation in rainfall: there was a sharp rise in the occurrence of new infections per individual within one to two months after the beginning of the first heavy rains of the season. This pattern coincides with the reproductive cycle of this nematode species. O. stephanostomum (95%) infections were associated significantly more frequently with medicinal plant use than either T. trichiura (50%) or S. fuelleborni (40%) infections. These observations are consistent with previous reports for the increased use of these plants during the rainy season and are consistent the hypothesis that medicinal plant use is stimulated by O. stephanostomum infection.