Annual fluctuation of Schistosoma japonicum infection in field rats, Rattus rattus mindanensis, in Dagami, Leyte, Philippines.

The annual fluctuation of Schistosoma japonicum infection in field rats, Rattus rattus mindanensis, in Leyte, the Philippines was investigated. One hundrend live-traps were set for one week every month from February, 1978 to February, 1979, in and around Central Elementary School II, Diagami, Leyte. Out of 199 rats, 163 (81.9%) were infected. The prevalence rate of S. japonicum infecton in rats was high during the relatively dry season, April to July, 1978. Meanwhile, the prevalence rate of schistosome cercariae in Oncomelania quadrasi of the study area increased during the relatively rainy season, October, 1978 to February, 1979. It is supposed that miracidia hatched from eggs discharged in the feces of rats during the relatively dry season would encounter snails, following which the emerging cercariae would penetrate the mammalian hosts, such as humans and rats, during the relatively rainy season. This might be the transmission dynamics of schistosomiasis in the endemic areas of Leyte. It is also suggested that this annual prevalence rate could be used as a parameter to formulate and evaluate control works. No infection was established in the youngest age group, the weanling. Then, the prevalence increased gradually by age. All rats of the older age group were infected. According to age composition, the field rat had no definite breeding season in Leyte. COP reaction was weak in sera of infected rats, but strongly exhibited in sera from rats possessing eggs in the spleen. Consequently, field rats not only maintain the infection as reservoir hosts, but also are an important source of infection as definitive hosts in the endemic areas of Leyte Island. Specifically, the rat role must be stressed, should human schistosomiasis ever be controlled.