The Nidality of Zoonoses

Summary Many infectious diseases show a natural nidality dependent on the presence of a suitable population which can maintain the infection in the ecosystem. Field investigations of Leptospira interrogans serovar ballum in rats showed a high prevalence of infection in ship rats ( Rattus rattus) of low population density. Although a similar prevalence of infection was detected in high population densities of Norway rats (R. norvegicus) , infection could not be demonstrated in populations of low density. Studies of both domestic stock and wildlife, showed that different serovars are restricted to specific groups of animals in the ecosystem. By relating information from these field studies, with the results of experimental infection of laboratory mice with different serovars, it was possible to define a maintenance host as an animal which is capable of acting as a natural source of infection for its own species. A maintenance population was defined as a population of a species of animal which acts as a continuous reservoir of infection in a specific ecosystem. It is suggested that the identification of such populations and individuals, is important in establishing the nidality of a disease.