Wild animals may help to spread epidemics that affect animals, especially domesticated ones, or may keep a disease enzootic when the epidemic has died out, thus causing serious difficulty in controlling animal diseases.
The responsibility of wild animals for the transmission of diseases to domestic animals is not known, but they are blamed for maintaining and spreading some of the most devastating diseases, such as anthrax, brucellosis, foot-and-mouth disease, Newcastle disease, liver fluke, rinderpest, trypanosomiasis, and rabies. Transmission may occur through direct contact, but is more likely to occur through pastureland and water infected by wild animals. Serious diseases of domestic animals, such as trypanosomiasis, may have a less serious effect on wild animals so that they remain alive and thus are able to transmit the disease more freely. The wild animal sector of disease is very difficult to control without mass extermination, which is undesirable from an ecological and cultural point of view, as well as being impracticable. Wild animals imported for zoos have introduced diseases to countries where it was not known before, such as American liver fluke brought to Italy by the importation of North American elk.