International movement of animals may cause the spread of diseases which are enzootic to other countries or continents. Control measures for certain very serious diseases such as trypanosomiasis may be such that the international movement of domestic animals for purposes of cross-breeding and agricultural improvement is severely hindered, giving rise to agricultural backwardness. Many serious epidemics of malaria and bubonic plague have been caused through man's accidental transport of infected animals.
Diseases caused by the international movement of animals may arise from the migration of wild animals or the import of either wild or domestic animals. The search for improved breeds of domestic stock such as cattle, sheep and pigs leads to cross-breeding and the importation of foreign breeds. The popularity of zoos in developed countries has led to an increase in the importation of wild animals which may set up diseases, as has the increased demand for exotic pets. Diseases from imported livestock in the USA include anthrax, scab, mange, blackleg, tuberculosis, fowl plague, fowl pox, and tick fever.