The toll taken by disease is most obvious when sudden outbreaks are accompanied by heavy mortality, either naturally or as the outcome of a slaughter policy of control. This, however, is not the worst aspect. Less apparent, but much greater, are the continuing losses which, year in, year out, are caused by lowered productivity - smaller and slower liveweight gains, depressed milk yields, poorer work output, diminished fertility, and mortality among young stock. (One estimate gives an annual loss of US$ 300 million in the USA due to animal parasites alone, representing about 15% of the value of the average annual consumption of livestock products). In the developing countries, a high animal-disease rate is the normal partner of low productivity, usually associated with poor management and inadequate feeding. As disease depresses production still further, a vicious cycle develops, with the result that the livestock becomes of such little value that investment either in disease control or in improved breeding or management appears very difficult to justify.
Diseases include those of a fungal, viral, bacterial, parasitic, infectious and communicable nature, and zoonoses. Contagious diseases of animals of particular concern include cattle plague, foot-and-mouth disease, contagious peri-pneumonia, anthrax fever, sheep-pox, rabies, glanders, dourine (sleeping sickness), swine fever and fowl plague.