Most of the future increase in livestock production is expected to occur in the tropical and subtropical regions of the world. Cattle are the most numerous of the ruminant species in the tropics and provide the largest quantity of animal food products. More than one-third of the world's cattle are found in the tropics. Disease is the major factor which prohibits full utilization of these regions for cattle production. Various infectious and transmissible viral, rick ettsial, bacterial, and particularly protozoan and helminthic diseases, are widespread in the tropics and exert a heavy toll on...