Worms, flies, lice, and ticks are the major parasites of cattle, sheep, pigs, and poultry. Diseases caused or carried by parasites constitute a major obstacle to the development of profitable livestock enterprises. Some of these parasites also have significance for human health.
Parasites and many parasitic diseases are influenced by climate. Diseases spread by insects are encouraged when climatic conditions, temperature and excessive moisture in particular, favor the propagation of the vector. Many parasites and parasitic diseases in cattle and sheep reach their peak incidence in warm, wet summers and are relatively rare in dry seasons. Internal parasites are similarly influenced by climate. The direction of prevailing winds is of importance in many disease outbreaks, particularly in relation to the contamination of pasture and drinking water by fumes from factories and mines and the spread of diseases carried by insects (Blood and Radostits, 1989).
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