Gastrointestinal Nematodiasis in Dairy Cattle

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Abstract
Infection with gastrointestinal nematode parasites appears to be widespread in the cattle population. The pathologically important genera of worms all belong to the family Trichostrongylidae. The most pathogenic species is Ostertagia ostertagi, which can produce profound changes in abomasal mucosa of affected animals. These nematodes show developmental adaptations, such as seasonally arrested development, that are of prime importance to their survival and persistence on a year-to-year basis. Nematode prevalence in cattle is high; their association with outbreaks of clinical disease is lower. This has focused attention on their so-called subclinical effects, particularly reduction of milk production in lactating cows. Evidence for this effect is still equivocal, but constant exposure to low infection apparently can depress milk production. Subclinical effects have been demonstrated in young animals and include depressed rates of gain and interference in energy and nitrogen metabolism. These effects may be partly due to hypersensitivity reactions at the mucosal level in sensitized animals.