Although there are many species of worm parasites harbored in the gastrointestinal and respiratory tracts of cattle, only a few target species are clinically and economically important. These include the brown stomach worm Ostertagia the coccidia Eimeria bovis and the lungworm Dictyocaulus. These are of great significance throughout the U.S. and worldwide, with Ostertagia considered the single most important parasite of cattle.

Clinically, the parasites of the stomach and intestine cause anemia, scouring, depression and even death, but clinical parasitism occurs infrequently. The effects of parasitism usually are insidious and subclinical, such as indigestion and poor feed conversion, less than expected weight gain and (for brood cows) decreased milk production. Lungworms cause verminous pneumonia and provide an environment conducive for viral and bacterial pneumonia, with labored breathing and anxiety evident. Depressed performance then may be a consequence of internal parasitism.

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