Worming dairy cows: myth or reality?

The rationale for treating dairy cows for gastrointestinal parasites in order to enhance production rests on a firm scientific base. The evidence includes: observations in abattoir studies of the frequent presence of Ostertagia ostertagi in the abomasum and lesions – often extensive – of parasitised gastric glands in the abomasum of adult cattle; associations between antibody titres to O. ostertagi and abomasal pathology, plasma pepsinogen, nematode epidemiology and milk production; the demonstration that nematode parasitism can reduce the feed intake of dairy cows; and production responses to anthelmintic treatment. Treatment at calving appears to be optimal in defining the returns on the investment from worming cows. Quantitative diagnostic tests can help discriminate amongst herds or individual cows that are likely to respond to treatment, thus facilitating targeted selective treatment (TST) approaches. In addition TST allows the maintenance of large refugia of unexposed worms that helps mitigate the risk of selection for anthelmintic resistance. None of this implies an imperative to worm dairy cows, but an understanding of the under-pinning science is useful if advice is to be informed, balanced and without subjective bias