Younger cattle, whether beef or dairy, are at the greatest risk of worm infection. Ostertagia infestations commonly occur during autumn, when the burden on fields is highest. Larvae are ingested from pasture and migrate to the abomasum where they burrow into the gastric glands and erupt after undergoing two moults. Calves can rapidly lose weight due to reduced appetite and diarrhoea. Later in the grazing season there is the risk of Ostertagiosis Type II, which occurs when ingested larvae pause theirdevelopment. Later in winter, or early spring, development is resumed and large numbers of adult worms can erupt at once, causing severe damage to the abomasum. COWS (Control of Worms Sustainably) states that cattle can take two grazing seasons to acquire immunity to Ostertagia, therefore it is imperative that good management is applied across all youngstock.
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