''O. ostertagi'' have a direct, non-migratory life-cycle. Calves are infected following ingestion of L3 that under moist conditions migrate from the faeces to pasture. The larvae are swallowed, pass through the forestomachs and develop in the abomasal gastric glands to the L4 stage, emerging as adult (L5) worms (brown, cotton-like, 1cm long) after nearly 3 weeks. Adult female worms then produce strongyle eggs which are passed out with the faeces. The prepatent period is 3 weeks (5-6 months if arrested at EL4 stage).
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''O. ostertagi'' have a direct, non-migratory life-cycle. Calves are infected following ingestion of L3 that under moist conditions migrate from the faeces to pasture. The larvae are swallowed, pass through the forestomachs and develop in the abomasal gastric glands to the L4 stage, emerging as adult (L5) worms after nearly 3 weeks. Adult female worms then produce strongyle eggs which are passed out with the faeces. The prepatent period is 16 - 23 days (5-6 months if arrested at early (E)L4 stage).
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====Epidemiology====
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The epidemiology of ''O. ostertagis'' varies in different regions due to the humidity, temperature and rainfall of the environment.