These facts are worth understanding because they have significance in our attempts to control this disease. Roundworm lifecycle small version large version Adult worms mature in the cavity of the abomasum and begin to produce fertile eggs about three weeks after the cow ingests infective larvae. Adult worms are thought to survive about 25 days in the abomasum. Adults do not survive outside the cow. Signs of disease occur in young animals when the number of infesting worms exceeds 50,000 to 100,000 with some individual calves parasitized by several million worms.
Signs occur as a result of damage by the developing larvae to the lining of the abomasum.
The damage reduces the production of hydrochloric acid and pepsin. Rumen pH increases and true indigestion develops. This pattern of the disease typifies what is referred to as Type 1 Ostertagiasis. Failure to gain or weight loss can occur in calves that do not develop severe Ostertagiasis as a result of less serious infestations. This slight weight loss or failure to gain is undoubtedly more common than severe clinical parasitism and is a potential problem for all calves the first time they graze on contaminated pasture.