They emerge as adults in about three weeks. While the larval development is arrested, no signs are seen except on the abomasal wall at post mortem examination. In early spring, arrested larvae mature, and signs of parasitism may occur when the worms emerge from the gastric glands. Type II Ostertagiasis results in clinical signs in a small percentage of animals. A wide range of factors within the cow interacts to affect the level of worm infestation.
Nutritional status is well known to have an effect on parasitism.
Well fed animals are more resistant and less likely to be affected by the adverse effects of parasitism than are poorly fed animals. Genetic resistance may play a role in some parasitic relationships, but as yet it is not practical to attempt to control dairy cattle parasites genetically. Sex and age of the animal are other factors that affect the worm burden, but knowledge in this area is limited so that no recommendations of practical significance can be made in these regards. Immunity to parasites is a significant feature of Ostertagiasis in cattle. It is evident that young animals are more susceptible to Ostertagia than older animals.