At about 9 months of age they start to acquire resistance by exposure to a gradual intake of larvae. Because lambs are most at risk they are the prime consideration in designing control procedures. Nutrition can have a major influence on the severity of parasite infection and some influence on the extent of acquired immunity. Parasites drain the sheep of nutrients, especially iron, trace elements and protein. Sheep on a diet deficient of these nutrients or pregnant ewes whose feed intake has not kept up with increasing demands from the foetus, will suffer disease sooner than sheep fed better.
Parasites also suppress the appetite of sheep, worsening the problem of nutrition.
Animals in poor body condition, as may occur after a drought have a poor immunity. On the other hand good diets directly promote host resistance. Stress Stress depresses host resistance. Stress in sheep leads to the release of hormones which have undesirable effects. Some hormones, such as the corticosteroids, suppress the immune system.