During Phase 1, the parasite has to develop and survive in the host. After ingestion, infective larvae lose their protective sheath and invade the mucosa, or lining of the abomasum, small intestine or large intestine, depending on which worm species is involved. While in the mucosa, larvae develop to the next larval stage and then return to the surface of the gut mucosa where they become adult worms.
The goat's major defense mechanism against parasites is his or her immune system. When infectious agents enter the body, the immune system reacts through a series of activities that mobilize various components, such as antibodies and killer cells, that then attack and kill the invaders. These components act on the larval stages in the mucosa and the adults. How strong the immune response is depends on several factors. The immune system has to mature with age,therefore, young animals are relatively susceptible to infection and become more resistant with age. Young animals usually harbor the heaviest infection levels and suffer the most severe consequences. Adult animals have developed stronger immunity and harbor lower infection levels. Geriatric animals tend to have poorer immune systems and become more susceptible to parasites such as worms.
One way to try to measure infection levels in individual goats is by counting the number of worm eggs being passed in the feces. Relatively high and low worm egg counts are usually seen in young and adult animals, respectively. However, some species of worms lay lots of eggs while other species lay far fewer. Another important measure of infection is whether or not your animals are showing symptoms of infection. Young animals are more subject to clinical disease where signs of infection like diarrhea, rough-hair coat, anemia, weight loss and bottle jaw are seen. In older animals, infection usually becomes more subclinical, and the only subtle sign may be reduced weight gain.
Nutrition and/or stress can alter the competance of a goat’s immune system. Under poor nutrition and/or stressful conditions, the immune system loses some effectiveness and cannot respond adequately. Therefore, no matter what the age of the animal, the effects of infection will become worse. The prepatent period of most worms is about three weeks, but this period can be extended for worms that have the capability to enter a period of delayed or arrested development within the host animal called "hypobiosis". This period generally takes place during the season of the year when the environmental conditions are unfavorable for development and survival of the free-living larval stages. In warm climates, it occurs either during summer or winter, depending on the worm. In colder climates, all worms capable of undergoing hypobiosis will do so in the winter as a way of overwintering in the host animal.