Sheep become infected with worms by eating food or drinking water contaminated with manure containing infective larvae. Within two to four weeks after eating the larva, the parasite develops into either a mature male or female worm. Later, usually in the spring, they will resume their development. Figure 2 . Life cycle of stomach worms in sheep.
Light infections cause little harm to sheep.
A large number of worms may cause anemia, poor growth, and occasionally, death. Although the disease may occur in all age groups, animals 2-24 months old are more commonly affected. Older sheep may have an acquired resistance to the parasite because of earlier exposure to them. The severity of the signs varies with the number of worms present. Sheep infected with large numbers of worms may show diarrhea, dehydration, reduced appetite, weight loss, anemia, and generally loss of body condition.