Lambs especially are susceptible to several kinds of parasites, and almost all harbor a few.
Some sheep parasites are blood feeders such as the barberpole worm, Haemonchus contortus, and bookworms, Bunostomum sp. The effects of the parasite on sheep are not always obvious. A few species can cause serious disease, especially in lambs. Others cause little or no harm unless found in large numbers. Infected sheep may eat less, further weakening the animal, and making it more susceptible to other diseases.