The nematodes are the endorparasites that cause the most severe economic losses in sheep. In some areas of the world liver fluke (a trematode endoparasite) also causes heavy losses in sheep production.

A knowledge of the complex life cycles of the endoparasites contributes to effectiveness of control by permitting development of well-timed control programs. Products available for control of endoparasites before ivermectin had some serious limitations.

Descriptions of the significant endoparasites of sheep are given in the following section.