During these months temperature and moisture conditions are most favorable for allowing the development and accumulation of large numbers of infective larvae on pastures. All live in the abomasum of sheep and suck blood while attached to the abomasal wall. One, or all three species, may be present in an animal, and a few to several thousands may be present. The life cycle of stomach worms includes both free living and parasitic stages. Adult worms are found close to the wall or attached to the abomasum.
Females lay microscopic eggs which pass to the ground in the manure.
Within a few days, a free-living larva develops within the egg which eventually hatches. This development may take only a few days in the warmer months or several weeks during cooler weather. 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.