These can be divided into large redworms and small redworms (cyathostomins). Both these parasites colonise the intestines after the horse grazes contaminated pasture. As adults in the intestine, they produce eggs which are passed in the droppings to further contaminate the pasture.
The large redworms hatch out of the eggs and migrate as larvae through the intestines, liver and mesenteric blood vessels before becoming adults. The small redworm larvae can form cysts in the intestinal wall to hibernate over the winter. They emerge again in the spring to form adults in the intestinal lumen. This is the dangerous time for horses as the mass emergence of larvae can cause severe damage to the intestines. This condition is known as larval cyathostominosis and can be fatal.