This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London, brokers, and their Kentucky agents.

The main internal parasites traditionally considered to be important in horses are bots, ascarids, large and small strongyles, and pinworms. Other species, such as tapeworms, stomach worms, and intestinal threadworms also can be of clinical importance. Of the approximately 100 species of internal parasites in horses, about one-half belong to the small strongyle group. The following discussion concentrates on the strongyles and ascarids.

Strongyles live as adults in the large intestine and lay eggs which pass in the feces of horses. In the environment, the egg embryonates into a first-stage larva, which hatches. It then undergoes further development into a second and then into a third-stage larva, which is the infective stage. Grazing horses ingest the third-stage larva after it crawls up on pasture vegetation, hay, or other food. In the horse, the third stage develops through two more stages (the fourth and then fifth, which is the adult). Large strongyles are more pathogenic than the small strongyles because the infective third stage migrates outside the large intestine into blood vessels and may go into internal organs. This may result in blockage of blood vessels and/or other problems that can result in debilitation or even death of the horse. Small strongyles in the infective stage migrate into the wall of the large intestine, where they encyst. These parasites can cause clinical problems (larval cyathostomiasis) and at times even result in killing the horse. During migration of large strongyles and encystment of the small strongyles, there is development to the fourth stage and, in some instances, fifth stage. During these latter stages, they return to the lumen of the large intestine, mature to egg-laying adults, and the life cycle continues.

Ascarid eggs that are passed in horse feces have a thick shell. Embryonation occurs in the egg in the environment to the infective larval stage; however, the larva does not hatch from the egg until ingested by a horse. These parasites migrate in blood vessels through the liver and then to the lungs, where they undergo further development before being swallowed again. They mature in the small intestine where they can cause problems, including blockage or rupture of the wall. Ascarids can be the cause of death in young horses