Roundworms are a serious infection of foals, responsible for not only typical signs of heavily parasitized horses but also pneumonia and obstruction colic. Developing resistance to commonly used anthelminics (dewormers) is causing a problem that we once thought we had under control. This article discusses the life cycle and diagnosis of this parasite including photos of all the life stages, and has links to treatment for roundworms. I would like to thank MSD-AgVet and Merck, makers of Eqvalan, for use of these photos.

The equine roundworm is a light colored, mature adults are heavy bodied worm measuring up to 9 inches and larger in length making it the largest intestinal parasite of the horse. Following infection of the horse by roundworm larvae it requires about 70 to 120 days before they mature and start producing eggs. These eggs are passed out with the stools and with warm conditions mature to infective larvae in about 10 days. The infective larva while still in the eggs are ingested and hatch in the intestines, see lifecycle below.

Roundworm infection is almost solely a problem of horses less than 2 years of age. Adult horses appear to acquire resistance and rarely are effs found in adults even when not regularly dewormed. Despite the resistance of adult horses there is generall a low level of infection and this in combination with the extreme enviromental persistance makes eradication from the environment difficult. Also unlike strongyles the eggs tend to stick to the body of the horse and the environment so foals are almost always exposed to them. In the foal the roundworms produce many eggs, recontamination the enviroment rapidly.

Foal roundworms can cause significant disease and besides general poor doing, includes chronic pneumonia, and obstructive colic, see below.