Creepy Crawlers – Equine Intestinal Parasites

By Dr. Caity Cosentino

Before being able to come up with an effective parasite prevention plan, it is important to know the main parasites infecting our equine friends. Some merely are passing through, while others can cause some serious harm. Knowing what types of parasites dewormers are targeting will bring a better understanding to the reasons that deworming is such an important part of herd management and horse health. All horses are dealing with a certain parasite load throughout the year. The goal is to maintain that level below where they will cause the horse harm.

Large Strongyles

There are several species that affect horses. The adults live in the large intestines, produce eggs and the eggs are then shed into the environment in the horse’s manure. The eggs hatch and mature into larvae. The larvae, living out in pastures, then migrate up blades of grass. Horses ingest them while grazing. One variety of large strongyle, Strongylus vulgaris, can migrate from the large intestine, into blood vessels and burrow into artery walls causing disruption to blood flow. Several other species of large strongyle will migrate through the liver and back to the intestines. Though these worms do exist, they are not commonly a problem in our domestic horses as deworming has been so effective in reducing their numbers. Horses who do become infected with these parasites may become sick, however, with diarrhea or colic.

Small Strongyles

The lifecycle of these worms is similar to that of the large strongyle, with one exception. Once in the large intestine they tend to burrow into the walls of the large colon and cecum and may become encysted. Encysted small strongyles are resistant to dewormers and may remain there for several years. Resistance to types of dewormers has led to a change in common deworming practices, with fecal egg counts being recommended prior to deworming. It is normal for most horses to carry a low worm burden and they will have no ill effect. High numbers may decrease the body condition of the horse, decrease performance and lead to colic due to inflammation or large populations emerging from the encysted state. These are the most common type of worm egg encountered during fecal tests.


Roundworms most commonly affect foals and young horses. Adult horses develop a natural immunity with age. Ascarid eggs are ingested from the environment and may be found contaminating surfaces inside barns, buckets and troughs. The eggs become larvae in the small intestines and migrate from there to the liver and the lungs. They are coughed up and re-ingested finishing maturation in the small intestine. Like the small strongyles, these worms are developing some resistance to common


Several species of tapeworm can be found in the horse. They have several hosts in their lifecycle. The eggs are first ingested by oribatid mites. Inside the mites the eggs develop into larvae. The mites are ingested by a grazing horse and then the worm is released into the small intestine. The adult worm travels to the ileocecal junction, located at the far end of the small intestine. The worms attach, leading to inflammation and possible ulcerations. Large numbers of these worms may cause colic.


Eggs from the pinworm are ingested by horses out in the field grazing. The worms develop and mature traveling through the intestines. Adult worms emerge out of the horse’s rectum and lay eggs on the skin causing irritation. Horses will often rub their tails causing hair loss and sometimes wounds. The parasite doesn’t cause damage inside the intestines and besides the irritation are not a major health concern.

Thread Worms

This parasite usually infects foals. They can acquire the parasite through mare’s milk, by ingesting it in the environment, or by it penetrating the skin. These worms also migrate to the lungs and live in the small intestines. Infection with this parasite can cause diarrhea in foals, but due to deworming practices it is uncommon to find this parasite causing problems.


Adult bot flies are external parasites that lay yellow oval-shaped eggs on a horse’s hair, especially the lower legs. The eggs hatch when they are exposed to the horse’s saliva, they then hatch into larvae and burrow into the gums of the horse, becoming an internal parasite. They migrate from the gums to the lining of the stomach where they stay approximately 9 months before being passed in manure. Horses may develop oral lesions and irritation to the stomach lining with a heavy burden. Major health problems are not generally caused by this parasite.

Owning horses means dealing with internal parasites. In adult horses, small strongyles and tapeworms are of most concern. Foals and young horses are more suceptible to other types as well and need to have special consideration taken when coming up with a deworming protocol. It is important to know how these parasites infect horses to be able to come up with a good strategy to cut down the parasite load on the farm and in all the horses living there. Removal of manure, cutting and dragging pastures and a good deworming program are all key to lessen the likelyhood of a heavy worm burden. Fecal egg counts play an important roll in today’s war against intestinal parasites as resistance to dewomers becomes more prevalent.