An Introduction to Internal Parasites in Horses

Parasites, organisms which live off the resources of a host body, can be found in each and every horse, no matter what breed or type. There are a number of common parasites which have evolved to live in the GI tract of the horse, and these are generally referred to as worms. These organisms are highly specialized to live and reproduce within the equine genus and are not able to live within other genera. Domestication of the horse, including time spent in a stall and shared grazing close to other horses, has increased the likelihood of heavy burdens developing by perpetuating the worm life cycle.

The horse is susceptible to more than 60 internal parasites and may harbor several species of worms. Only a few parasite species, however, are capable of any serious harm. It is important to realize that worms are often present in low numbers in many horses and cause no ill effects. Occasionally, though, horses develop heavy worm burdens which can be damaging and even fatal. When we consider how quickly a parasite may reproduce, it is not an exaggeration to say that a horse could discharge up to 25 million eggs per day! Healthy worms are rarely passed in the feces but instead their eggs are transmitted, allowing the parasite to spread and infest other horses, ensuring the survival of the parasitic species.  A fecal worm egg count under the microscope can determine what type of parasite is present and estimate the population of the worm infestation inside the horse.

Familiarity with the basic worm types and the ways in which they can appear will help you determine whether your horse has worms and how best to treat them. Read on for a basic overview of ten of the main intruders.


Common parasites in horses

Large strongyles (Strongylus vulgaris) are colloquially known as bloodworms. The larvae migrate through the abdominal arteries which can lead to a rupture or circulation blockages and is sometimes the cause of colic. The horse can lose weight, become anemic and suffer poor coat and condition.

Small strongyles (cyathostomes) will burrow into the wall of the intestine as larvae and lay dormant for many months. In spring, they hatch and shed in order to reproduce and spread to new hosts.  Signs of infection include weight loss and failure to thrive; large numbers of small strongyles emerging at once can cause diarrhea and in some extreme cases may be fatal.

Ascarids (Parascaris equorum) are intestinal worms found predominately in foals. Ascarids may cause weight loss, diarrhea and/or colic.

Tapeworms in horses can be huge!

Tapeworms (cestodes) are large parasites. They have been known to cause colic and contribute to severe intestinal blockage.  There are a number of different species of tapeworms which can live inside the GI tract of a horse. Their larvae can take around four to six weeks to mature, after which the segments come loose and shed in the horse’s manure.


Botflies (Gasterophilus spp.) lay tiny yellow eggs on the hairs of a horse's legs. The horse will lick at these, permitting the eggs to burrow into the mouth for a prolonged period. Botfly larvae hatch in spring and then migrate to the stomach, where they attach to the wall until they are ready to be ejected. Once passed in droppings, the larvae bury into ground and emerge later in the year as adult pests. 


Less common parasites in horses


Eye worms (Thelazia lacrymalis) live in the tear ducts of horses. You may notice a discharge from the eye and infestations can lead to light sensitivity or conjunctivitis.

Filarids (Onchocerca spp.) are thin, small worms transmitted by biting flies. They live in the neck ligaments and skin tissues, which in some horses can lead to an immune response manifesting as an excessive itch.

Lungworms (Dictyocaulus arnfieldi) are the most common parasite in donkeys, but horses who share their grazing can also become infected. The larvae migrate through the intestinal wall and into the lungs, where they lay eggs. They often cause a severe cough.

Pinworms (Oxyuris equi) burrow into the wall of the intestine during their larval stage, which can lead to damage. The horse often suffers irritation in the anal area and will rub incessantly on gates, doors and even owners. This may be the only symptom.

Stomach worms (Habronema muscae) are transmitted by biting flies and are responsible for summer sores. The larvae will burrow into open sores, resulting in inflammation and pain. This is known as Habronemiasis or, "Jack Sores."


An effective deworming program can prevent your horse from becoming infested with parasites and in turn, prevent internal damage and related problems. It is also very important to follow good pasture and barn management practices to prevent the spread of parasites – in the next installment in our series we will highlight the best practices for parasite prevention.  As always, we advise you to consult with your local veterinarians to determine the best parasite fighting strategies for your area. Come back soon to find out more! 

*Image courtesy of Wikimedia Commons