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A parasite is an organism that lives on or in an organism of another species, known as the host, from which the parasite obtains nutriment (Yuck!).  Internal parasites of horses, therefore, live within the bodies of our horses and grow, feed, and reproduce there.  Although there are a number of internal parasites, worms are the ones most horse owners think of when parasites are mentioned.  We’ll focus on the different types of worms that infect our horses and how to treat for them and to control their numbers in the environment.

As the name implies, parasites gain their nutrition from their hosts and give nothing in return.  However, it is in their best interest not to kill their food source.  Horses in the wild live a nomadic, grazing lifestyle, covering large areas in search of food.  These horses carry a small worm burden, but their exposure is relatively low due to their constant movement that keeps the worm egg population low in any given area. Domestic horses, on the other hand, often carry very high, potentially health and life threatening worm burdens, due to high exposure and re-infection from confinement on small areas of pasture.

Each parasite has a specific life cycle, but in general they follow a similar pattern.  Eggs or larvae are passed in the manure and contaminate the environment. Horses grazing in that environment ingest the egg or larvae.  The larvae develop and mature in the digestive tract (stomach or intestines).  Some larvae migrate to other internal organs such as the lungs and blood vessels and can cause damage there, before returning to the intestines as mature adults to lay eggs.  The adult worms lay eggs that are passed out in the manure (or develop into larvae that are passed) to start the cycle again.

What are the signs of parasites?
Clinical signs of worm parasitism generally include weight loss, anemia, poor growth in younger horses, colic, diarrhea, poor hair coats, and death.  Many horse owners are familiar with the typical  pot-bellied, dull coated worm infested horse.  Although this clinical picture may be easy to recognize, most infected horses will have much subtler signs even with serious infections that can result in death. It is very important to do a fecal egg count on your horse.  A fecal egg count is more informative than a simple fecal floatation.  This will tell you whether your horse is carrying any internal parasites, how many, and how effective your de-worming program is.  A fecal flotation test to detect eggs will not detect most tapeworm infections, however.   A blood test to determine exposure has been used to determine the prevalence of tapeworm infections 

What types of worms are common in horses? 
The most common and important worms that affect equine health are:

Roundworms (Ascarids) – Parascuaris equorum or roundworms usually infect young horses age 3-15 months.  Adult horses acquire a fairly strong immunity to ascarids.  These worms have an unusual life cycle that involves the larvae migrating through various internal organs (causing damage as they travel) until they reach the lungs.  The larvae are then coughed up into the horse’s mouth where they are swallowed and continue their life cycle in the digestive tract.  Mature round worms can be several inches long and as thick as a pencil. These worms can cause an impaction of the intestines or stomach when found in large numbers.  Caution should be taken when de-worming a heavily parasitized horse, as the dying roundworms can also cause an impaction.  Consult your veterinarian if you suspect your horse has a heavy roundworm infection.  Broodmares should be de-wormed thirty days prior to and /or at foaling to help decrease the foal’s exposure.

Large Strongyles – Strongylus edentates, Stongylus vularis.  Both the larval and adult stages of the large strongyles cause damage.  The larvae migrate through the vessels of the intestinal mesentery and can cause thrombi (clots), infarctions, or inflammation of the vessels that, in turn, result in damage to the intestine.  The adults are plug feeders, that is, they take bites of the intestinal lining that result in anemia and decreased absorption of nutrients.  Fortunately, these worms are susceptible to most de-wormers.

Small Strongyles – Referred to as the cyathostomes, the larvae of this group are of the most importance.  The larvae burrow in to the intestinal lining and become encysted for several months before completing their life cycle.  During this time, the larvae are resistant to de-worming.  The intestinal mucosa is damaged when the larvae burrow in, especially when large numbers are present.  Horses exhibit signs of weight loss, poor hair condition, lethargy, poor growth, and colic.  Adult worm infestations also cause similar signs. Before and after they become encysted, they are susceptible to most de-wormers.

Tapeworms – Anapolcephala perfoliata, Anophlocphla magna.  Tapeworms were once thought not to be of much significance in horses, with a few exceptions in foals.  Recent research has shown that large numbers of horses are infected.  Tapeworms have an affinity for attaching at the ileum and ileocecal junction.  This can lead to inflammation and ulceration, which impairs normal intestinal function.  This intestinal malfunction can lead to three common types of colic – ileocecal intussusception, ileocecal impaction, and spasmodic colic.  Fortunately, praziquantel has recently been approved for the treatment of tapeworms in horses and is very effective.

Bots – Gasterophilus spp affect horses age two months and older.  Horses may develop mouth ulcers where the eggs burrow in after entering the mouth.  Some horses can develop stomach ulcers where the bots attach.  Sensitive horses may have a decrease in performance due to the ulcers.  Good grooming to remove the eggs from your horses legs will help to reduce their exposure, as will a targeted ivermectin treatment that is very effective in killing bots.

The following worms are less of a threat to your horse’s health, but should be taken seriously and considered in your prophylactic de-worming program or when a problem arises.



Pinworms– Oxyuris equi lay their eggs around the horse’s anus resulting in persistent itching and tail rubbing.

Stomach worms

Thread worms

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