Equine Parasites

Wherever there are horses, there are parasites that must be keep at a controllable level. In the past, deworming horses was performed almost exclusively by veterinarians since it involved pumping the medication directly into the horse's stomach through the animal's nose. In recent years, deworming products, known as anthelmintics, have become much easier to administer. They can simply be squirted into the horse's mouth or added to daily feed. While there are many positive aspects to this modernization, it has resulted in owners taking over the deworming process, frequently without veterinary consultation. The serious downside to this is that owners have not kept up with recent changes in the parasites or the methodology to eliminate them. As a result, parasites have become drug-resistant.

Parasite Control Guidelines

With this in mind, The American Association of Equine Practitioners (AAEP) has recently compiled parasite control guidelines, both for veterinarians and for horse owners. It includes valuable information, but is still no substitute for actual consultation with a professional veterinarian.

According to the AAEP guidelines, the goal of parasite control is to keep animals healthy by limiting parasitic infections, rather then eliminating all parasites from a particular horse. This is an important distinction because not only is total extermination impossible, but the attempt inadvertently encourages the development of drug resistance in the parasites.

Types of Equine Parasites

There are many types of parasites that infect the gastrointestinal tracts of horses. Most are ingested by the horses as they graze. The most common are:

  • Small strongyles (redworms)
  • Large strongyles
  • Tapeworms
  • Roundworms (ascarids)
  • Pin Worms
  • Bots

Other parasites that may affect horses include stomach worms, lung worms, hair worms, and threadworms.

Symptoms of Equine Parasitic Disease

There are many symptoms that may indicate the presence of equine parasites, some associated with general malaise and some specific to gastrointestinal infection. These symptoms may include:

  • Slow growth or inability to maintain normal weight
  • Rough coat
  • Anemia
  • Pica (appetite for odd items such as wood or paper)
  • Diarrhea
  • Enlarged abdominal girth

Any horse showing these symptoms should be checked by a veterinarian to determine whether parasites, or some other disease factor, are the cause.

Diagnosis of Equine Parasites

In addition to observing their horses for symptoms of parasites, owners and handlers should administer a simple, inexpensive test, called a Fecal Egg Count Reduction Test (FECRT), twice a year. This test involves an examination of fresh manure in order to take a fecal egg count. Normally, two FECRTs are administered, one before and one after deworming, in order to make sure the medication has been effective.

The FECRT, however, does not produce accurate results of tapeworm infestation. To diagnose equine tapeworm, a blood test for antibodies should be performed annually or as needed.

Treatment of Equine Parasites

All horses are infected with some parasites, particularly small strongyles, but most do not become ill unless the level of infection is very high. For most adult animals, infrequent, but appropriately timed, treatments with anthelmintics, such as invermectin and moxidectin, are sufficient to keep them healthy.

Such doses should be given at times of the year when the parasites tend to proliferate, usually in the spring. It is always wise to consult a veterinarian concerning proper methods of dealing with equine parasites.

For purposes of treating equine parasites, horses are divided into two categories: low-shedders or high-shedders.


Low-shedders have a strong positive immune response to intestinal parasites. Even without medication, their bodies tend to keep their parasites in check. About 70 percent of the horses in the United States fall into this category. Low-shedders should be treated semi-annually to prevent parasites from making them ill.


The remaining 30 percent of the country's horse population does not, unfortunately, have this natural ability to keep their parasites at bay. While the proliferation of parasites may not necessarily cause these animals to become ill, the ever-increasing number of parasites in their bodies may cause them to contaminate their surroundings. For these horses, deworming must be performed at least three or four times annually.

Treating horses for parasites should always include treating the pasture, where most of the parasites reside, as well. The two most effective methods of treating areas where animals graze are removing manure as frequently as possible and making sure that the grazing area is not overpopulated.

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