Equine Parasite Control


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Why equine veterinarians routinely address parasite control measures and interventions for their patients, and how distributor reps can help ensure they have the right products for the job.

There are a lot of variables involved in how many cases of internal parasites a veterinarian may diagnose and treat among his or her client base. “I think the best way to preface this answer is to say: ‘it depends,’” stated Kenton Morgan, DVM, associate director, equine technical services, Zoetis.

It depends upon the type of practice, which also determines the kinds of horses a particular veterinarian would see on a regular basis. The age of the horse, use of the horse, housing, pasture management, manure management, stocking density, and the geography and climate where the horse resides can all impact their respective risk for internal parasitism. “As a consequence, all of these factors will impact what an equine veterinarian will encounter when it comes to internal parasites in horses,” Dr. Morgan said. “But in general, and even though these risk factors can vary, internal parasites are a concern for virtually all horses, and an equine veterinarian routinely addresses parasite control measures and interventions for their patients.”

In an interview with Veterinary Advantage, Dr. Morgan discussed the risk that internal parasites pose to horses, common signs, keys to an effective parasite control program, and questions that distributor reps can ask veterinary clients to better determine their needs.

Risks vary

The risk internal parasites pose depends on the age of the horse, Dr. Morgan said. In foals (and weanlings), the parasite of primary concern is the roundworm, sometimes referred to as ascarids. “This parasite is rather ubiquitous in foal populations and remains a concern until the foal is about 6-8 months of age,” he said. Around that age, the foal begins to develop acquired immunity to this parasite. “In fact, it is rather unusual to find roundworms in healthy adult horses.” This parasite resides in the small intestine of the young horse and is the largest of all equine internal parasites. Significant numbers of these parasites in the young horse can lead to the “classic” signs of internal parasitism, those being; unthriftiness, stunted growth, pot belly, long hair, and general poor appearance. On occasion, the physical presence of large numbers of these worms can lead to small bowel blockage and, rarely, small intestine rupture. Another aspect of this parasite is that the immature larval stages migrate through the liver and lungs as part of its normal developing life cycle. This can lead to airway inflammation with cough and nasal discharge.

In adult horses, the parasites of primary concern include the small strongyle and tapeworm. “There are other internal parasites, but these two pose the greatest potential health risks to adult animals,” Dr. Morgan said. The adult stage of the small strongyle does not pose a significant risk to the host horse – it is the immature larval stages that are the problem. These larval stages can burrow into the wall of the large bowel of the horse and form a small “cyst” or capsule. While in this cyst, the larva may slow its development or continue to develop and enlarge. At some point (and due to multiple factors), this larval stage will “hatch” or excyst into the lumen of the large colon and cecum of the horse and continue maturation into adulthood. It is this process of hatching or excystment that poses a risk to the host because this can cause significant inflammation of the lining of the large bowel. In severe cases (which fortunately are rare), this can lead to diarrhea, colic, losing protein through the gut wall, weight loss, fever, and on rare occasions even death.

The tapeworm is also a concern. A few tapeworms pose little health risk to the horse, but larger numbers can result in an increased risk of a specific kind of colic in these animals. This risk of colic is a result of where these parasites like to take up residence within the intestinal tract of the horse. The predominate tapeworm affecting horses in the U.S. likes to congregate at the junction of the small intestine and large bowel of the horse. When large enough numbers of these parasites are present, they can impact the function of the intestine at this point, which may result in the bowel “telescoping” on itself (a condition we call intussusception). The result of which can be a life-threatening colic that requires surgery to correct.

“The problem is that there is no good way to determine how many tapeworms may reside in the intestine of the horse,” Dr. Morgan said. “We know that tapeworms are a risk to horses throughout the entire country. As a result, it is recommended that we deworm adult horses at least once per year for tapeworms, and the best time to do this is at the end of the grazing season. Treatment of tapeworms requires a specific ingredient (praziquantel) which is not found in all deworming products.”

Common signs

In adult horses, with rare exceptions, it is challenging, if not impossible, to look at an animal and determine if they have significant numbers of internal parasites, Dr. Morgan said.

But one of those exceptions can be pinworms. Horses with pinworm infections will sometimes rub their tails and hindquarters. This is not the consequence of any damage the pinworm causes but rather a result of where and how the adult female pinworm lays her eggs – around the rectal area of the horse with a “sticky” glue-like substance which can result in “itchiness” that then induces the rubbing of the hindquarters.

“Now, there can be other causes of horses rubbing their tail, but one of the things we want to rule out/address when we see this is whether this animal may have pinworms,” he said. “But again, in general, it is very difficult to look at an animal and draw any conclusions about its internal parasite burden. And that is one of the reasons why running fecal egg counts are so important.”

Effective parasite control

In order to have an effective parasite control program for horses, first and foremost, a veterinarian must be involved, Dr. Morgan emphasized. This is important due to a number of crucial factors:

  • They can run fecal egg counts on the horses and then interpret those results, which will help direct the treatment/control efforts.
  • They know the parasite life cycles.
  • They know which deworming products are effective against which specific parasites and the best time of the grazing season/year to use those products. This is especially true for the encysted stages of the small strongyle and for tapeworms. Additionally, their knowledge or parasite resistance in their practice area can greatly impact treatment and control efforts.
  • They are familiar with each individual operation and can also assess other risk factors which can impact control efforts such as stocking density, manure management, age of the horses, etc.


“In a very succinct summary, the key elements include: a veterinarian, fecal egg counts, and an owner willing to follow the recommendations of their veterinarian,” Dr. Morgan said.

Indeed, there are no “calendar-based” or cookie-cutter recommendations, he said, such as deworming every horse on the farm every three or four months. “Those types of programs perhaps served us well in the past, but they have also contributed to the current serious situation with respect to parasite resistance,” he said. “As a result, the days of these types of programs are over. We must use a more focused and individualized approach which requires fecal egg counts and veterinarian oversight.”


Questions to Ask


Dr. Morgan said the following are potential questions reps can ask veterinary clients to better determine their needs related to deworming products and services.

  • Do you take an active role in parasite control for your clients, or do you leave that up to them? If yes, then are you currently providing anthelmintics? If not, do you think there is an opportunity here for you to provide this input to your clients?
  • If the practice is not dispensing anthelmintics – ask them why not.
  • How many fecal egg counts do your run per year versus how many Coggins tests you run or how many doses of WN vaccine you give? Can you do more fecal egg counts for your clients?
  • Do you promote fecal egg counts to your clients as an essential diagnostic?
  • When you do run fecals, do you then recommend which anthelmintic products they should use? If so, is there an opportunity for you to dispense those products rather than them purchasing elsewhere?
  • Are you familiar with the AAEP’s internal parasite control guidelines? Do you use those as a reference for your parasite control efforts? aaep.org/guidelines/internal-parasite-control-guidelines


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