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Controlling Parasites and Equine Dental Care


By Nancy Brannon, Ph.D.

Allison Forbes, DVM of Full Circle Equine Veterinary Services was the featured speaker at the Purina Horse Owners Workshop (HOW) at Hall’s Feed and Seed in Collierville, Tenn. on Tuesday, April 4, 2017. Owners Beverly and Jimmy Thompson treated all those who attended to supper, and Rusty Bane of Purina Animal Nutrition gave his presentation, “What is your horse nutrition IQ?” There were door prizes and free items for everyone to take home, including a dog dish with a sample of Exclusive dog food.

Forbes spoke about the two factors that are the main causes of a horse losing weight: teeth problems and parasite infestation. When your horse is losing weight, the first two things the veterinarian checks for are parasites and dental problems.

Dr. Forbes emphasized how important a deworming program is to your horse’s health care. Then she discussed the life cycle of parasites, such as large and small strongyles, which are the largest and most significant parasites in horses, and how they affect the horse’s health. These parasites are considered responsible for the vast majority of serious parasite-related health problems in adult horses.

Size is the main difference between the large and small strongyles. Large strongyles are relatively stout worms up to two inches long, whereas small strongyles are small, hair-like worms; both can be seen with the naked eye.

Although their life cycles outside the horse are practically identical, the large and small strongyles have very different approaches to infection once they’re in the horse's gastrointestinal tract. Large strongyle larvae take the grand tour of the equine interior, leaving the intestine soon after infection and migrating through various tissues for the next 6 to 11 months. The best-known large strongyles are Strongylus vulgaris, whose larvae invade the lining of arteries supplying the gut. Forbes explained how small strongyles lodge in the intestine wall like a cyst. She said most dewormers can’t penetrate the cyst.

Large strongyles in horses are also known as blood worms. Adult large strongyles have large buccal capsules and are active blood feeders; they ingest mucosal plugs as they move about in the intestine. The associated blood loss may lead to anemia, weakness, emaciation, and diarrhea.  S. vulgaris damages the cranial mesenteric artery and its branches. As a result of the interference with the flow of blood to the intestine and thromboembolism, any of several conditions may follow, including colic; gangrenous enteritis; or intestinal stasis, torsion or intussusception, and possibly rupture.

Diagnosis of mixed strongyle infection is based on demonstration of eggs in the feces. Specific diagnosis can be made by identifying the infective larvae after a fecal culture.

Strongyle eggs hatch in the fecal pile when environmental temperatures range from 45-85° F. Temperatures below the stated range are too cold for hatching to occur, and freezing is usually fatal to strongyle eggs. The rate at which strongyle eggs hatch and larvae develop is directly proportional to the environmental temperature. In warm weather, eggs can hatch and yield infective larvae in as little as three days, but the process might take several weeks in cooler months.

Horses pick up strongyle larvae through the normal process of grazing, as larvae crawl up blades of grass. The examination of a single early-morning dewdrop on a grass blade might reveal thousands of them. Horses can also ingest the larvae directly from the soil or from drinking contaminated water.

Other types of parasites include ascarids, which are round worms that are seen mostly in young horses who have the classic pot belly and a dull coat. Ascarids, or roundworms, are comparatively large parasites, ranging in width from one to two millimeters (P. equorum, size of a pencil lead) to three-eighths of an inch, and in length from one to 14 inches. Ascarids are easily observed in the manure of infected horses, especially after treatment with an effective anthelmintic.

P. equorum adults usually hang out in the small intestine. When a horse swallows an infective ascarid egg, the larva inside is released in the small intestine. This larva then invades the gut wall and is carried to the liver, either through the circulation or the lymphatic system.

Once in the liver, the ascarid larva migrates through hepatic (liver) tissues for about one week and molts to the next larval stage. It proceeds from the liver, riding the bloodstream to the lungs, where it enters the terminal air sacs, or alveoli. The ascarid larva then migrates up the airways, through bronchioles and bronchi, and eventually up the trachea. This process is greatly accelerated whenever the horse coughs.

When the migrating ascarid larva reaches the back of the throat (pharynx), it is swallowed and returns to the small intestine, where the larva becomes mature and begins to reproduce. Female ascarids generally start to lay eggs within 10-12 weeks after infection.

Three species of tapeworms are found in horses, involving mites as intermediate hosts. They are found in the small intestine, at the ileocecal junction, in the cecum, and in the ileum. In heavy infections, GI disturbances may be seen, as well as unthriftiness and anemia. Dewormers such as Quest Plus, Zimecterin Gold, and Pyrantel are effective against tapeworms.

Less common parasites are lungworms (usually seen in donkeys); pin worms, that irritate the rectum/anus area (not the same kind that children can get); bots; and threadworms (usually seen in young foals; cause diarrhea).

The takeaway is to develop an individualized deworming program – a plan based on fecal egg counts (FEC) that your veterinarian can perform. Forbes warned that a zero FEC does NOT mean the horse is parasite free; the horse is just not shedding at that time. Horses are determined to be low, medium, or high shedders, based on the FEC.

Forbes said that NO dewormer is 100% effective and that it is considered normal for a horse to have some parasite load. Nonetheless, there are two types of deworming programs: continuous and strategic. Strategic deworming takes place only at the times of the year when the FEC goes up (and shedding).

Three types of dewormers are oral paste, feed additive, andnasogastric intubation, which has now fallen out of favor.

The environment is a factor in parasite infestation, too. Forbes said that parasites can’t tolerate extreme heat and dryness, but they can survive freezing temperatures. So it’s during those hot dry times when using a harrow to break up manure in the pasture is effective at killing parasites in the soil and pasture.

Forbes provided a printed Recommended Equine Deworming Schedule for all the attendees to take home.

Forbes asked the audience: how often do have your horse’s teeth floated? Do you use an equine dentist or a veterinarian? Regular dental care is essential for the sound health, comfort, and longevity of the horse. Forbes says using the veterinarian is better because the vet looks at the whole horse and at all the structures related to the teeth.

The upper jaw of the horse is wider than the lower jaw, which results in efficient crushing of food particles. The chewing surfaces of the teeth are at an angle of 10–15 degrees to each other. The grinding action of the molar teeth tends to produce sharp points on the outside of the upper molars and on the lingual aspect of the lower molars. Forbes demonstrated how this occurs. Floating to balance the arcade is required for general maintenance of the horse’s teeth. Good dental care is important to improve chewing, relieve pain, and to promote general good health.

Irregularities are most common in the incisor and molar teeth of mature and geriatric horses. The mouth of the horse changes throughout the life span, but the greatest changes occur from ages 2–5; therefore, semiannual or annual treatment is important for younger horses. Forbes talked about the importance of management of baby teeth to address any malocclusion. Early recognition of dental disease is most important!

Forbes described common dental problems: sharp enamel points, which can harm the tongue and cheeks; malocclusions; hooks (upper teeth); ramps (lower teeth); lost or fractured teeth. Forbes said horses are really good at adapting to their discomfort.

Sharp points and uneven wear can cause problems not initially considered to be tooth related; e.g., mouth ulcers, choke, gastric ulcers, back pain, or erratic head carriage when the bit is in the mouth. Behavior problems may be a result of long, sharp teeth, an uneven arcade, or abscesses in the teeth or gums. A foul odor in the horse’s mouth is an indicator of advanced dental disease. A complete oral examination, including visualization of the premolars and molars, requires sedation, a speculum, and good lighting.

It is important to perform a full physical exam first and to determine if the horse is healthy enough to sedate.  Sedation will be required for teeth floating and other dental procedures. Forbes recommended teeth floating only after the veterinarian examination and when necessary. Radiographs and/or X-rays can determine how much of the tooth is involved in the problem.

Forbes lives by the “8-8-8 mantra,” she said. Check the teeth at 8 days, 8 weeks, and 8 months in foals. Most of the baby teeth have been replaced by adult teeth at about age 2 ½ years. By age five, horses have all their adult teeth; males have 40 and females have 38.

Adult horses should have an annual oral exam. For horses under five years old, check their teeth every six months from day one, she recommended. Senior horses may get by with yearly exams, or others may need an exam every six months.

There was a question about wolf teeth. Forbes said they don’t always need to be removed. Many horses lose them at age 6 without any intervention. But if they do need to be removed, the procedure is considered surgery, and is not always a safe, simple procedure.

The takeaway is that a regular, yearly oral exam by your veterinarian is an essential part of your horse’s health care.

Horse Nutrition IQ
Rusty Bane has worked with Purina for 20 years. Bane said he “got bitten by the horse bug when he was 12 years old” and now his whole family is into horses. He said his wife rides anything from cutting horses to dressage. She is also a country-western musician and performer.

Bane said he got the idea for a quiz testing horse owners’ Horse Nutrition IQ from Cosmopolitan magazine. He has observed how interested folks are in the various types of quizzes that the magazine offers, and decided that would be his approach in his HOW presentations this year.

For detailed information about the quiz, see the April 2017 Mid-South Horse Review article, “Purina HOW at R&J Feed.”


Briggs, Karen. “Strongyles: The Worst of the Worms.” The Horse.
Briggs, Karen. “Ascarids: A Growing Problem.” The Horse.
Klei, Thomas R., PhD. “Large Strongyles in Horses.” Merck Veterinary Manual.
Klei, Thomas R., PhD. “Tapeworms in Horses.” Merck Veterinary Manual.
Lane, Thomas J., DVM. “Dental Care of Horses.” Merck Veterinary Manual.

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