13 Guidelines to Follow When Deworming Horses

When deworming horses, timing is key

Horses less than three years old are more susceptible to parasite infection.

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Controlling internal parasites in patients is a primary concern of equine practitioners. The advent of readily available, easily administered and effective deworming agents—along with recognition that a particular parasite, S. vulgaris, causes a destructive colic problem (verminous arteritis)—has resulted in a deworming frenzy, particularly among horse owners. 

However, due in part to constant deworming, parasite resistance has become a significant problem. Thus, veterinarians must work with clients to develop effective deworming strategies.

Given deworming practices of the past, parasites that were once of extreme concern—S. vulgaris and other large strongyles—are now rare. Today, the focus with adult horses is on small strongyles (cyathostomins) and occasionally tapeworms. For horses less than three years of age, ascarids (Parascaris equorum) are the biggest problems. 

The current recommendation is that horses should only be treated if they show signs of a heavy parasite load.

Most adult horses develop immunity to parasites, some better than others. Horses with a high level of immunity don’t shed many eggs, so deworming all horses on the same schedule makes little sense. On the other hand, horses less than three years old are more susceptible to parasite infection and are at greater risk of developing parasite-related disease.

Deworming every couple of months, or rotating dewormers each time, or every other year, do not control internal parasites effectively. Such strategies waste time and money, and they build parasite resistance.

While the goal once was to rid an individual horse of all parasites, it’s impossible. The current thinking is to limit parasite infections so horses and other equids remain healthy and that clinical illness does not develop. Here are some guidelines:

1. Horses, especially those over three years old, should be treated as individuals and not according to some routine. The baseline program should be one or two yearly treatments, depending on climate and whether the patient lives with other horses. If the horse lives alone or in a stable, the frequency could be less.

2. Ivermectin and moxidectin are the best choices to control strongyle parasites. Pyrantel, fenbendazole and oxibendazole are good for treating ascarids in young horses. Ivermectin resistance is common in ascarids.

3. In herd situations, fecal egg counts should be used to select moderate and high egg shedders for deworming. These are the only horses that should be treated. Not all horses need to be tested. Choosing at least six in each pasture should give a good picture of the situation.

4. High shedders almost certainly need more than one or two treatments a year. Moderate and high egg shedders may need a third or fourth treatment for small strongyles, ideally using moxidectin and a daily feeding of pyrantel tartrate, or a dose of moxidectin. Any additional treatments would be given on an as-needed basis.

5. Low shedders likely need only one or two treatments a year, at most.

6. Deworm at appropriate times.

Don’t deworm during the temperature extremes of winter or summer and during droughts. Parasites can’t reproduce effectively under such conditions.

Worm-control programs are best viewed as a yearly cycle starting when worm transmission to horses changes from negligible to probable.

7. To test a deworming program’s effectiveness, conduct fecal exams at appropriate intervals.

If the interval is too short, the results of the fecal egg count will show only how well the last dewormer worked rather than measure how well the horse’s immune system reduced levels of cyathostomin egg shedding. For example:

  • After moxidectin, wait at least 16 weeks to collect a fecal.
  • After ivermectin, wait at least 12 weeks to collect a fecal.
  • After benzimidazoles (fenbendazole/oxibendazole or pyrantel), wait at least nine weeks to collect a fecal.

8. Stabled horses that do not mingle with other equids in pasture may require deworming very infrequently, if at all.

9. Horses less than three years old are more susceptible to parasite infections compared with older horses.

Among specific guidelines for youngsters:

  • During the first year, foals should get at least four deworming treatments. The first should be carried out at two or three months of age, and a second three months later. Check for eggs at weaning to see what, if any, parasites are in the foal.
  • Third and fourth treatments should be considered at about nine and 12 months old, targeting the worms found. Tapeworm treatment should be included in one of the latter treatments.
  • Perform yearly fecal tests to evaluate how well the dewormers are working.
  • Don’t deworm an 8-day-old foal. The diarrhea isn’t caused by worms.
  • Turn out recently weaned foals on the cleanest pastures.
  • Yearlings and 2-year-olds should be treated as high shedders and should receive three to four yearly treatments with drugs shown to be working by a fecal exam.

10. Horses showing signs of being parasitized should be dewormed using either moxidectin, which carries little evidence of parasite resistance, or possibly a larvicidal regimen of fenbendazole (10 mg/kg for five consecutive days).

11. Don’t rely on dewormers alone. Environmental control is critical. Manure should be picked up and not spread in the pasture. If cattle or goats are present, allowing them to rotate onto the horse pastures for a few weeks will help remove eggs.

12. Two other parasites of concern are bots, which almost never cause problems, and pinworms, which can cause horses to itch. Bot treatment is traditionally performed 30 days after the first frost. Pin-worm treatment is initiated only when a problem occurs.

13. Don’t bother with alternative dewormers such as diatomaceous earth. They have been shown not to work.

There is no one-size-fits-all deworming program. Controlling parasites involves setting up a reasonable program and sticking with it.

For more information, see the American Association of Equine Practitioners’ guidelines at their website


Originally published in the August 2016 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today! 

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