Horse owners and veterinarians have focused on control of equine internal parasites using chemical deworming strategies for decades, especially with the advent of readily available, easily administered oral paste deworming agents. However, due at least in part to rather constant deworming, parasite resistance to deworming agents is now recognized as a significant problem. Thus, it is increasingly important veterinarians work with their clients to develop effective deworming strategies.
Due to indiscriminate deworming practices of the past, the parasites that were of most concern—S. vulgaris and other large strongyles—are now actually pretty rare. Currently, the major parasites of concern in adult horses are small strongyles (cyathostomins) and occasionally tapeworms. For horses younger than 3 years of age, ascarids (Parascaris equorum) cause the biggest problems.
It’s now recommended that horses should be treated only if they are showing signs of a moderate to heavy parasite load. Most adult horses develop some immunity to parasites. Horses with a high level of immunity shed few eggs. Thus, in a group of horses, it makes little sense to deworm all horses on the same schedule. On the other hand, horses younger than 3 years of age are more susceptible to parasite infection, are more at risk of developing parasite-related disease, and may require more frequent deworming (in addition to other parasite control measures).
Strategies of frequent deworming or “rotating” dewormers do not control internal parasites effectively, they waste time and money, and they build parasite resistance. A goal of trying to eradicate all parasites by deworming alone is essentially impossible. Current thinking suggests parasite control should limit parasite infections so that animals remain healthy and clinical illness does not develop.
General deworming guidelines
- Treat horses—especially those younger than 3 years of age—as individuals. The baseline program should be one or two yearly treatments, depending on climate, and whether or not a horse cohabitates with others. If the horse lives alone, or in a stable, deworming can be done less frequently, if at all.
- Use ivermectin and moxidectin to control strongyles. Use pyrantel, fenbendazole, and oxibendazole to treat ascarids in young horses (ivermectin resistance is common in ascarids).
- Employ quantitative or semiquantitative fecal egg counts in herd situations to select moderate and high egg shedders for deworming. Moderate and high egg shedders are the only horses that should be treated. Not all horses in a herd need to be tested; testing at least six in each pasture should provide a good picture of the situation.
- Treat high shedders frequently. Moderate and high egg shedders also 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.
- Refrain from treating low shedders more than once or twice a year, at most.
- Deworm at appropriate times. Don’t deworm during winter or summer temperature extremes or during droughts; parasites can’t reproduce effectively under such conditions. Worm control programs should be a yearly cycle starting at the time of year when worm transmission to horses changes from negligible to probable.
- Always perform fecal exams at an appropriate interval after deworming to test the effectiveness of a deworming program. If the interval is too short, test results will show only how well the last dewormer worked, rather than measuring how well the horse’s immune system resolved levels of cyathostomin egg shedding.
- After moxidectin—Wait at least 16 weeks
- After ivermectin—Wait at least 12 weeks
- After benzimidazoles (fenbendazole/oxibendazole or pyrantel)—Wait at least 9 weeks
- Evaluate whether deworming is needed in stabled horses or horses that do not mingle with others in pasture.
Guidelines for horses 3 years or younger
- Perform deworming treatments for foals at least four times during the first year. First deworming should be done at about 2 to 3 months of age, with a second treatment 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 9 and 12 months of age, targeting the worms that are found. Depending on prevalence, tapeworm treatment probably should be included on one of the latter treatments.
- Perform yearly fecal tests to help evaluate how well dewormers are working.
- Do not deworm a young foal at 8 days of age. The diarrhea the foal is experiencing at this time isn’t caused by parasites.
- Turn out recently weaned foals on the cleanest pastures.
- Treat yearlings and 2-year-olds as high shedders, and give them three to four yearly treatments with drugs that are shown to be working by fecal exam.
- Deworm horses showing signs of being parasitized, using either moxidectin (the dewormer for which there is least evidence of parasite resistance) or possibly a “larvicidal” regimen of fenbendazole (10 mg/kg for five consecutive days).
- Pick up manure—don’t spread it in the pasture; manure spreading distributes eggs. Allowing cattle, goats, or sheep to rotate onto the horse pastures for a few weeks will help remove equine parasite eggs.
- Consider two other parasites: bots (which almost never cause problems) and pinworms (which can cause horses to itch). Bot treatment is traditionally done 30 days after the first frost. Pinworm treatment is initiated only if there’s a problem.
- Stay away from “alternative” dewormers, such as diatomaceous earth. No evidence exists that it works.
No single deworming program fits all horses. Controlling parasites involves establishing and sticking to a reasonable program and involving an equine healthcare professional to promote and improve the veterinarian-client relationship.
View the American Association of Equine Practitioners’ parasite control guidelines for information and recommendations on deworming programs at bit.ly/2JU4YKA.
Dr. David W. Ramey is a Southern California equine practitioner who specializes in the care and treatment of pleasure horses. Visit his website at doctorramey.com