By Kathi Billard, HBSc, BEd,
Equine Research Centre
Writers Bureau

Parasite resistance to equine dewormers

Drug resistance occurs by overexposing a pathogen to a medication. This overexposure gives the pathogen more opportunities to find a way to avoid the effects of the medication. Evidence has been presenting itself around the world that resistance of equine parasites to
current medications is already occurring. The most well known case is the resistance of small strongyles to benzimidazoles (e.g. Panacur). This is a particular problem within North America, but also occurs around the world. In parts of the southern U.S. resistance to Pyrantel, a drug used to kill strongyles and other intestinal parasites of horses, has also been documented. Recent studies in Holland have identified incidences of total resistance of roundworms to the class of drugs called macrocyclic
lactones (see Table1).

A farm being studied here in Ontario has also presented evidence suggestive of resistance of roundworms to macrocyclic lactones. Many of the common deworming practices support the indiscriminate use of dewormers, increasing the rate at which the worms become resistant. Crowding horses into small pastures multiplies the problem and potentially results in the development of serious health problems. Another issue is that many people go out and buy a dewormer without consulting their veterinarian. Owners administer it and hope for the best. However, this practice will not necessarily target the type of parasite your horse is harbouring. It may also be unnecessarily overexposing harmless low-levels of parasites to the medication, thereby increasing their chance of becoming resistant.

“Strategic Regime” for deworming

Andrew Peregrine, BVMS, PhD, DVM (UK) Associate Professor in the Department of Pathobiology at the Ontario Veterinary College, University of Guelph, suggests the following practices be adopted to slow the progress of parasite resistance:

1. Rotate deworming products every one to two years. Make sure the drug you are switching to is from a different class of drugs (see table 1).
2. Do not overuse dewormers. A horse may be able to develop immunity to parasites through controlled exposure to them.
3.Within the Canadian climate, adult horses should only be dewormed (following manufacturer’s directions) during the first four months of the grazing season.
4. Do annual fecal checks in the summer to determine the presence of eggs and to monitor the efficacy of treatment. Once the parasite(s) identity has been determined the vet can recommend the best treatment regime.
5. Dosing according to the manufacturer instructions is extremely important. Too low a
dosage may not completely eliminate all the worms. Too high a dose may lead to drug
6. Management of manure: situate manure piles far from water sources and grazing areas; follow composting procedures (high temperatures inside compost will kill parasites); muck stalls on a daily basis to avoid exposure due to confinement; remove manure from pasture twice per week; on larger acreage harrow the land only in dry, hot weather or below freezing (but snow-free) to kill parasites by exposing them to unfavourable elements.
7.Avoid overcrowding your pasture; overcrowding increases the concentration of shed parasites.
8. Keep hay and grain off the ground or far away from manure deposits.
9.To ensure they are not contaminated by fecal matter, clean and disinfect buckets and water troughs periodically.
10. Isolate new horses from the rest of the herd until they have been dewormed and do a fecal 10-14 days later to confirm that the treatment worked. All horses on a farm should be on the same deworming medication.
11. Consult your veterinarian to set up a treatment program appropriate for your situation.

The author gratefully acknowledges Dr. A. Peregrine for his help and assistance in the creation and preparation of this article.

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Updated: October 2005.