By Kathi Billard,
Equine Research Centre
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.
Regime for deworming
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 manufacturers 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
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
11. Consult your veterinarian to set up a treatment program appropriate
for your situation.
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.