Horse deworming schedule (Canada): egg counts, resistance on Canadian farms, and Quebec's prescription rule

By Sebastien Kister

Last reviewed 6 November 2026

If you were taught to deworm every horse every six to twelve weeks with a different product each time, Canada's own national code says that habit may be behind the resistance it now has to manage, and Canadian farms have the egg counts to prove it. What replaced it is a fecal egg count, one or two treatments a year for most horses, and a test to see whether the product still works. Where you buy that product depends on the province: over the counter in Ontario, on a vet's prescription in Quebec.

What the national code requires: a program, not a calendar

The Code of Practice for the Care and Handling of Equines has one Requirement on parasites: "A parasite control program to prevent parasite related disease must be in place." It recommends building that program with a vet, around the horse's age, the stocking density, drug-resistant parasites and the season, and it is candid about what went wrong: parasite resistance to several dewormers, it says, "may be related to the traditional approach of deworming all horses every 6-12 weeks with rotating products". Quebec, which made most of the Code binding in 2024, left this Requirement out of the obligations, along with record-keeping; in Quebec the program is recommended rather than required.

What replaced the calendar

The American Association of Equine Practitioners revised its guidelines in May 2024. Their messages:

• Count first. Fecal egg counts once or twice a year sort adult horses into low, moderate and high shedders, and the same few horses tend to shed most of the eggs year after year.
• A baseline for every horse: one or two treatments a year, whatever its count, against the parasites an egg count does not show.
• More only for high shedders: above 500 eggs per gram, one or two extra treatments, given in the transmission season, not through the winter.
• Test the drug: a fecal egg count reduction test every year.
• Stop fixed-interval treatment of every horse all year, and stop "blindly rotating" drug classes.

The Western College of Veterinary Medicine in Saskatoon gives prairie owners the same shape in plainer terms: most horses need deworming only once or twice a year, with an egg count before the spring treatment, a second count in the fall "strongly recommend[ed]", and the fall treatment given in late fall, after a hard frost.

Resistance, measured on Canadian farms

The two halves of that table pull in opposite directions, which is the point of the egg count. Treat too often and the small strongyles and roundworms become resistant; treat too rarely, or never test, and the large strongyle that frequent deworming had nearly eliminated comes back. The AAEP guidelines add that the time before eggs reappear after ivermectin or moxidectin has shortened, from 8 to 16 weeks in the 1990s to 4 to 5 weeks now. The AAEP says that is not necessarily resistance, but it means egg shedding is held back for a shorter time.

Check that your dewormer still works

A fecal egg count reduction test counts eggs on the day of treatment and again about two weeks later. The AAEP's 2024 thresholds for small strongyles, read as the reduction a fully effective product should reach and the level below which it has failed:

The test wants an average of at least 40 eggs counted per horse before treatment, which is why it is run on horses with positive egg counts, and why it is a farm result rather than a horse's.

What a Canadian winter does not do

It is tempting to think a prairie winter sterilizes the pasture. It does not. The AAEP guidelines note that below 0 °C the developing larvae die, but undeveloped eggs and infective larvae "can survive and persist for long periods (i.e. months)", and that it is repeated freezing and thawing that harms them. A Saskatchewan study in the 1980s found infective larvae surviving the winter on pasture. In Ontario, larvae on the grass peaked from late August to mid-October, and transmission ran mainly from mid-summer to mid-fall, which is why the fall egg count and the late-fall treatment matter more than any midwinter dose.

The winter has a danger of its own. The small strongyle larvae that wait in the gut wall can emerge together, and an Ontario review of 24 cases of this larval cyathostominosis found them between September and May, most of them from October to December. A clean egg count does not rule them out, because those larvae lay no eggs. Moxidectin and a five-day course of fenbendazole are the only drugs registered against them, though the AAEP says both remove fewer than 85 percent of those larvae, and your vet decides whether a horse needs one.

Foals, tapeworms and bots

• Foals are treated for roundworms, not strongyles: the AAEP suggests two treatments, at about two to three months and about five months, with a benzimidazole, a first egg count at five to six months, and an ivermectin or moxidectin with praziquantel at six to nine months. For roundworms, resistance to ivermectin and moxidectin should be assumed until shown otherwise, which is exactly what the Ontario farms found. The Western College's foal schedule is more frequent, every two months from two months of age, and notes that on the prairies foals do not need deworming through a freezing winter.
• Tapeworms: praziquantel at least once a year on green pasture, in late fall once the cold has stopped transmission. In Canada praziquantel for horses is sold combined with ivermectin or moxidectin. An Ontario study of 234 horses, half of them colic cases, found 56 percent with tapeworm antibodies; the Western College calls tapeworms uncommon on the prairies.
• Bots: the eggs are laid on the coat in late summer and early fall, and the fall ivermectin or moxidectin treatment helps reduce them; the AAEP finds ivermectin may work better than moxidectin against the larvae.

Who can sell you a dewormer

Federally, Health Canada lists every horse dewormer as a non-prescription drug. The 2018 move of "medically important antimicrobials" to prescription covered antibiotics, not dewormers. What happens at the counter is provincial. Ontario requires a licence to sell livestock medicines, which include "preparations for the control of external and internal parasites", and its own page notes that farm supply, feed and tack retailers frequently sell livestock medicines. Quebec is different, as set out in the French version: there the common horse dewormers are sold only on a veterinarian's prescription, and a feed store was fined in 2019 for selling a moxidectin-praziquantel gel that only a pharmacist or a vet may sell.

The active ingredients marketed for horses in Canada, according to Health Canada's drug database: ivermectin, ivermectin with praziquantel, moxidectin, moxidectin with praziquantel, fenbendazole, pyrantel pamoate, and piperazine, in one powder labelled for horses, swine and poultry. There is no oxibendazole product on the market, and no daily pyrantel feed of the kind sold in the US.

The withdrawal times nobody reads

Because Canadian horses can enter the food chain, every dewormer given in a horse's last 180 days goes on its Equine Information Document, with the product's DIN, the date and the withdrawal period. Where a label gives no withdrawal period specific to horses, CFIA publishes provisional ones: 7 days for pyrantel, 13 for fenbendazole, 28 for ivermectin, with or without praziquantel, and 36 for moxidectin. No dewormer is on CFIA's list of drugs that bar a horse from the food chain for life, but the dates still have to be written down.

Why the plan drifts

An egg-count program is only as good as the record behind it. Last spring's counts, the product used in the fall, the reduction test that showed pyrantel failing on your farm: a year later, at a new barn or with a new vet, nobody remembers them, and the horse is back on a calendar by default. The high shedder is the same horse year after year, which is only useful if somebody can see last year's number.

Sources

• National Farm Animal Care Council — Code of Practice for the Care and Handling of Equines (2013), section 4.1.3, and MAPAQ — guide to Quebec's welfare regulation, article 52 (in French)
• American Association of Equine Practitioners — Internal Parasite Control Guidelines (2024)
• Western College of Veterinary Medicine — Deworming your horse
• Ontario resistance studies: Slocombe, Coté and de Gannes, Canadian Veterinary Journal (2008); Slocombe and de Gannes, Veterinary Parasitology (2006); Slocombe, de Gannes and Lake, Veterinary Parasitology (2007); Hearn and Peregrine, JAVMA (2003)
• Prince Edward Island: Butler et al., Veterinary Parasitology: Regional Studies and Reports (2021) and Stoughton et al. (2023); Alberta: Domshy et al., Canadian Veterinary Journal (2024)
• Climate and season: Polley, American Journal of Veterinary Research (1986), Slocombe et al., Canadian Journal of Veterinary Research (1987) and Peregrine et al., Canadian Veterinary Journal (2006) (larval cyathostominosis in Ontario); tapeworms: Trotz-Williams et al., Veterinary Parasitology (2008)
• Health Canada — Responsible use of medically important antimicrobials and the Drug Product Database
• Ontario — Livestock medicine sales in Ontario and O. Reg. 583/20
• Ordre des médecins vétérinaires du Québec — notice of illegal practice (2019) (in French)
• CFIA — Equine Information Document (provisional withdrawal periods, section 8)

Guidelines are revised and resistance moves farm by farm; agree the program with your vet before applying it.

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