Horse Deworming Calculator

How this plan is built

Current AAEP guidance works on two tiers. Baseline treatments go to every horse once or twice a year to cover large strongyles, roundworms, bots and tapeworms. Everything beyond that is surveillance based.

Surveillance-based treatments exist for one reason: to reduce the number of small strongyle eggs a horse drops onto pasture. Horses that shed few eggs contaminate little, so treating them more often buys nothing.

The goal is not a worm-free horse. Small strongyles are present in essentially every grazing horse, they are usually mild pathogens, and chasing eradication mainly produces drug resistance.

Why calendar rotation was dropped

Rotating drug classes every eight weeks was standard advice for decades. It was designed for large strongyles, a parasite that intensive treatment has since pushed to near-undetectable levels across much of the United States.

AAEP now advises stopping fixed year-round intervals and abandoning blind rotation. Rotating between classes that a farm has already selected resistance to does not restore efficacy, it just spreads the selection pressure across every class.

The replacement is fewer treatments, chosen by test results, and timed to when parasite transmission is actually happening in your climate.

What a fecal egg count does and does not show

A fecal egg count measures eggs per gram of manure. It tells you how much a horse contributes to pasture contamination, and in foals it tells you whether roundworms or strongyles dominate.

It does not measure worm burden. It does not diagnose disease. AAEP is explicit that there is no correlation between egg counts and the parasite stages that cause clinical illness, so a normal count never rules parasites in or out for a sick horse.

Counts also miss things: encysted small strongyle larvae in the gut wall, migrating larvae, and pinworms, whose eggs sit around the anus rather than in manure. Standard techniques detect fewer than one in ten tapeworm infections.

Samples should be fresh, sealed, refrigerated straight away, tested within about a week, and never frozen.

The fecal egg count reduction test

The FECRT is how you find out whether the dewormer you are buying still works on your farm. Count eggs before treatment, treat at the labeled dose, then count again 14 days later.

AAEP recommends running one every year. At least five horses grazing together gives a usable result, and each horse should contribute around 40 counted eggs before treatment for the numbers to mean much.

Thresholds now come from the 2023 WAAVP guidelines and differ by drug class. For small strongyles, macrocyclic lactones are expected to reach 99.9%, benzimidazoles 99%, and pyrantel 98%, with resistance indicated below 92%, 90% and 80% respectively.

For a single horse, such as a new arrival in quarantine, a simpler rule applies: any effective product should cut strongyle and roundworm counts by more than 95% at 14 days.

Where resistance currently stands

Small strongyle resistance to benzimidazoles and to pyrantel is widespread worldwide and should be assumed on any farm until testing shows otherwise. Early macrocyclic lactone resistance has now been reported too.

Roundworms run the other way. Macrocyclic lactone resistance in this parasite is widespread, which is why foal treatments are built around benzimidazoles instead.

Pinworm resistance to macrocyclic lactones is widespread, and treatment failure of praziquantel and pyrantel against tapeworms has been reported in central Kentucky.

Egg reappearance periods have collapsed alongside this. Ivermectin once suppressed egg output for 8 to 10 weeks and moxidectin for 12 to 16. Studies since 2017 report 4 to 5 weeks for both.

Foals, weanlings and young horses

Roundworms, not strongyles, are the main threat in foals, and they are the most important worm pathogen in this age group. Heavy burdens can cause small intestinal impaction.

Foals are treated twice for roundworms, at roughly 2 to 3 months and again around 5 months, using the benzimidazole class. A first fecal egg count at about 5 to 6 months shows whether roundworms are giving way to strongyles.

Weanlings turned out on green pasture get their first strongyle treatment at about 6 to 9 months, with praziquantel included. Yearlings are treated as high shedders regardless of their counts, and two to three year olds usually receive three to four treatments a year.

Selective, count-driven treatment is specifically not recommended under five years old. Young horses have not yet developed the immunity that keeps shedding low in mature horses.

Timing to the transmission season

Strongyle eggs need moderate temperature and moisture to reach the infective stage. Development stops below about 43°F and free-living stages die rapidly above 104°F.

That means the useful treatment window is completely different in North Dakota and in south Texas. Treatments aimed at cutting pasture contamination belong inside the active season, because outside it most eggs die on their own.

Pasture management does real work here. Removing manure frequently, composting before spreading, and grazing cattle or sheep on contaminated ground all reduce infective larvae. Fresh manure should never go onto grazed pasture.

When your veterinarian has to be involved

Anything clinical: weight loss, diarrhea, repeated colic, ventral edema or a horse that is simply doing badly. Parasitic disease is diagnosed from the whole clinical picture, never from an egg count alone.

Also product selection and dosing. Dose by an accurate bodyweight rather than an estimate, since underdosing selects for resistance faster than almost anything else. Our horse weight calculator gives a measurement-based figure to work from.

Herbal and organic dewormers have never demonstrated efficacy in controlled trials, and they are not regulated the way approved anthelmintics are. They do not replace a tested program.

Parasite control sits alongside the rest of routine care. See the horse vaccine schedule and the horse care guide for how the pieces fit together across the year.

Sources

• AAEP Internal Parasite Control Guidelines, revised May 2024
• Full guidelines document (PDF), AAEP 2024
• FECRT analysis tool, University of Copenhagen
• AVMA: AAEP updates internal parasite control guidelines