Deworming Strategy for Horses: Fecal Egg Counts and Targeted Treatment
Parasites are still a real health risk for horses, but the way we control them has changed. Dewormers are losing effectiveness because worms are becoming resistant. This guide explains the current approach based on fecal egg counts, and how to plan it.
Why the old rotation is gone
For decades horse owners rotated dewormer products every two months. Many of the worms have since become resistant to some of the drug classes, and no new classes are on the way. The American Association of Equine Practitioners now recommends a strategic approach: test, treat the horses that need it and treat at the times of year when treatment works best. Over-deworming healthy low shedders speeds up resistance without helping the horse.
Horses are not all equal
In most herds, a small share of adult horses, often cited as about 20 to 30 percent, pass the majority of the parasite eggs onto the pasture. Many others pass very few, and a horse's shedding pattern tends to stay the same from year to year. That is why treating every horse on the same schedule wastes drug and promotes resistance.
The fecal egg count
A fecal egg count (FEC) is a lab test that counts the strongyle eggs in a manure sample, expressed as eggs per gram (epg). Your vet or a diagnostic lab runs it. A commonly used set of categories for adult horses is shown below.
| Count (epg) | Category | Typical approach |
|---|---|---|
| Under 200 | Low shedder | Treat once or twice a year, often at the start and end of the grazing season, and retest |
| 200 to 500 | Moderate shedder | Treat two or more times a year and retest |
| Over 500 | High shedder | Treat more often, and retest after treatment to check that it worked |
These are guidelines. Your veterinarian should set the program for your climate, pasture density and horse population.
Check that the dewormer works
A fecal egg count reduction test compares counts before treatment and 10 to 14 days afterward. If the count does not fall by the expected amount, the worms may be resistant to that drug class, and your vet can recommend another. Doing this test once a year or so keeps the program honest.
Young horses and special cases
Foals, weanlings and yearlings are more vulnerable to roundworms (ascarids) and need a different schedule, normally set by a vet. Tapeworms and bots are not detected by a standard egg count, so a tapeworm blood or saliva test or a seasonal treatment may be recommended. Pregnant mares, stressed horses and horses with poor condition may need a different approach as well.
Dose by weight
Giving too little dewormer is another way to create resistance. Use the weight from a tape or a scale, see How to Estimate Your Horse's Weight, and follow the label. Most syringes are marked in weight increments, and the label says whether to round up. The Deworming Schedule Planner builds a yearly plan from your horse's age, count category, climate and pasture, and the Deworming Approach Finder helps you pick between a fecal-test program and a standard rotation.
Manage the pasture too
- Pick up manure from pasture at least twice a week if you can; this is one of the most effective controls.
- Avoid overstocking. Fewer horses per acre means fewer parasites. See Pasture, Acreage and Fencing.
- Rotate pastures and rest them, or graze them with other species such as cattle or sheep.
- Do not spread fresh manure on grazing areas, and compost it before use.
- Feed hay and grain off the ground.
- Quarantine new horses and test them before they join the herd.
A yearly outline
- Spring: run a fecal egg count on each adult horse before treating, then treat as the count and your vet advise.
- Summer: retest if the count was high or the horse is in a hot, wet area.
- Autumn: test again, and treat for bots or tapeworms after the first hard frost if your vet recommends it.
- Always: dose by weight, and record dates, products and counts.
Common mistakes
- Rotating dewormers on a fixed schedule without testing.
- Underdosing because the weight was guessed.
- Skipping the post-treatment test on high shedders.
- Ignoring manure management.
Parasite control depends on your region and herd. This guide is general information, so build the program with your vet.
Frequently asked questions
How often should I deworm my horse?
Under current guidance, it depends on the horse's fecal egg count rather than a fixed calendar. Low shedders may need treatment once or twice a year, while high shedders need more frequent treatment and retesting. Your vet can set the schedule.
What is a fecal egg count?
A lab test that counts parasite eggs in a manure sample, reported as eggs per gram. It shows how many eggs a horse is shedding and which horses need treatment.
Can I use the same dewormer every time?
Worms can become resistant to a drug class that is used too often or at the wrong dose. A fecal egg count reduction test, run 10 to 14 days after treatment, tells you whether the product is still working.
Tools used in this guide
- Deworming Schedule Planner
- Deworming Approach Finder
- Horse Weight Calculator
- Acres per Horse Calculator
More guides
- How to Estimate Your Horse's Weight Without a Scale
- Pasture, Acreage, Fencing and Turnout Planning
- Feeding Horses: Hay, Grain and Supplements
- Body Condition Scoring with the Henneke Scale
- All HorseCalcs guides
Figures in this guide are general guidance for planning. Every horse is different, so work with your veterinarian, farrier or trainer on health, hoof and training decisions, and read our disclaimer.