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Horse Parasites and Flies

ByJamie Textor, DVM, PhD, DACVS, DACVSMR
Reviewed ByManuals Staff
Reviewed/Revised Modified Aug 2025
v102883122

Horses can get parasites from grazing on grass. These parasites can cause stomach problems, diarrhea, and even colic (abdominal pain and digestive upset).

Common Signs of Digestive System Parasites

Strongyles: Weakness, dry coat, diarrhea

Roundworms: Weight loss, intestinal problems, possible effects on lungs in young horses

Pinworms: Tail rubbing, itching around the rear

Bots: Weight loss, stomach pain

Tapeworms: Mild diarrhea, weight loss

Horses should be dewormed if a parasite egg count in their feces is high enough and your vet recommends it. A healthy adult horse that has very few parasite eggs or no eggs in its feces might not need deworming more than once or twice a year. Young horses need regular deworming, because they are more likely to have parasites.

Tips for Decreasing Digestive System Parasites

Mow and break up manure in pastures to limit the accumulation of parasite eggs passed into feces on pastures.

Rotate pastures with other livestock (like cows or sheep) to decrease the exposure of horses to specific parasite types.

Keep the number of horses per acre low to decrease the shedding of parasite eggs in feces in any particular location.

Feed hay and grain from a feeder to keep it off the ground, where parasites often live.

Remove bot (fly) eggs from the horse’s coat regularly before they hatch into adult flies.

How Do I Prevent Flies?

Flies can bother horses and cause sores. To control flies, keep stalls clean and frequently remove manure (where flies often live). Many insect sprays and lotions are available for direct application to horses to prevent flies. Talk to your vet about the best options for your area.

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