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Parasite Control


Virtually every horse owner knows that horses harbor parasites in their bodies. These parasites can live in most areas within the body, from the eye to the nuchal ligament to the GI tract to the muscles. Mostly we concern ourselves with the types that live in the gastrointestinal tract and can be seen passing in the manure or that cause GI upset in the forms of colic and loose stools. As with rapidly evolving viruses and bacteria, we are in a sort of arms race to control parasites. Lacking new pharmaceuticals to eliminate parasites from the body, we are in a precarious position that requires strategic application of the available treatments.

Targeted Treatment

Today, best practices in parasite control target diagnosis and treatment of at-risk horses, horses showing evidence of infection, and “high shedders” identified through fecal egg counts.

Young horses are often more at risk for parasites than adult horses. Not only can parasites be passed to foals through their mother’s milk, but they are more likely to ingest infective larvae as they investigate their environment. They should be tested and treated for parasites from a very young age. Geriatric horses are also at higher risk for parasites, as their immune system is not as robust and can be suppressed by other diseases like PPID.

Evidence of gastrointestinal parasitism can be as minor as poor hair coat appearance. Poor weight gain or weight loss are common. Symptoms can be more serious, including diarrhea or fluctuations in stool consistency, general ill-thrift, or recurrent colic.

Not every horse that has a high parasite load will exhibit the stereotypical clinical signs, and some may be completely asymptomatic. This is why each horse should be treated as an individual for testing and treatment, and why fecal egg counts are so useful for identifying high shedders. 


Gone are the days of rotational deworming on a fixed schedule. Overuse and misuse of currently available anthelminthic drugs have led to medication-resistant parasites. Not every parasite sheds eggs into the manure and they do not all have the same length of life cycle. The current recommendation is to deworm once or twice a year as-needed, based on the fecal egg count (FEC) test. But diagnostic testing needs to be timed correctly to maximize the chances of finding the parasites that are present. Timing of the FEC should account for the last deworming and the parasites’ life cycles, which can be as short as four weeks from ingested larva to egg-laying adult.

Performing the FEC twice yearly is useful to identify those individuals that are moderate to high shedders. Deworming care can be focused on those individuals. If there is some suspicion of medication resistance, perform a follow-up fecal about 10 days after the dewormer treatment. The egg count should be reduced by at least 90%. If it wasn’t, then we know that there is a resistance issue and plans need to be made to find an effective treatment.

Manure Management

All horses are not equally affected by parasites - 90% of the parasite load is carried by 10% of the horse population. The Pacific Northwest does not get cold enough, hot enough, or dry enough to naturally kill parasite larvae. Aside from well-timed fecal egg counts, manure management is your strongest tool for parasite control.

Frequent removal of manure from horses’ living spaces is important to keep the reinfection rate low.

At least twice a day, manure should be removed from the area where the horse eats. Ideally the pasture should also have the manure removed daily. Most parasites that can live in the ground survive freezing and aren’t killed unless they have been through several freeze-thaw cycles.

Beyond Worms

Parasites may also cause dermatitis. Some parasites are brought into a wound by flies and may prevent the wound from healing. Habronema and Drashia are common culprits for causing summer sores. The larvae can cause intense itching and inflammation when present in a wound. If your horse has a wound that hasn’t healed as expected; that suddenly becomes much more inflamed; or that produces small, granular, yellow debris it may have a summer sore caused by a parasite. Treatments often involve both systemic deworming and topical medications.

Botflies are a special case among parasites. Their life cycle can take as long as 8-11 months, and they cannot be diagnosed with fecal egg counts, as the flies deposit their eggs externally on the horse’s hair shafts. Their presence can only be confirmed with gastroscopy, but bots are assumed to be present in the stomach if eggs are seen on the coat.

Parasite Prevention

Aside from keeping the environment as free from fecal contamination as possible and routine monitoring of fecal egg counts, important prevention methods include frequent application of fly spray/fly sheets and fly masks and removal of bot fly eggs from the haircoat. Verification of negative status post-deworming and isolating new horses until their parasite status has been identified are also best practices to prevent and control parasites in our herds.  

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Pilchuck Veterinary Hospital


11308 92nd St SE Snohomish, WA 98290


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