Katie Hennessy

Deworming Your Horse

Controlling gastrointestinal parasites & deworming 


Parasites can be a significant problem in horses and with increased resistance to current deworming medication, it is critical to create an effective strategy and plan. There are numerous species of worms that can infect horses, with three types being the most common. While some changes are visible, they may also cause internal damage without creating any clinical signs. Following a schedule specific to your horse will keep your horse’s parasite load low while minimizing the chance of resistance to the various dewormers.


Common Internal Parasites


Roundworms are a common parasite in young horses, usually under 2 years of age. As horses age, they develop immunity to the roundworm. While these parasites are in the intestines, they also migrate through the trachea, lungs, and liver. Symptoms are an unthrifty appearance with a rough/dull hair coat, a cough or possible nasal discharge, slow growth, colic, and a potbelly appearance. 


Small and large strongyles are found in the large/small intestine, the cecum, and migrate thru the walls of the arteries or liver.  They cause dramatic symptoms such as fever, diarrhea, loss of appetite, weight loss, colic, and death if untreated. In addition, high numbers of strongyles often cause repeated bouts of colic. 


Tapeworms live in the small intestine and cecum. They are interesting because they need an intermediate host called a forage mite to complete their lifecycle.  The highest number of forage mites are found on the pasture in the fall. They may not show up in fecal tests so detection may be hard to determine. They can cause slow growth, loss of body condition, diarrhea, and recurrent colic. 


Deworming Recommendations for Florida


Internal parasite control begins in the fall as the weather starts to cool. First, a fecal analysis determines if the horse is a low, moderate, or high shedder. The “shedder” describes the level of fecal egg counts and an estimate of the horse’s parasite burden or worm load. 


Most horses should be administered a dewormer containing praziquantel in the fall, which treats tapeworms. Low and moderate shedders should have a fecal test performed twice a year and then dewormed accordingly. High shedders should have a second fecal test performed 10-14 days after a dewormer is administered to check for possible dewormer resistance.


Communicate with your veterinarian about scheduling a fecal test during their regular exams.

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