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Parasites: An Old Problem with a New Solution


By Sarah Cates, DVM

Do you have questions about your horse’s health? The veterinarians at Full Circle Equine Services – Drs. Kakki Wright, Sarah Cates, and Ellen Yungmeyer – answer your questions. Submit your questions to their Facebook page, Full Circle Equine Services ( Those that aren’t selected for publication in this feature column will be answered on Facebook.

You vaccinate your horse every year. You call the veterinarian when he’s sick. You deworm him every three months. You are giving him the best possible veterinary care, right?  The answer is: maybe. Many deworming practices are based on information from 40 years ago.  At that time Strongylus vulgaris, a large strongyle, was the predominant concern.  This blood worm causes damage to the intestinal blood supply and can, subsequently, cause colic. With the advent of Ivermectin, this parasite is no longer a major concern for most horse populations.  Instead, small strongyles are now the primary parasite of concern for most adult horses. 

While high burdens of small strongyles can cause diarrhea, weight loss, poor hair coats, and other symptoms, high infection rates are relatively uncommon and frequent deworming treatments are not needed to keep the majority of horses healthy.  Additionally, the practice of frequent deworming has contributed to a high degree of drug resistance among parasites.  Because of these unintended consequences, new parasite control strategies are needed to keep our horses healthy and treatments effective.  The answer: individualized deworming.

Every horse is an individual and has unique needs.  Many factors contribute to parasite burdens, including the horse’s individual immune system, egg shedding status, stocking density, and environmental conditions.  All of these factors must be taken into consideration when designing a deworming plan for your individual horse.  Your veterinarian can collect history, note important environmental factors, and perform a fecal egg count to help decide what treatment plan is best for your horse. 

A fecal egg count (FEC) is a reflection of the innate ability of the horse’s immune system to regulate levels of egg shedding.  The fecal egg count must be performed at a specific time relative to the horse’s last deworming to truly be accurate and not just a gauge of how successful that last treatment was.  Based on the results of the fecal egg count, a horse can be placed in one of three groups: low, moderate or high shedder.  Once the degree of egg shedding is calculated, a deworming schedule can be determined.  For example, a low shedder (the category approximately 70% of the adult horse population will be included in) may only need to be dewormed twice a year.  Depending on the current deworming practices, this may result in fewer dewormings, which can mean monetary savings for the owner and avoidance of unnecessary treatment for the horse.

Fecal egg counts address small strongyles, but several other parasites must be considered when developing a thorough deworming protocol.  Tapeworms are relatively common amongst adult horses- approximately 50% of horses will be affected.  Although a rare event, tapeworms can be a cause of colic.  Tapeworm infections are difficult to diagnose using common methods.  Because of these combined factors, it is recommended to treat all adults once a year with a dewormer that is effective against tapeworms.  Common options include praziquantel or pyrantel pamoate.  Pinworms can cause pruritus (itching) of the tail and perineal areas.  These infections can be diagnosed by using scotch tape to collect eggs and then examining them under a microscope.  Bots are usually not associated with any disease, but may be incidentally seen during endoscopy of a horse's stomach.

Roundworms are an important consideration for younger horses (typically less than a year of age).  They can cause poor growth, airway inflammation and impaction colic. Unfortunately, roundworms are now showing resistance to ivermectin and moxidectin worldwide.

Not only is complete parasite eradication impossible, it is also not the goal of a successful deworming program.  A successful deworming program should strive to limit number of parasites so clinical signs of infection are not present. This is achieved by correct drug selection and properly timed dewormings to limit the amount of eggs that are transmitted into the environment.  By analyzing the combination of environmental factors and an individual horse’s needs, dewormers can be used efficaciously with the goal of avoiding further drug resistance.

If you have any questions or want further information, please contact Dr. Sarah Cates via email ( or through the clinic’s Facebook page
Additional information is available from the AAEP:  and from the UT College of Veterinary Medicine:

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