Emily Decker, DVM

March 2006

Chewing tobacco is toxic to horses, so why would anyone give it to their horse?  Many years ago, tobacco was used as a dewormer for horses.  Owners had to risk the sometimes deadly side effects of tobacco to prevent problems with parasites.  Thanks to major improvements in veterinary medicine, a multitude of safe, effective and affordable dewormers are available to horse owners and we no longer need to take this risk.  In fact, so many dewormers are now available at tack and feed stores that choosing the right dewormer can be a daunting task.  Many people ask us, “Which dewormer should I give?".  Just as every horse and stable are different, so are the answers to our clients’ questions.  The right deworming program for your horse is dependent on your horse, your farm or stable, and the activities you and your horse enjoy together.  Taking all of these variables into account may seem complicated, but once we understand commonly encountered parasites in horses, controlling parasites becomes common sense.

Know Your Enemies

Strongyles

The first troublesome class of parasites is the strongyles.  The strongyles are split into two groups- large strongyles and small strongyles.  Large strongyles start life as eggs passed in the feces.  These eggs hatch and grow into larvae which are able to survive northern winters on pasture or in hay.  During eating, horses ingest the larvae.  Once ingested by the horse, the larvae pass through the liver, lungs or intestinal arteries as they mature.  As adults, the larvae then migrate to the large colon or cecum and use their teeth to latch on to the horse’s intestinal mucosa.  They lay eggs, which pass into the feces and continue the cycle.  These parasites often do permanent damage as they migrate through important organs in the horse.  Before the current dewormers were available, these parasites were one of the leading causes of death in horses.  Strongyles are susceptible to many dewormers including ivermectin and pyrantel. 

Small strongyles have a similar life cycle to large strongyles with one important difference.  During their larval stage, small strongyles are able to form a capsule or cyst and stop maturing for as long as three years.  These cysts are resistant to regular doses of anthelmentics and often, the larvae will re-emerge in large numbers after a regular deworming, leading to diarrhea, colic, or weight loss.  In order to rid your horse of larval small strongyles, a double dose of the dewormer fenbendazole for 5 days in a row is required.  If you are worried about these parasites, talk to your veterinarian about purchasing this dewormer.

Bots

Another parasite that horse owners often combat during warm weather months is the bot fly.  These flies lay tiny, yellow, sticky eggs on a horse’s hair coat.  Grooming, heat and moisture from the horse’s mouth trigger these eggs to hatch.  The larvae migrate into the horse’s nasal and oral tissues and begin to grow.  Eventually, the maturing larvae migrate to the stomach, where they mature into a grub-like creature.  These larvae eventually pass out with the feces into the environment, where they pupate to become bot flies and start the cycle over again.  Ivermectin is an excellent dewormer for bots, and is very effective in breaking the bot life cycle if used after the first frost and during the winter, when no adult flies are present.

Pinworms

A third class of parasites is pinworms.  These parasites actually crawl out of the anus of a horse and lay their eggs around the horse’s anus and vulva.  The eggs are very sticky; they hang onto your horse until the larvae hatch, and then fall to ground.  Once on the ground, the larvae are again ingested and start the life cycle over again.  The process of hatching can be very uncomfortable and is often the cause of tail rubbing in horses.

Roundworms           

Roundworms are another troublesome group of parasites.  Their eggs can persist in the soil for years until ingested by a horse.  Once inside the horse, these worms do damage as they migrate through the liver and lungs.  Roundworms are susceptible to pyrantel and ivermectin.  Young horses are especially vulnerable to roundworms, and infestations can lead to organ damage or death in youngsters that are not dewormed regularly.  For this reason, we recommend deworming once a month in horses less than one year of age.

Tapeworms

One last class of parasites that has recently become important in equine medicine is the tapeworms.  The early stages of tapeworms are present in orbatid mites in the soil.  During grazing, horses ingest the mites and the tapeworms are able to mature in the horse’s digestive tract.  Tapeworms prefer to migrate and attach to one very specific area of the gastrointestinal tract, known as the ileocecal valve.  High numbers of parasites can in this area are very irritating and have been associated with colic and diarrhea.  Not all dewormers are effective against tapeworms, but several new combination dewormers containing the drug praziquantal are very effective at killing these parasites.

Attack on All Fronts

Obviously, using the appropriate dewormer is a big step in keeping parasites under control.  However, farm management can be just as important as medication in parasite control.  By knowing the life cycle of our parasites, we can see that most parasites contaminate the environment through manure.  The more manure a horse is exposed to, the better chance that horse has of ingesting parasites.  In an ideal world, keeping a small number of horses on a large acreage is the best way to prevent parasite ingestion.  However, this is not always compatible with our living situations and schedules.  Therefore, as owners, we need to compensate for these changes by diligent removal of manure from stalls, paddocks, and arenas.  In addition, dragging these areas and smaller pastures breaks up manure and allows it to dry, killing many parasite eggs and larvae.  Another way to lower the number of parasites our horses ingest is by feeding horses off the ground and away from manure.  How can you tell if your management practices are working?  The best way is to have your veterinarian perform a fecal count on your horse.

In reading about each parasite, you have probably noticed that more than one dewormer is required to kill different kinds of parasites.  Giving several different dewormers at once is neither cost effective nor safe for your horse.  In addition, giving the same dewormer month after month can also lead to resistant parasites.  Therefore, the ideal solution to these issues is to rotate types of dewormers.  When purchasing dewormers, make sure you read the medical name for the dewormer and not just the brand name.  Although two dewormers may have different brand names, they may not contain different medications.  The correct dewormer to rotate and how often to use them differs by region, by farm, and by horse.  For our area, we recommend deworming every six to eight weeks, even in the winter, with rotating dewormers.  However, horses that are in high parasite risk situations or that tend to have higher parasite loads, such as older or younger horses, may require more frequent dewormings or a daily dewormer to control parasites.  Therefore, when starting a deworming program on your farm, it is an excellent idea to consult with your veterinarian.  By doing so, you can determine the parasite risks of your farm and identify horses that may be more susceptible to parasites.