Is it time to deworm your horse?

April 25, 2018

We are frequently asked questions such as “When to worm my horses?”, "How often do you worm a horse?", and “Which dewormers affect what parasites?”, but in order to answers those questions we’ve got to do more digging in on how horses get worms, what are the signs to look for in your horse, and how worms affect your horse.

How does a horse get worms?

Horses typically get worms when turned out with previously infected horses or when they are turned out in a contaminated pasture. In both situations, it is very likely the horse will become infected, as well. Pastures become contaminated with the eggs and larvae or parasitic worms through the manure of an infected horse which then mixes in the grass of the pasture. As your horse grazes, the eggs and larvae are ingested. A pasture can stay infected for a considerable amount of time so always keep that threat in mind.

How do I know if my horse has worms?

Even if a horse appears to be in good health, it still can be infected with worms. Common signs of worm/parasite infection in both younger and older horses include:

- Lethargy

- Loss of weight

- Loss of condition

- Diarrhea Colic

- Lack of appetite

- Dull coat

The best way to confirm whether or not your horse has worms is to have your vet perform a fecal egg count and blood test, which will confirm the species of parasite and will help you decide which dewormer will be most effective.

What are worms and how they can affect your horse?

There are four common types of internal parasites: Strongyles, Roundworms, Tapeworms and Bots. Each species of parasite affects a horse in its own way.

– Strongyles (blood or red worms) – Can be identified as S. vulgaris, S. edentatus and S. equinus. Strongyle infection occurs by ingestion of the larvae, which begin their transformation into parasites as they travel down the horse’s intestine. The S. vulgaris can cause damage in the cranial mesenteric artery, eventually causing colic, gangrenous enteritis, or intestinal stasis and possibly rupture. The S. edentates and S. equinus are active blood feeders that can lead to anemia, weakness, emaciation and diarrhea.
– Roundworms – The larva of this nasty worm start its growth in the small intestine and then migrate through the liver, the lungs and finally, the pharynx or throat where it gets swallowed again. The worm returns to the small intestine to mature and reproduce. Roundworms are an issue with younger horses up to about 15 months of age because of their lack of immunization against the worms. A small infestation will probably have a negligible impact on the horse’s health; however, a heavy infection can trigger weight loss, stunt the young horse’s growth, give a rough hair coat and/or pot-bellied appearance, and cause lethargy and/or colic.
– Tapeworms – Tapeworms take a different approach to infecting your horse. Forage mites in the grass eat tapeworm eggs; the tapeworm larvae then develop within the mites. The horse ingests the forage mites during grazing. Now that the larvae are in the horse’s gut they can develop into maturity. They adhere themselves to the gut wall at the ileo-caecal junction, thusly increases the risk of intestinal obstruction or rupture due to inflammation at the attached site.
– Bots – Adult flies lay yellow-colored eggs to the horse’s forelegs, chest and shoulders. As the horse grooms itself, the horse’s saliva releases the egg adhesive and the larvae then enter the mouth. Once ingested, the larvae travel and attach to the lining of the stomach when it causes irritation, digestive issues and obstruction. After 8-10 months, the larvae are passed in the feces and then burrow into the ground to pupate. They surface from the ground as adult flies and repeat the cycle.

Which type of dewormer removes which parasites? 

How often should a horse be wormed?

Traditionally, veterinarians recommend worming your horse every two months. However, there is a lively debate about the effectiveness of repeated use of the same wormers. Before beginning a worming schedule, it is wise to have a serious discussion with your vet about the best possible worming schedule for your horse.  Here's a chart for you to reference to help keep your horse on a rotation of dewormers and the correct times to deworm.  * Please refer to your veterinarian for an effective wormer schedule.

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