Horses And Parasite Control

For many years it was thought that the large strongyles were the only real threat to horses’ health. The large strongyles caused considerable damage to the blood vessels supplying the intestine during the migration stage. The damage caused by migrating large strongyles caused many horses to become chronic poor doers or colickers.

Strongyles (Blood Worms)

With the introduction of avermectin-type dewormers that killed migrating strongyles, the danger of permanent damage to blood vessels was greatly decreased. However, horses continued to colic and do poorly and we began to realize that the small strongyle species was equally damaging although in a different way.
The small strongyles penetrate the wall of the intestine and become encysted there until conditions become favorable for them to emerge. These encysted larvae are resistant to dewormers, even the avermectins that kill migrating large strongyles.

Large and small strongyles are resistant to many dewormers on the market today. My drug of choice is Strongid pasteT (pyrantel). This is a very safe product that has been on the market for years and has a proven track record. In healthy horses it stays in the digestive tract where it kills the adult worms. It is not absorbed systemically. IvermectinT, ZimectinT, or QuestT (avermectins), on the other hand, are absorbed, which allows them to kill migrating parasites but also increases their toxicity. StrongidCT (pyrantel tartrate) is designed to be fed on a daily basis to kill worm larbae as they are ingested and to kill small strongyles as they emerge from the gut wall. Most small strongyles are resistant to benzimidole dewormers such as PanacurT.

Ascarids (Roundworms)

These worms are rarely a problem in horses over 2 years old. They can, however, be deadly for youngsters. After the ascarid eggs are ingested, they migrate through the liver and lungs. Many of the “colds” and coughs of babies are actually related to the inflammation in the lungs from roundworm migration. The adult worms can become quite long, and in large numbers cause blockage of the intestine. Roundworms in the intestine are killed by pyrantel, avermectin, and benzimidole dewormers. Avermectins are reported to be effective against adult and migrating ascarids, but based on my experience, I prefer pyrantel or benzimidoles.


It has been thought that tapeworms do not cause much damage to horses, but recently they are being looked at more closely. Tapeworms attach to the intestine at the junction between the small intestine and cecum. This is already a potential area of impaction in the horse, and it is believed by some that tapeworm infestation compounds the problem. Unfortunately tapeworm eggs do not show up on routine fecal exams, so if regular deworming is not resulting in thriftiness, consider giving pyrantel at 2-3 times the normal dose; this should kill any tapeworms.

Oxyuris (Pinworms)

These worms do not cause serious disease but can be irritating to horses. Stabled horses are most at risk. The female worm lays eggs around the perianal area, causing the horse to show symptoms of tail rubbing and hair loss. The best treatment is to wash the area with a mild soap and apply a soothing ointment.


These worms cause mild diarrhea in young horses. Foals become infected via the mare’s milk. Benzimidazole dewormers are safe and effective to use to treat foals. Control involves removing moist damp bedding where the worms breed.


These are not worms at all, but fly larvae. The female botfly, which looks like a bee, lays eggs around the face or legs of the horse and the eggs hatch when exposed to moisture. The larvae migrate to the stomach and attach there to develop. The larvae cause little damage, but the flies are extremely irritating to horses. Avermectin-type dewormers are excellent for controlling bots and treatment is only needed if the small yellow eggs are seen on the legs. Wetting the eggs with warm soapy water or scraping them off the hair is an easy, non-chemical approach to bot control.


Article used with permission. More information about Madelyn Ward, DVM, can be found at her website, Holistic Horsekeeping.

Ruben Martizano

Horses are one of the most amazing animals (after dogs that is…) I always wanted to learn more about them. I been blogging for the last 10 years and creating professional sites for my clients. This one is strictly about my hobby. Hope you will enjoy it. — Ruben Martizano

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