Parasitic worms can live in the intestines of most horses, with small numbers of worms causing no effect on the horse’s overall health. Larger infestations of worms can cause a wide range of problems however, including ill thrift, colic, digestive issues, diarrhea and death. 

While nearly all horses have some level of parasitic infestation, deworming and other management practices can help to keep a horse’s internal parasite load low. This lowers the likelihood of clinical disease arising from parasites that are present in the horse. If serious infestations do develop, worms can damage a horse’s intestines and other internal organs, causing irreversible harm and potentially fatal consequences.

The life cycle of the worm species involved will determine the exact method of transmission of worms to the horse. In most cases, worm eggs are ingested by a horse from an infected pasture and develop inside the digestive tract or lungs, where they may or may not cause disease. Eggs produced by adult worms are then shed in the horse’s feces where they can potentially infect other horses.

The most common worm infections of horses include:

Large Strongyles – These are the most dangerous but least common of the equine parasites. Horses acquire large strongyles by grazing pasture that is covered with the worm larvae, ingesting the grass and the immature stage of the worm. The larvae migrate through the horse’s gastrointestinal system and may invade the blood vessels of the cecum and colon, where they compromise gut function and vitality.

Small Strongyles – More common than large strongyles, the larvae of these worms encyst in the lining of the horse’s small intestine during winter after the horse has grazed contaminated pasture. The following spring the encysted larvae emerge within the horse’s gut, which can cause significant damage to the intestines.

Ascarids – These worms live in the small intestine, are common in foals, and are also acquired from grazing. The worms penetrate through the gut wall and migrate to the liver and/or lungs. Subsequently, they are coughed up and swallowed, where they develop into adults within the gastrointestinal system. Since these worms can migrate to the lungs, the horse may show symptoms of respiratory disease.

Bot Flies –In the summer months, bot flies lay small eggs on the horse’s legs and coat. The eggs are ingested when horses groom each other. These transform into the larval stage within the horse’s mouth and then migrate to its stomach, where they attach to the gut lining. 

Pinworms – These worms live in the horse’s rectum and exit only to lay their eggs around the perineum. This causes itching, so the most common sign of pinworms is a horse that is continuously rubbing its rear end.

Habronema – These are stomach worms that cause inflammation of the stomach and sores on the skin. Transmitted through an intermediate host, horses are infected by ingesting flies that carry the larval stage of the worm. 

Tapeworms – Several species of tapeworm can infect horses, with each species potentially colonizing a different part of the horse’s gut. The most common tapeworm, Anplocephala perfoliata, can cause impactions at the ileocecal junction, which causes the horse to show signs of colic.¹

Common signs of worm infections in horses can include:

• Lethargy
• Loss of weight
• Loss of condition
• Diarrhea
• Colic
• Lack of appetite
• Dull coat

Fenbendazole for Horses

In horses, fenbendazole is labeled for removal of the following parasites: large strongyles (Strongylus edentatus, S equinus, S vulgaris, Triodontophorus spp), small strongyles (Cyathostomum spp, Cylicocyclus spp, Cylicostephanus spp, Cylicodontophorus spp), ascarids (Parascaris equorum) and pinworms (Oxyuris equi). Benzimidazole resistance has been noted in Cyathostomum spp found in the United States.³

In susceptible parasites, fenbendazole’s mechanism of action is believed to involve a disruption of intracellular microtubular transport systems by binding selectively and damaging tubulin, which prevents tubulin polymerization and inhibits microtubule formation.⁴ Fenbendazole has activity against adult life cycle stages of susceptible parasites and may have larvicidal and ovicidal activity against certain parasites.^4,5

Where to buy Fenbendazole

Fenbendazole is available in the U.S. through veterinary custom compounding companies. 

This product carries numerous potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹thehorse.com.

³Mason ME, Voris ND, Ortis HA, Geeding AA, Kaplan RM. Comparison of a single dose of moxidectin and a five-day course of fenbendazole to reduce and suppress cyathostomin fecal egg counts in a herd of embryo transfer-recipient mares. J Am Vet Med Assoc. 2014;245(8):944-951.

⁴Daniels SP, Proudman CJ. Ovicidal efficacy of fenbendazole after treatment of horses naturally infected with cyathostomins. Vet Parasitol. 2016;227:151-156.

⁵Pittman JS, Myers GH, Stalder KJ, Karriker LA. Effect of fenbendazole on shedding and embryonation of Ascaris suum eggs from naturally infected sows. J Swine Health Production. 2015;23(5):252-263.