Parasitic worms can infect a horse’s specific tissues, organs and systems, obtaining nutrients from the horse so that they can continue grow, develop and reproduce. Many can live in the intestines of most horses, with small numbers of worms being tolerated and causing no real effect on the horse’s overall health. Larger infestations of intestinal parasites can cause a wide range of problems however, including ill thrift, colic, digestive issues, diarrhea and, in some cases, death. 

Nearly all horses have some degree of parasitic infestation, but along with other management best practices, deworming helps to keep a horse’s internal parasite load low. Doing so lowers the likelihood of clinical disease arising from parasites that are present in the horse. 

The class of internal parasites that most often cause health problems for horses are nematodes (e.g., large and small strongyles), roundworms and tapeworms.¹ Other internal parasites such as threadworms, pinworms and botfly larvae do cause problems in some horses, but are less common.

The precise method of transmission of worms to a horse is dependent upon the life cycle of the worm species involved. 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 to pastures where they can infect other horses.

The clinical signs of parasitic worm infections in horses tend to be common across the various species of worms, and include lethargy, weight loss, diarrhea, colic, lack of appetite and dull coat. The best method for confirming whether or not a horse has worms is a fecal egg count and blood test. These tests will confirm the species of parasite, provide an idea of how many adult worms are in the intestine and give an estimate on how badly a pasture is infested.¹

Fenbendazole for Horses

Fenbendazole is a broad-spectrum methylcarbamate benzimidazole anthelmintic that is used for the reduction and removal of nematode and protozoal parasites.² In horses, fenbendazole is labeled for the removal of: 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.⁴

Fenbendazole’s mechanism of action is believed to lie in the disruption of intracellular microtubular transport systems by binding selectively and damaging tubulin, which prevents tubulin polymerization and inhibits microtubule formation. Fenbendazole has activity against adult life cycle stages of susceptible parasites, and may have larvicidal and ovicidal activity against certain parasites.^5,6

Where to buy Fenbendazole

Fenbendazole is available in the U.S. through veterinary pharmacies and 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.

¹extension.okstate.edu.

²thehorse.com.

⁴Mason 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.

⁵Daniels SP, Proudman CJ. Ovicidal efficacy of fenbendazole after treatment of horses naturally infected with cyathostomins. Vet Parasitol. 2016;227:151-156.

⁶Pittman 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.