Parasitic worms can live in the intestines of most horses, with small numbers of worms being tolerated and causing no demonstrable 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 death. 

In horses, proper pasture management and medications can assist the horse's immune system in keeping the intestinal population of worms in manageable proportions. With serious infestations, worms can damage a horse’s intestines and other internal organs, causing irreversible harm with potentially fatal consequences.

The exact method of transmission of worms into the horse depends on the lifecycle 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 can cause disease. Eggs produced by adult worms are then shed in the horse’s feces where they can potentially infect other horses.

Although a horse may appear to be in good health, it still can be infected with worms. Common signs of infection in both younger and older horses can include:

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

The best method for confirming whether or not a horse has worms is to have a veterinarian perform 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 agent that is useful for reduction and removal of nematode and protozoal parasites in a variety of species.² 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 may be 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.^4,5

After PO administration in horses, peak blood levels of 0.11 µg/mL and 0.07 µg/mL, respectively, were measured. Absorbed fenbendazole is metabolized to the active compound, oxfendazole (sulfoxide), and sulfone (inactive), and vice-versa.²

At labeled dosages, fenbendazole does not typically cause any side effects. Its use is considered safe in pregnant animals. Hypersensitivity reactions secondary to antigen release by dying parasites may occur, particularly at high dosages. Infrequently, side effects of fenbendazole in dogs or cats may include salivation, vomiting, and diarrhea. Rarely, allergic reactions can develop.² If an animal shows signs of an allergic reaction (facial swelling, itchiness, hives, diarrhea, seizures, or shock), veterinary care should be sought immediately.

Praziquantel for Horses

Praziquantel is an anticestodal anthelmintic agent with activity against a broad spectrum of trematodes and cestodes, typically prescribed to treat tapeworms. Although praziquantel’s exact mechanism of action against cestodes has not been determined, it may be the result of interacting with phospholipids in parasite integument, causing ion fluxes of sodium, potassium, and calcium. At low concentrations in vitro, the drug appears to impair the worm’s sucker function and stimulates the worm’s motility.²

At higher concentrations in vitro, praziquantel contracts and paralyzes the worm’s strobilla. Praziquantel also causes irreversible focal vacuolization, with subsequent cestodal disintegration at specific sites of the cestodal integument. The parasite ultimately becomes susceptible to digestion. In schistosomes and trematodes, praziquantel directly kills the parasite, possibly by increasing calcium ion flux in the worm. This is followed by focal vacuolization of the integument, and the parasite is phagocytized by the host.²

Praziquantel is considered safe to use in pregnant animals. Oral administration of praziquantel can cause anorexia, vomiting, lethargy, or diarrhea in dogs; the incidence of these effects is less than 5%. In cats given oral praziquantel in field trials, adverse effects were rare (ie, less than 2%); salivation and diarrhea were reported.²

Where to buy Fenbendazole + Praziquantel

Fenbendazole + Praziquantel + Pyrantel Pamoate is available in the U.S. through veterinary custom compounding companies. FENBENDAZOLE 68 MG/ML + PRAZIQUANTEL 68 MG/ML, ORAL SUSPENSION 250ML by NexGen Pharmaceuticals provides superior relief from parasitic worms in the horse.

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.

²Plumb’s Veterinary Drugs.

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.