Parasitic worms can live in the intestines of most species, with small numbers of worms being tolerated and causing no demonstrable effect on an animal’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 companion animals (primarily dogs and cats), intestinal parasites are fairly common, and are quite often treated in small animal veterinary practices.

In horses, proper pasture management and medications can assist the horse's immune system in keeping the intestinal population of worms in manageable proportions. Veterinarians and researchers have found that horses’ immune response to worms varies; thus, some horses are more susceptible to worm infestation than others. When present in low numbers, worms cause minimal problems in horses. However, when worms reach moderate or larger numbers, they can severely affect the horse’s health. In serious cases, they can damage a horse’s intestines and other internal organs, causing irreversible harm with potentially fatal consequences.

The exact method of transmission (into the horse) depends on the lifecycle of the worm. In general, eggs are ingested by the horse from an infected pasture and develop inside the horse’s digestive tract or lungs, which is where they can potentially cause disease. Eggs produced by adult worms are then shed in the horse’s feces where they can potentially infect other horses.

While a horse may appear to be in good health, it still can be infected with worms. Common signs of parasite 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 your vet perform a fecal egg count and blood test. These tests confirm the species of parasite; provide an idea of how many adult worms are in the intestine; and give an estimate on how badly your pasture is infested. The blood test measures chemicals in the blood produced by inflammatory responses to the migration of the larvae.¹

Classes of Medications

The classes of medications commonly used to treat worm infestations include:

• Benzimidazoles (Fenbendazole, Oxibendazole)
• Macrocyclic Lactones (Ivermectin, Moxidectin)
• Tetrahydropurimidines (Pyrantel Pamoate, Pyrantel Tatrate)
• Isquinoline-pyrozines (Praziquantel)

Fenbendazole

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 dogs, fenbendazole is labeled for removal of the following parasites: roundworms (Toxocara canis, Toxascaris leonina), hookworms (Ancylostoma caninum, Uncinaria stenocephala), whipworms (Trichuris vulpis), and tapeworms (Taenia pisiformis). It is not effective against Dipylidium caninum. Fenbendazole has also been used clinically to treat Eucoleus aerophilus (formerly known as Capillaria aerophilia), Filaroides hirthi, and Paragonimus kellicotti infections in dogs. Fenbendazole has been used extra-label in domestic cats, small mammals, reptiles, sheep, pet birds, and camelids and may be approved in countries outside the United States for these 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

Fenbendazole: Pharmacology

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. Benzimidazoles can also act at higher concentrations to disrupt metabolic pathways within the helminth and inhibit metabolic enzymes, including malate dehydrogenase and fumarate reductase. Benzimidazoles may be considered time-dependent antiparasitic agents.² 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 calves 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

Fenbendazole: Adverse Effects, Drug Interactions and Warnings

At labeled dosages, fenbendazole does not typically cause any side effects. 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), seek veterinary care immediately.

The following drug interactions with fenbendazole have either been reported or are theoretical in humans or animals and may be of significance in veterinary patients. Unless otherwise noted, use together is not necessarily contraindicated, but the potential risks must be weighed and additional monitoring performed when appropriate.

BROMSALAN FLUKICIDES (eg, dibromsalan, tribromsalan; none available in the United States): Oxfendazole and fenbendazole should not be given concurrently with bromsalan flukicides; abortions in cattle and death in sheep have been reported when these compounds are used together.

METHIMAZOLE: In sheep, methimazole has increased fenbendazole (parent) concentration and delayed the formation of sulfoxide and sulfone metabolites.

TRICLABENDAZOLE: In an in vitro study, triclabendazole reduced the rate of oxfendazole formation but increased oxfendazole accumulation.²

Praziquantel

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 (ie, chain of proglottids). In addition, praziquantel 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: Adverse Effects, Drug Interactions and Warnings

The following drug interactions have either been reported or are theoretical in humans or animals receiving praziquantel and may be of significance in veterinary patients. Unless otherwise noted, use together is not necessarily contraindicated, but the potential risks should be weighed and additional monitoring performed when appropriate.

ALBENDAZOLE: May increase albendazole sulfoxide concentration

CIMETIDINE: May increase the serum concentration of praziquantel

CYP3A4 INDUCERS (eg, dexamethasone, phenobarbital, rifampin): May decrease praziquantel levels.11 In humans, it is recommended to discontinue rifampin 4 weeks before administering praziquantel.

GRAPEFRUIT JUICE: Can significantly increase praziquantel serum levels

KETOCONAZOLE, ITRACONAZOLE: May increase praziquantel levels

OXAMNIQUINE: In humans, synergistic activity reportedly occurs with praziquantel and oxamniquine in the treatment of schistosomiasis; the clinical implications for veterinary patients is not clear.²

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

Pyrantel Pamoate

Pyrantel Pamoate is a pyrimidine anthelmintic used primarily for ascarids in a variety of species. Due to its poor absorption as compared to other anthelmintic agents, it is only considered to be useful for GI parasites.²

Pyrantel pamoate is FDA approved in horses for the removal of Strongylus vulgaris, S equinus, and Parasacaris equorum.⁶ It has variable activity against Oxyuris equi, S edentatus, and small strongyles. Pyrantel pamoate at twice the recommended dose is active against ileocecal tapeworms (eg, Anoplocephala perfoliata); however, resistance has been reported. Resistance to this medication is an ongoing concern,⁷ particularly for large strongyles⁸ and cyathostomins.⁹ It is recommended to perform fecal egg count reduction (FECR) testing for strongyle nematodes. A value of greater than 90% in treated horses is the suggested cutoff for determining pyrantel pamoate’s efficacy in an individual horse and on a given farm; FECR testing values less than 90% indicate resistance.¹⁰

Pharmacokinetically, pyrantel pamoate is a cholinergic agonist that binds to the nicotinic receptors of susceptible parasites to cause depolarization and acts as a neuromuscular-blocking agent to paralyze the organism. It also inhibits cholinesterase.²

Pyrantel Pamoate: Adverse Effects, Drug Interactions and Warnings

The following drug interactions have either been reported or are theoretical in humans or animals receiving pyrantel and may be of significance in veterinary patients. Unless otherwise noted, use together is not necessarily contraindicated, but weigh the potential risks and perform additional monitoring when appropriate.

LEVAMISOLE: Because of similar mechanisms of action (and toxicity), do not use concurrently with pyrantel.

MORANTEL: Because of similar mechanisms of action (and toxicity), do not use concurrently with pyrantel.

ORGANOPHOSPHATES: Increased risk for adverse effects

PIPERAZINE: Pyrantel and piperazine have antagonistic mechanisms of action; do not use together.²

Adverse effects are unlikely when pyrantel pamoate is administered at recommended doses. Emesis, diarrhea, or inappetence can occur in small animal species receiving pyrantel pamoate and may be related to expulsion of parasites.²

Where to buy Fenbendazole + Praziquantel + Pyrantel Pamoate

Fenbendazole + Praziquantel + Pyrantel Pamoate is available in the U.S. through veterinary custom compounding companies. In combination, Fenbendazole 68 mg/ml / Praziquantel 68 mg/ml / Pyrantel Pamoate 68 mg/ml oral suspension by NexGen provides superior relief from a wide range of endoparasites.

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.

⁶Strongid T [product label]. In. Kalamazoo, MI: Zoetis Inc.; 2013.

⁷Peregrine AS, Molento MB, Kaplan RM, Nielsen MK. Anthelmintic resistance in important parasites of horses: Does it really matter? Vet Parasitol. 2014;201(1-2):1-8.

⁸Smith MA, Nolan TJ, Rieger R, et al. Efficacy of major anthelmintics for reduction of fecal shedding of strongyle-type eggs in horses in the Mid-Atlantic region of the United States. Vet Parasitol. 2015;214(1-2):139-143.

⁹Scare JA, Lyons ET, Wielgus KM, Nielsen MK. Combination deworming for the control of double-resistant cyathostomin parasites – short and long term consequences. Vet Parasitol. 2018;251:112-118.

¹⁰AAEP. American Association of Equine Practitioners Parasite Control Guidelines. AAEP website. https://aaep.org/sites/default/files/Documents/InternalParasiteGuidelinesFinal5.23.19.pdf. Updated May 2019.