Appendix B: Anthelmintic Formulations Available
Table 1. Anthelmintic drug classes currently marketed in the U.S., and their general gastrointestinal parasite label claims. Drug class
Label claims*
Benzimidazoles (fenbendazole, oxibendazole)
Tetrahydropyrimidines (pyrantel)
Macrocyclic lactones (ivermectin, moxidectin)
Isoquinoline-Pyrozines (praziquantel)
*Red = Widespread resistance *Blue = Resistance reported *Black = No resistance reported
Benzimidazoles: These drugs interfere with a worm’s energy metabolism on a cellular level. They bind to beta tubulin and prevent its polymerization into microtubules. They are available in paste, liquid and pelleted formulations.
Tetrahydropyrimidines: Pyrantel pamoate and pyrantel tartrate act at the neuromuscular junction causing an irreversible rigid paralysis. Pyrantel salts only affect adult parasites that reside in the lumen of the gastrointestinal tract. Pyrantel pamoate is available in suspension and paste formulations, while pyrantel tartrate is formulated in alfalfa pellets and must be fed on a continual basis, serving only as a preventive, not a purge dewormer.
Heterocyclic Compounds: Several dewormers are classified as heterocyclic compounds, but piperazine is the only one used in horses. Piperazine works by depolarizing muscular membranes, which renders them resistant to the action of acetylcholine. The action of piperazine is limited to adult parasites. Piperazine is used infrequently in horses, and there is currently no formulation marketed for equine usage in the U.S., but one product is listed in Canada. It was available as a liquid or powder formulation which required nasogastric intubation.
Macrocyclic Lactones: These act on glutamate-gated chloride channels in nematode nerve and muscle cells, disturbing the normal transmission of nervous stimuli to muscles. The result is flaccid paralysis. Macrocyclic lactones are the most potent killers of worms, being effective at less than one-tenth the dosage of other classes of dewormers. They also have the unique quality of killing arthropod parasites such as bot larvae, lice, and mites. Macrocyclic lactones are available in paste (ivermectin) and as an oral gel (moxidectin).
Isoquinoline-Pyrozines: Praziquantel is the sole member of the isoquinolone class used in horses. It is also unique in that it has no activity against nematodes. Praziquantel is effective only against tapeworms. In
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Cyathostomins, large strongyles, ascarids, pinworms, threadworms
Cyathostomins, large strongyles, ascarids, pinworms, tapeworms
Cyathostomins, large strongyles, ascarids, pinworms, threadworms, bots
Tapeworms
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