Praziquantel is a member of the pyrozine group of dewormers that is effective against cestodes which is the family of helminths called tapeworms. There are three species which infect horses and horses only: Paranoplocephala mamillana, Anoplocephala magna, and Anoplocephala perfoliata. As we learn more about tapeworms, our understanding grows that they are an important cause of both medical and surgical colic cases in equines.

In 2003 there was an explosion of combination products containing praziquantel in combination with ivermectin and moxidectin (Quest). These products bear names like Equimax, Quest Plus, Eqvalan Gold, Zimectrin Gold etc. No other approved deworming products at regular doses are effective against tapes in horses. This article is about the use, pharmacology, effectiveness, toxicity, and dosage of praziquantel and the new combination products.

Though how praziquantel work is uncertain it is believed to destroy the outer layer of the worm making it susceptible to attack by the immune system. At lower concentrations it appears to impair the function of the tape's suckers and stimulates the worm's motility. At higher concentrations it increases the contraction (irreversibly at very high concentrations) of the worm. Also seen is degeneration of the tape worms outer casing which may allow the immune system to attack the worm. In schistosomes and trematodes, praziquantel directly kills the parasite, possibly by increasing calcium ion flux into the worm. Focal vacuolization of the integument follows and the parasite is phagocytized.