Deworming may seem complex and overwhelming with all of the different types of horse dewormers available. Not every horse dewormer treats every kind of parasite, and drug resistance is a problem in some areas.

Avermectin (Ivermectin)

“Ivermectin has a broad spectrum of activity against both intestinal and insect parasites and is effective against drug-resistant strongyles,” say James M. Giffin, M.D. and Tom Gore, D.V.M. in their book Horse Owner’s Veterinary Handbook. “It also kills migrating larvae. It is effective against the tissue stages of large strongyles, ascarids, Onchocerca, and bots.”

Benzimidazole (BDZ)

Benzimidazoles are highly effective against pinworms and strongyles. “The emergence of small strongyles resistant to BZDs has become a significant problem,” say Giffin and Gore. Oxibendazole, however, is still effective against small strongyles. With the exception of thiabendazole, they are also effective against ascarids. They are safe for use in pregnant mares, foals, and sick horses.

Carbamate

Pyrantel pamoate and pyrantel tartrate are safe for use in mares during late pregnancy, foals, and sick horses. They are not effective against bots, but are useful for treating other common worms and tapeworms.

Organophosphate

Dichlorvos and trichlorfon are excellent for use against ascarids, bots, and pinworms, and moderately effective against strongyles. Organophosphates can be toxic, so they should not be used in foals younger than 4 months and in pregnant mares who have reached the midpoint of their pregnancy. Some insecticides contain organophosphates and should not be applied to the horse within two weeks of deworming with dichlorvos or trichlorfon.

Piperazine

Piperazines are highly effective against ascarids and works rather quickly. For this reason, horses with major ascarid infestations should not be treated with piperazines for fear of impaction.

“It is not possible to control all worms with just one wormer,” says Colin Vogel in his book, The Complete Horse Care Manual: The Essential Practical Guide to All Aspects of Caring for Your Horse. “You must target-worm for the exceptions, and then have one drug for routine use throughout the year. There are no resistance problems with ivermectin and pyrantel, but in some parts of the world, there is resistance to the benzimidazole wormers.”

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