Ever wonder which deworming medication you should use and when? When you know which ingredients are designed to target specific parasites, the decision is easier.

Dewormers, technically called anthelmintics, are based on their mode of action. They fall into one of several classes. 

Deworming medication classes
Benzimidazoles These include fenbendazole, oxibendazole, and thiabendazole. The intended targets are ascarids (roundworms), strongyles, and pinworms.

Tetrohydropyrimidines These include pyrantel pamoate and pyrantel tartrate. The intended targets are ascarids, strongyles, and pinworms.

Avermectins These include ivermectin and moxidectin. The intended targets are ascarids, strongyles, pinworms, and bots. 

Other equine anthelmintics include praziquantel, which is efficacious against tapeworms, and piperazine, which targets roundworms, but is no longer used.

Today’s Reality
Large strongyles, once the bane of horse health, have been effectively reduced with aggressive parasite control programs over the past 30 years. The three primary drug classes listed above still work to control them. Tapeworms are also effectively reduced with a single dose of praziquantel.

Recent data shows that some anthelmintics that used to be efficacious against other worm species have lost their ability to reduce internal parasite infection. For example, avermectins no longer work well on ascarids; pyrantel is inconsistent; benzimidazoles are still effective.

Currently, small strongyles (cyathostomins) are the most important parasite to control since parasite larvae of this worm encyst in the large intestinal wall and set up inflammatory conditions in the colon. According to the Merck Veterinary Manual, there are more than 40 species of several genera of small strongyles that have been found in the cecum and colon of the horse.

Historically, avermectins were quite effective against small strongyles, but they are becoming less so now. Single doses of pyrantel and benzimidazoles do not work at all. There is still some effectiveness from the five-day dosing of oxibendazole and better effectiveness from moxidectin.

In general, however, it is important to have your veterinarian perform regular fecal egg count reduction testing to determine how well a particular dewormer is performing on the horses on your farm. The efficacy of any anthelmintic depends on specific geographic areas and climate, certain seasons, and each individual horse.

By Nancy S. Loving, DVM