Implementing deworming medications in your horse’s health program is a mainstay strategy on the farm. Every 3-4 months, we are faced with using some kind of anti-parasitic (anthelmintic) product. Often there is confusion as to which deworming product to use and when. This can be simplified if you think of drugs in terms of classes that are based on their mode of action.
What are the Classes?
Benzimadazoles 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 anthelminitics include praziquantel, which is efficacious against tapeworms, and piperazine, which targets roundworms, but is no longer used.
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; benzimadazoles 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 benzimadazoles 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. Efficacy of any anthelmintic depends on specific geographic areas and climate, certain seasons and each individual horse.