While internal parasites in horses have been recognized for centuries, there had been little scientific basis for their control until the early 1900s. Some methods for controlling parasites in the past are quite frightening: a recommendation from the 1600s advised horse owners to slice the horse's palate with the thought that the ingested blood would kill any internal parasites.
In the 1940s through the '80s, new classes of antiparasitic compounds were developed about every decade. However, now there are only a few classes of dewormers commercially available in the United States: macrocyclic lactones (ivermectin and moxidectin alone or combined with praziquantel), benzimidazoles (fenbendazole and oxibendazole) and tetrahydropyrimidines (pyrantel pamoate and pyrantel tartrate). Because of this, drug resistance among parasites has become a problem for some types of worms.
Some types of worms are less resistant or less common than others. Recent guidelines from Maxwell H. Gluck Equine Research Center at the University of Kentucky suggests the following outline for managers to address parasites without worsening issues with drug:
- Establish a strongyle eggs per Gram (EPG) profile for individual horses rather than blanket deworming entire herds of horses
- Treat foals every eight weeks for ascarid infections until they become yearlings (oxibendazole is currently considered the drug of choice, then fenbendazole)
- Control strongyloides with ivermectin or oxibendazole and tapeworms with praziquantel or pyrantel pamoate/tartrate
- Use ivermectin or moxidectin alone or in combination with praziquantel twice a year (usually the spring and fall) for strongyles
Read more at Equine Disease Quarterly
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