Numerous research papers and presentations over the last several years have noted that internal parasites in horses are growing more resistant to the deworming drugs (anthelmintics) we have available. A study of European horses presented at the 2009 American Association of Equine Practitioners Convention, held Dec. 5-9 in Las Vegas, Nev., provided the latest evidence in support of the same grim reality.

"In the past, the selection pressure generated by erroneous use–or even abuse–of anthelmintics has led to the spread of drug-resistant parasitic populations," began Donato Traversa, DVM, PhD, Dipl. EVPC, of the University of Teramo, Italy. "This is particularly true for horse cyathostomins (small strongyles); recent evidence of reduced efficacy has been reported in Europe, the United States, and Latin America."

For this study, 1,704 horses with cyathostomin infection on 102 properties in Italy, the United Kingdom, and Germany were split into four groups. Each group was dewormed with fenbendazole, pyrantel, ivermectin, or moxidectin