When a weanling on a North Carolina breeding farm was diagnosed with larval cyathostominosis, Dianne Little, BVSc, MRCVS, equine surgery resident and PhD candidate in the Department of Clinical Sciences in North Carolina State University’s (NCSU) College of Veterinary Medicine, was suspicious of anthelmintic resistance. Parasites resistant to dewormers and associated diseases, such as larval cyathostominosis, aren’t unusual in her experience.

“A horse may display signs of recurrent, low-grade colic, weight loss, poor performance, colitis, or edema,” she reports. A fecal examination revealed a fecal worm egg count (FWEC) of 2,650 per gram (target is less than 100), with 100% small strongyles. For nine years, the farm rotated three dewormers–fenbendazole, pyrantel pamoate, and ivermectin at eight-week intervals–a common practice.

Further testing revealed that the small strongyles found were resistant to both fenbendazole and pyrantel pamoate. Little, along with other researchers, including Sarah Gardner, DVM, PhD, Dipl. ACVIM, assistant professor of equine medicine at NCSU’s College of Veterinary Medicine, designed a study to closely monitor fecal egg count and egg reappearance time (ERT) for small strongyle eggs after deworming in a group of mares and foals.

Rather than simply treating mares and foals every eight weeks with ivermectin, Little and Gardner’s group performed FWECs at intervals, treating when FWECs were greater than 200 in mares and greater than 100 in foals. This type of “targeted” deworming helps decrease the number of treatments per year, perhaps limiting development of further resistance. Over the course of the three-year study, this resulted in a 77.6% decrease in the number of dewormings in mares at a predicted cost savings of about 25%. There was a decrease of 53.3% in the number of dewormings in foals, although predicted costs increased 16.2% because of FWECs performed. Perhaps most important was the realization that rapid rotation deworming is no longer the best approach to protecting horses and making the best use of available deworming products