When ivermectin was introduced into the veterinary market in the early 80’s it revolutionized animal parasite treatment. Ivermectin paralyzes parasites by disrupting the function of the neurotransmitter gamma-amino-butyric acid (GABA), which is peculiar to invertebrates. This accounts for the high level of safety noted with use of this drug. Doses of up to 9X the recommended dose in horses don’t cause symptoms of toxicity.¹ In addition, the wide variety of parasites affected is astonishing. It is labeled for treatment and control of large and small strongyles, pinworms, ascarids, hairworms, large-mouth stomach worms, bots, lungworms, intestinal threadworms, summer sores (caused by Habronema and Draschia spp.), and dermatitis (caused by neck threadworm microfilariae, Onchocerca sp.).² Widespread use of ivermectin horse dewormer and other antiparasitic drugs has been accompanied by a persistent call for judicious use and rotation of products as veterinarians apply these strategies to slow the inevitable development of resistance.

Parasites have been less likely to develop resistance to ivermectin compared to other anthelmintics. In a prospective study of 80 horses on 10 Georgia farms it was found that cyathostomes (small strongyles) were more likely to develop resistance to fenbendazole and pyrantel pamoate than to ivermectin.³ Fenbendazole resistance was most prevalent with its use being ineffective in reducing fecal egg counts on 9 farms and equivocal on one farm. Pyrantel pamoate, on the other hand was effective in reducing fecal egg counts on 7 farms, ineffective on 2 farms, and equivocal on one farm. Ivermectin was effective in reducing fecal egg counts on all 10 farms. A larger study of 786 horses on 44 farms in the southern United States revealed similar results, but the prevalence of resistance to pyrantel pamoate was higher (~40%).⁴ In addition, multi-drug resistance was high. The authors recommended use of fecal egg count reduction tests for each individual farm to better evaluate the level of resistance and treat horses appropriately.

While rotating between classes of horse dewormers is important, keep in mind that rotating too frequently can result in parasites that are resistant to more than one drug. According to the American Association of Equine Practitioners,⁵ most parasitologists recommend rotating no more often than yearly, for example using pyrantel pamoate one year and ivermectin the next. On the other hand, some researchers state that the slow rotation approach can no longer be recommended and that drugs need to be selected on the basis of a variety of factors including drug efficacy against parasites that are most prevalent at the particular location and the season of the year.⁴ Vets often recommend fecal egg counts to help them determine what deworming program will be most effective. The bottom line is that it is essential to get advice from your local veterinarian regarding the deworming program that will be best for your situation.

1. Plumb DC. Veterinary Drug Handbook. 3rd ed. Ames, Iowa: Iowa State University Press; 1999.
2. Bimeda. Bimectin Product Insert; 2004.
3. Tarigo-Martinie JL, Wyatt AR, Kaplan RM. Prevalence and clinical implications of anthelmintic resistance in cyathostomes of horses 1. J.Am.Vet.Med.Assoc. 2001;218(12):1957.
4. Kaplan RM, Klei TR, Lyons ET, et al. Prevalence of anthelmintic resistant cyathostomes on horse farms. J Am Vet Med Assoc. Sep 15 2004;225(6):903-910.
5. Chamberlain L. Ask the vet: Deworming April 07.  2007;  Available at: http://www.aaep.org/ask_the_vet_questions.php?category_id=81; Accessed February 15, 2008.