With many horse owners puzzled by the thought of parasite resistance — and a number of different parasite control products available — it’s difficult to know which products will work for their horses.

“Of the three classes of dewormers, many experts believe that macrocyclic lactones are the only class of medications to which key equine parasites of adult horses have not developed resistance in the United States,”1,2 says Hoyt Cheramie, DVM, MS, manager, Merial Veterinary Services. “Within that class, there are different products for horse owners to choose from — the two most common being ivermectin and moxidectin — making the choice more difficult.”

Macrocyclic lactones, which are used for parasite control in cattle, sheep and horses, all have a similar mode of action but are different in their chemical properties.3 These differences may affect the likelihood of resistance occurring.3

“Equine parasite resistance is becoming more of a concern as we continue to investigate how well each anthelmintic class and product is currently working,” Dr. Cheramie says. “While we have seen resistance in other anthelmintic classes,4 we had not seen it pop up in macrocyclic lactones until recently. I and other equine veterinarians I’ve spoken with are particularly concerned with moxidectin resistance, which we’ve seen occur in other species and in horses in other countries.”3,5

A study that was conducted between 2001 and 2004 — published in 2006 — of Australian sheep farms that were using only a moxidectin drench for parasite control were found to be nearly three times more likely to have resistance to brown stomach worms than those using ivermectin.3 Additionally, prevalence of resistance was significantly higher — up to 77 percent — on the sheep farms where moxidectin had been used for more than two of the preceding five years.3

With reports of resistance to moxidectin in sheep3 and Brazilian horses,5 some experts in equine parasitology caution that moxidectin use in horses should also be limited.6 Based on theories of selection for resistance, some experts believe that regular use of moxidectin may increase the risk of producing macrocyclic lactone resistance.6

Even though ivermectin and moxidectin are both members of the same anthelmintic class, they do have differences. In horses, ivermectin is not labeled for control of encysted larval stages of small strongyles. Therefore, encysted small strongyles should be slow to develop resistance to ivermectin as a population that hasn’t had an opportunity to be exposed to the drug.5 However, while moxidectin is extremely effective in eliminating larval stages of small strongyles, that also may mean small strongyles may be more likely to develop resistance to moxidectin.5

“If macrocyclic lactone resistance were to develop in one product, it would most likely occur with all macrocyclic lactone products,”7-13 Dr. Cheramie says. “There are some differences in relative effectiveness of each macrocyclic lactone to different species of parasites, but all products would be adversely affected if resistance were to develop.”7-13

With no new parasite control products or classes on the horizon, adds Dr. Cheramie, it’s increasingly important to use all parasite control products carefully.

“One-size-fits-all parasite control programs no longer work to eliminate all parasites,” Dr. Cheramie says. “Instead, horse owners and veterinarians should work together to create a parasite control program that reduces transmission, keeps worm burdens low and treats clinically affected horses.”

Horse owners should work with their veterinarian to identify which horses on their property are shedding the most worm eggs by asking a veterinarian to complete quantitative fecal egg count tests. Once identified, deworming treatments can be directed at those animals.2

“Horse owners should talk to their veterinarian about products that will be best for each horse,” Dr. Cheramie says. “In addition, choosing effective products from a trusted manufacturer will allow horse owners to make sure any concerns will be addressed.”

Some products, like ZIMECTERIN® Gold (ivermectin/praziquantel), are backed by a
100 percent product satisfaction guarantee that can help make deworming decisions easier.* ZIMECTERIN Gold is a broad-spectrum dewormer that controls more species and stages of equine parasites — including the tapeworm Anoplocephala perfoliata — than any other brand.14,15

“Merial continually works with internationally recognized parasitologists to be a part of the solution to parasite resistance,” Dr. Cheramie says. “By working together with horse owners and veterinarians, we can help keep our parasite control products as effective as possible.”

For more information, contact Rachel Torbert, Bader Rutter, at (402) 434-5307 or rtorbert@bader-rutter.com

For more information about the 100 percent product satisfaction guarantee for ZIMECTERIN Gold, please go to www.zimecterin.com.
Warning: Not for use in humans. Keep this and all drugs out of reach of children. In horses, there have been rare reports of swelling and irritation of the mouth, lips and tongue following administration of ZIMECTERIN Gold. These reactions have been transitory in nature. Do not use in other animal species as severe adverse reactions, including fatalities in dogs, may result.  ®ZIMECTERIN is a registered trademark of Merial Limited. ©2009 Merial Limited. Duluth, GA. All rights reserved. EQUIZIM945 (9/09).

1Lyons ET, Tolliver SC, Ionita M, Collins SS. Evaluation of parasiticidal activity of fenbendazole, ivermectin, oxibendazole and pyrantel pamoate in horse foals with emphasis on ascarids (Parascaris equorum) in field studies on five farms in central Kentucky in 2007. Parasitol Res 2008;104:569-574.

2Kaplan RM. These ain’t your father’s parasites: An evidence-based medical approach to equine parasite control. The Practitioner October 2008.

3Rendell DK, Rentsch TE, Smith JM, Chandler DS, Callinan APL. Evidence that moxidectin is a greater risk factor than ivermectin in the development of resistance to macrocyclic lactones by Ostertagia spp. in sheep in southeastern Australia. New Zealand Veterinary Journal. 2006;54(6):313-317.

4Kaplan RM, et al. Prevalence of anthelmintic resistant cyathostomes on horse farms. JAVMA2004;225(6):903-910.

5Schumacher J, Taintor J. A review of the use of moxidectin in horses. Equine Veterinary Education2008;20(10):546-551.

6Coles GC, et al. Anthelmintic resistance and use of anthelmintics in horses. The Veterinary Record 2008;163:679.

7Shoop WL, Haines HW, Michael BF, Eary CH. Mutual resistance to avermectins and milbemycins: oral activity of ivermectin and moxidectin against ivermectin-resistant and susceptible nematodes. The Veterinary Record 1993;133:445-447.

8Craig TM, Miller DK. Resistance by Haemonchus contortus to ivermectin in Angora goats. The Veterinary Record 1990;126:560.

9Kieran PJ. Moxidectin against ivermectin-resistant nematodes – a global view. Australian Veterinary Journal 1994;71:18-20.

10Pomroy WE, Whelan NC. Efficacy of moxidectin against an ivermectin-resistant strain of Ostertagia circumcincta in young sheep. The Veterinary Record 1993;132:416.

11Conder GA, Thompson DP, Johnson SS. Demonstration of co-resistance of Haemonchus contortus to ivermectin and moxidectin. The Veterinary Record 1993;132:651-652.

12Le Jambre LF, Gill JH, Lenane IJ, Lacey E. Characterization of an avermectin-resistant strain of Australian Haemonchus contortus. International Journal for Parasitology 1995;25:691-698.

13Sangster NC, Dobson RJ. Anthelmintic resistance. In: Lee DL, Ed. The Biology of Nematodes. London: Taylor & Francis, 2002:531-567.

14Based on data provided in FDA Freedom of Information summaries.

15Based on data provided on the ZIMECTERIN Gold product label.