Choose Equine Dewormers Carefully

Choose Equine Dewormers Carefully

Veterinarians, horse owners should collaborate for a parasite control program that works

When confronted by shelves and shelves of dewormers, many horse owners can be confused about their choices – and the issue of equine parasite resistance adds even greater confusion.

“Multiple studies from across the country have shown entire classes of dewormers are no longer working against small strongyles, which are a serious equine parasite threat,”1,2,3 says Frank Hurtig, DVM, MBA, director, Merial Veterinary Services. “Now, the challenge to every person who cares about horse health is to help preserve the remaining dewormer classes – while keeping their horses healthy.”

There are only three major chemical classes of dewormers. Dr. Hurtig notes that well-documented resistance to small strongyles has been demonstrated against benzimidazoles, which is one of the older classes of dewormer.1,4,5 In addition 40.5 percent of farms surveyed in a study had small strongyles that were resistant to pyrantel, a second chemical class of dewormer.4

Macrocylic lactones represent the third chemical class, containing ingredients like ivermectin and moxidectin. Studies show that this class is still effective against small strongyles and other key parasites.4,6 However, because ingredients like ivermectin and moxidectin work in similar ways, experts believe that if parasites were to develop resistance to either ingredient, it would leave virtually all dewormers ineffective against this serious parasite threat.7,8,9,10

“It’s time to choose and use deworming products with greater thought and care,” Dr. Hurtig urges. “The first two concerns for all horse owners should be to identify which horses are shedding the most worm eggs and to determine the performance of the product most recently used at their barn or on their farm. Then, horse owners can selectively treat horses and this may help to slow the further development of parasite resistance.”11

Dr. Hurtig recommends starting by talking to your veterinarian about doing fecal egg count tests to identify the horses on your property that are shedding the most worm eggs. Once identified, deworming treatments can be directed at these animals.12

“About 20 to 30 percent of the horses on a farm, or in a barn, put out about 80 percent of the eggs,”12 Dr. Hurtig says. “This apparently relates to individual animal variability with regard to immunity to some parasites but much remains unknown on the topic.”

“The goal of today’s deworming program should no longer be to eliminate all worms,” Dr. Hurtig says. “One approach, is to reduce transmission and limit worm egg output. If owners or veterinarians can identify and selectively treat horses with high worm egg output, they can help control parasites on the farm and help reduce the possibility of those parasites developing resistance. Of course, one’s first instinct often is to try to eliminate all parasites from every horse, but it may not be the best option – now or in the future.”12

Finally, horse owners should choose a product from a reputable manufacturer so any concerns can be addressed fully, Dr. Hurtig says.

“For example, Merial backs every syringe of ZIMECTERINR (ivermectin) and ZIMECTERIN Gold (ivermectin/praziquantel) with a 100% Product Satisfaction Guarantee,” Dr. Hurtig says. “Not only do we stand behind the product’s performance, but we will work with the horse owner and their veterinarian to thoroughly address any concern.”

Small strongyles are considered both a health concern for horses and susceptible to developing resistance. ZIMECTERIN Gold is effective against small strongyles and controls more species and stages of equine parasites – including the tapeworm Anoplocephala perfoliata – than any other brand. 13,14

“Parasite control is an important part of every horse health program, and Merial encourages horse owners to discuss creating a specific, customized program with their veterinarian,” Dr. Hurtig says. “The staff at Merial routinely works with local veterinarians and independent parasitologists. If you have a concern, you can trust that we’ll do the same for you. Owners should be able to trust that their dewormer is working, even while we consider the future performance of deworming products.”

Horse owners with questions about the ZIMECTERIN Brands Product Satisfaction Guarantee can call 1-888-MERIAL-1, option 3, for more information.

Chemical Classes of Common Equine Dewormers

Chemical Class

Active Ingredient






Pyrantel pamoate

Pyrantel tartrate


(Macrocyclic Lactones)



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.

1Kaplan RM, Hodgkinson JE, Thamsborg SM, Nielsen MK. Background and goals. In: Kaplan RM, Nielsen MK, eds. Proceedings of the Equine Parasite Drug Resistance Workshop 2008:3.

2von Samson-Himmelstjerna G. Anthelmintic resistance in equine parasites-potential clinical relevance and implications for control. In: Kaplan RM, Nielsen MK, eds. Proceedings of the Equine Parasite Drug Resistance Workshop 2008:10.

3Kaplan RM. Biological considerations in evaluating drug efficacy and resistance in equine strongyle parasites using fecal egg count data. In: Kaplan RM, Nielsen MK, eds. Proceedings of the Equine Parasite Drug Resistance Workshop 2008:14.

4Kaplan RM, et al. Prevalence of anthelmintic resistant cyathostomes on horse farms. JAVMA


5Woods TF, Lane TS, Zeng QY, Courtney CH. Anthelmintic resistance on pleasure horse farms in

north central Florida. In: Proceedings 42nd Annual Meeting of the AAVP 1997:88.

6Lyons ET, Tolliver SC, Collins SS. Probable reason why small strongyle EPG counts are returning “early” after ivermectin treatment of horses on a farm in central Kentucky. Parasitol Res 2009;104:569-574.

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.

8Conder GA, Thompson DP, Johnson SS. Demonstration of co-resistance of Haemonchus contortus to ivermectin and moxidectin. The Veterinary Record 1993;132:651-652.

9Le 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.

10Sangster NC, Dobson RJ. Anthelmintic resistance. In: Lee DL, ed. The Biology of Nematodes. London: Taylor & Francis, 2002:531-567.

11Martin PJ. Development and control of resistance to anthelmintics. International Journal for Parasitology, 1987(17):493-501.

12Kaplan RM. These ain’t your father’s parasites: An evidence-based medical approach to equine parasite control. The Practitioner October 2008.

13Based on data provided in FDA Freedom of Information summaries.

14Based on data provided on the ZIMECTERIN Gold label.

RZIMECTERIN is a registered trademark of Merial Limited. C2009 Merial Limited, Duluth, GA. All rights reserved. EQUIZIM912-R2 (07/09)