Resistance is a real threat, and has been for years.

What is parasite resistance?

  • Parasites that used to be easily controlled by a dewormer start surviving treatment.
  • A dewormer doesn’t work as well as it used to.
  • Parasite resistance is a farm situation, not an individual horse situation.

Parasite resistance is not new.

  • Resistance of small strongyles (one of the major parasites in horses) has long been a concern1 — that’s why rotation was initially recommended.
  • Even 30 years ago, there was documented resistance to equine dewormers.2,3

How bad is parasite resistance in horses today?

  • Entire classes of equine dewormers no longer work well against small strongyles.1,2,4,5
  • Small strongyles — the major target of parasite control in mature horses — have demonstrated widespread resistance to two of the three major dewormer classes (benzimidazole and pyrantel products).1,2,4,5
  • While products in the third class (macrocyclic lactones) still control small strongyles,2,3,6 resistance could develop from overuse.7,8,9,10
Chemical classes of common equine dewormers
Chemical Class Active Ingredients Common Product Names
Benzimidazoles Fenbendazole
Tetrahydropyrimidines Pyrantel pamoate
Pyrantel tartrate
STRONGID® products
Macrocyclic lactones Ivermectin
QUEST® Plus*

*These products also include praziquantel, an active ingredient that specifically controls tapeworms (Anoplocephala perfoliata)

Results of a study documenting resistance.2

bar chart

Based on the largest survey of equine dewormer resistance reported to date. Forty-four farms/stables in five states.2

Rotation is no silver bullet.

If we keep doing what we’re doing …

What do independent equine parasitologists recommend?

Strategic deworming instead of a blanket one-size-fits-all approach.

The goals of strategic deworming.

Strategic deworming: A resistance management plan to discuss with your veterinarian.

HorsesStep 1.

  • Have your veterinarian monitor to determine which parasites exist on your farm and which products are effective against them.
  • Use fecal egg count reduction tests (FECRTs) to monitor product efficacy and worm burden by horse.3,12,13,14

Step 2.

  • Selectively treat horses with products known to work on your farm.
  • Give more treatments to higher egg shedders and fewer treatments to lower egg shedders13 (20 percent to 30 percent of the horses on any farm shed about 80 percent of worm eggs12,13,15).
  • Foals, weanlings and yearlings may need more treatments because they are more vulnerable to small strongyles and ascarids.16

Step 3.
Be aware, this is a different direction than deworming programs have ever taken before.

  • Maintain a population of nonresistant parasites on the farm — a refugia — which are left untreated, so that susceptible parasites (those not resistant to dewormers) remain to breed with resistant parasites.2
  • Another way to think of this: Dilute the resistant parasites in a large “pool” of nonresistant parasites.
  • You actually NEED nonresistant parasites to keep resistance from taking off.2
  • Refugia is the most important factor in slowing the rate of development of resistance.13

Parasites of concern: The top three troublemakers.
Others are considered case by case.

Small strongyles …

  • Can cause mild colic (may be chronic), weight loss, diarrhea, loss of appetite, poor coat condition and intestinal ulcers17
  • Virtually all grazing horses are infected12
  • Horses never develop total immunity12
  • Well-documented resistance to benzimidazole and pyrantel products1,2,4,5

Roundworms (ascarids) …

  • The greatest concern for horses under 6 months of age16
  • After 6 months, healthy horses will develop immunity, but can still shed eggs
  • Adults cluster in the small intestine causing impaction, often with colic, that can result in a ruptured gut and death16

And tapeworms.

  • Virtually all grazing horses are at risk
  • Contribute to colic by causing inflammation, ulceration and bowel obstruction
  • No fecal test can reliably diagnose active tapeworm infections18

ZIMECTERIN® Gold (ivermectin/praziquantel) controls more species and stages of parasites than any other product.19,20

  • Effective against small strongyles resistant to benzimidazole products20
  • More than 99% effective against natural tapeworm infections (Anoplocephala perfoliata)19
  • Approved for use in adult horses and foals as young as 2 months old20
  • 100% product satisfaction guarantee

Count on your veterinarian’s expertise.
Talk to your veterinarian about a customized deworming program for the horses in your stable.


1Barger IA and Lisle KA. Benzimidazole resistance in strongyles of horses. Aust Vet J 1979;55:594-595.

2Kaplan RM, et al. Prevalence of anthelmintic-resistant cyathostomes on horse farms. JAVMA 2004;225(6):903-910.

3Lyons 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;103:287-291.

4Woods 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.

5Kaplan 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.

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.

11Uhlinger CA, Kristula M. Effects of alternation of drug classes on the development of oxibendazole resistance in a herd of horses. J Am Vet Med Assoc 1992;201:51-55.

12Reinemeyer CR. Rational approaches to equine parasite control. Equine Parasite Control Kentucky Equine Research, Inc. 64-72.

13Kaplan RM. These ain’t your father’s parasites: An evidence-based medical approach to equine parasite control. The Practitioner October 2008.

14Kaplan R. Recommendation for standardization of fecal egg count reduction tests in cattle. AAVP 2007. Washington, D.C. Abstract 78:84-85.

15Brazik EL, et al. Pyrantel pamoate resistance in horses receiving daily administration of pyrantel tartrate. JAVMA 2006;228:101-103.

16Clayton HM. Ascarids: recent advances. In: The Veterinary Clinics of North America: Equine Practice. Philadelphia, Pa.; 1986;2(2):313-328.

17Merck Veterinary Manual, Ninth edition, 2005:268-269.

18Reinemeyer CR. Update on equine tapeworms presentation notes. Louisiana Veterinary Medical Association Equine Committee. September 21-23, 2003.

19Based on data provided in FDA Freedom of Information summaries.

20Based on data provided on the ZIMECTERIN Gold product label.

21Reinemeyer CR, et al. A prevalence survey of antibodies to Anoplocephala perfoliata in horses from the United States. Proc WAAVP 2003:18.