Controlling Parasites in Horses, Without Contributing to Resistance

By Dr. Kenton Morgan, managing veterinarian, Equine Technical Services, Zoetis

For horse owners in parasite-prone areas of the country such as the Southeast, their deworming program could be teetering on the fence of responsibly controlling parasite burden or unintentionally contributing to parasite resistance.

Within temperate environmental conditions that are key for parasite survival, horse owners may be tempted to deworm every few months in an attempt to reduce their horse’s parasite risks; however, by deworming every few months, horse owners can inadvertently do more harm than good — increasing the horse’s risk for parasite resistance. When parasites are overexposed to certain active ingredients, they can become resistant, leaving fewer effective treatment options.

To determine prevalence of anthelmintic (deworming treatment) resistance, a 2004 study examined 786 horses on 44 farms across Florida, Georgia, South Carolina, Kentucky and Louisiana. Fecal egg count reduction tests were conducted, and results demonstrated farms with anthelmintic-resistant cyathostomes (encysted small strongyles) was highest at 97.7% for fenbendazole (Panacur®, Safe-Guard®). The prevalence of resistance found in the study is higher than previously reported, which suggests a growing problem of parasite resistance.1  

The Gluck Equine Research Center evaluated the safety of moxidectin, the active ingredient in Quest® and Quest® Plus Gel, as well as other products. Examining inflammatory reaction within the intestinal walls, the studies suggest moxidectin presents very little inflammatory response in the large intestinal walls, thus offering a safer choice for treatment of horses with large burden of encysted small strongyles. “Similarly, moxidectin may qualify as drug of choice for treatment of larval cyathostominosis, and could possibly replace the long-term treatment regimes involving several alternations between fenbendazole and ivermectin that were previously suggested. However, further studies are needed to verify these assumptions fully,” according to the report.2 

With just a single dose, Quest and Quest Plus Gel continue to demonstrate safety and efficacy in treating and controlling encysted small strongyles, bots and roundworms. 3,*

Because every horse is unique, I encourage horse owners to work with their veterinarian to perform an annual fecal egg count (FEC) test to determine their horse’s parasite levels and efficacy of treatment. The American Association of Equine Practitioners (AAEP) considers an FEC test to be the best assessment of parasite burden to identify the frequency of treatment needed. Once a baseline is established, horse owners can work with their veterinarian to develop an Individualized Deworming™ plan tailored to their horse’s needs.

The following factors contribute to equine parasite risk and, with an FEC test, can be used to determine your horse’s individualized deworming plan:

  • Environmental conditions: In rainy, temperate areas, horses may experience a higher incidence for parasites, as such conditions are an ideal  environmental support of the parasite, lending itself to more parasite life cycles per season.
  • Horse’s age: The AAEP recommends foals receive their first deworming at 2 to 3 months of age and be redosed every three months. Strongid® Paste is safe for foals and pregnant mares, and helps to effectively remove roundworms, the No. 1 parasite concern for foals.4
  • Manure management: For minimal parasite risk, horses can be stabled in a paddock with little to no grass or in a frequently cleaned stall. Removal of manure from the premises can reduce the number of parasite eggs and larvae within the environment.

Yearlings and 2-year-olds should continue to be treated as “high” shedders (those with an FEC of 500 or above) and receive three to four yearly treatments. Horse owners can determine deworming needs for mature horses with an FEC test, conducted by the horse’s veterinarian, to identify the frequency of deworming treatment needed to target small strongyles, tapeworms, bots and spirurid nematodes, which are responsible for causing summer sores (Habronema spp. and Draschia spp).

For additional information and resources, visit

Do not use Quest Gel or Quest Plus Gel in foals less than 6 months of age or in sick, debilitated and underweight horses. Do not use in other animal species, as severe adverse reactions, including fatalities in dogs, may result.

About Zoetis
Zoetis is the leading animal health company, dedicated to supporting its customers and their businesses. Building on more than 60 years of experience in animal health, Zoetis discovers, develops, manufactures and markets veterinary vaccines and medicines, complemented by diagnostic products, genetic tests, biodevices and a range of services. Zoetis serves veterinarians, livestock producers and people who raise and care for farm and companion animals with sales of its products in more than 100 countries. In 2016, the company generated annual revenue of $4.9 billion with approximately 9,000 employees. For more information, visit

For more information, contact:

Kassi Hoxmeier
Bader Rutter

1 Kaplan RM, et all. Prevalence of anthelmintic resistant cyathostomes on horse farms. JAVMA. 2004:225(6);903- 910. 
2 Betancourt A, Lyons ET, Horohov DW. Characterization of the inflammatory cytokine response to anthelmintic treatment in ponies. Equine Veterinary Journal. 2014.
3 Mason ME, Voris ND, Ortis HA, et al. Comparison of a single dose of moxidectin and a five-day course of fenbendazole to reduce and suppress cyathostomin fecal egg counts in a herd of embryo transfer-recipient mares. J Am Vet Med Assoc. 2014;245(8):944-951.
4 American Association of Equine Practitioners. AAEP Parasite Control Guidelines Accessed August 15, 2017.
* This study compared Quest (moxidectin) Gel with Panacur Powerpac (fenbendazole).



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