Delaying Dewormer Resistance: Advice Offered in Study

Nonchemical parasite control strategies, fecal egg count monitoring, and controlled quarantine treatments all can delay the development of anthelmintic resistance (AR) in horses.

These were the recommendations made by a group of German researchers from the University of Veterinary Medicine in Bünteweg, Hannover, after investigating parasite control measures on 76 farms. Participating farms all had more than 10 horses each available for fecal sampling and pasturing, and they were willing to withhold deworming for at least eight weeks prior to commencement of the study.

In total, 2,000 foals, yearlings, and mature horses from riding farms, stud farms, and small holdings were investigated. The authors reported that:

  • Yearlings on stud farms had a twofold higher risk of being positive for strongyles;
  • Anthelmintic drug administration three times per year was associated with reduced strongyle rates in adult horses, but not foals or yearlings;
  • Foals residing on farms that fertilized pastures with horse manure had a significantly greater risk of being positive for roundworms (Parascaris equorum); and
  • Yearlings residing on stud farms more commonly had incomplete fecal egg count reductions after deworming compared to yearlings on other farm types.

According to the study authors, "To delay the development of AR, sound management practices are needed. These should include optimized parasite control strategies employing FEC-monitoring, control of treatment efficacy with FEC (fecal egg count) reduction test on a regular basis, aiming to reduce treatment frequencies supported by improved pasture and stable hygiene, as well as parasite control orientated pasture and farm management."

Average treatment frequencies (per year) were 4.52 in foals, 3.26 in yearlings, and 2.72 in mature horses.

Parasite control in horses continues to be an important area of equine research as anthelmintic resistance is increasing worldwide. Widespread resistance of small strongyles against benzimidazole-type drugs (e.g., fenbendazole) and P. equorum resistance has been reported for macrocyclic lactones (e.g., ivermectin), particularly on stud farms.

The study, "Endoparasite control management on horse farms -- lessons from worm prevalence and questionnaire data," is scheduled to be published in an upcoming edition of the Equine Veterinary Journal. The abstract is currently available online.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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