Deworming horses

Veterinarians know that strongyle parasites and the diseases they can cause are issues for grazing horses worldwide. However, there’s been little research on their precise prevalence and geographic distribution in the U.S.

To bridge that knowledge gap, researchers recently examined the results from the National Animal Health Monitoring Systems Equine 2015 study to determine strongyle parasite egg shedding patterns in the U.S. equine population.

In the study, horse owners from 28 states completed a questionnaire about parasite management practices at their facilities. Those responses, paired with fecal egg count (FECs) results, revealed that:

  • Most adult equids in the U.S. were low or moderate strongyle egg shedders;
  • 30% of the horse population sheds 80% of the eggs;
  • Horses in the Southeast had the greatest odds of having strongyle eggs present on a FEC;
  • Horses in the Western U.S. (Arizona, California, Colorado, Montana, Oregon, and Wyoming) had had significantly lower strongyle prevalence than did horses residing in the rest of the country;
  • Season impacted population levels—horses across the country had higher fecal egg counts in summer and fall;
  • Horses with daily pasture access in the previous 30 days had higher odds of egg presence and greater median egg counts;
  • Time since last deworming and the type of dewormer used was significantly associated with strongyle egg prevalence—equids treated with macrocyclic lactone class of drugs had lower odds of strongyle egg presence at 120 days since the last deworming; and
  • Younger horses had higher egg count levels than their older counterparts.

“This is consistent with findings elsewhere in the world,” said researcher Martin Nielsen, DVM, PhD, Dipl. EVPC, ACVM, associate professor of parasitology and Schlaikjer Professor of Equine Infectious Disease at the University of Kentucky’s Maxwell H. Gluck Equine Research Center, in Lexington. “Confirming this information helps us design good parasite control strategies to maintain effectiveness and avoid further parasite resistance.”

Specifically, the data support recommendations that horses require individualized deworming treatment plans.

“For example, we know that equine parasite control has to be approached very differently in the western part of the country,” he said. “Outside this region, it is also very useful to know that parasite infection patterns are very uniform.”

Ultimately, Nielsen said, recognizing the variance in strongyle populations can help us refine parasite control recommendations depending on region, pasture access, and age.

The study, “Risk factors associated with strongylid egg count prevalence and abundance in the United States Equine Population,” was published in Veterinary Parasitology.