Parasite Load Not Related to Horse Age, Researchers Say

Contrary to common belief, older horses do not carry higher worm burdens than their younger counterparts, said Oklahoma State University veterinarians.

"Despite the belief that older horses are more susceptible to internal parasitism than younger horses, the effect of advanced age on resistance to parasites has not been adequately studied," explained lead researcher Dianne McFarlane, DVM, PhD, Dipl. ACVIM, from the Department of Physiological Sciences at Oklahoma State's Center for Veterinary health Sciences.

McFarlane and colleagues dewormed 29 healthy horses residing in similar environments. These horses ranged in age from 4 to 35 years, 11 of which were more than 20 years. Fecal egg counts (FECs) of small strongyles were measured before deworming the horses with ivermectin and again at two, four, six, eight, 10, and 12 weeks post-deworming.

"Age did not affect FECs at any point during this study, suggesting that age alone does not mandate designing special parasite control programs."
--Dr. Dianne McFarlane

At each of the time points considered there was no statistically significant relationship between age and FECs, either when all 29 healthy horses were compared or when researchers looked at horses on a single farm.

"Thus, age did not affect FECs at any point during this study, suggesting that age alone does not mandate designing special parasite control programs," said McFarlane.

Since this inaugural study of the effect of age on internal parasitism of horses used only a small number of horses, follow-up study using a larger number of horses with a broader age range and residing on the same pasture is needed.

This study is also interesting as resistance of small strongyles to ivermectin was not noted.

"All horses in the study had a 99% to 100% reduction in FECs for up to four weeks post-deworming," reported McFarlane. "Although we found an excellent response to ivermectin in this study, there is enough evidence from the work of others that development of parasite resistance to anthelmintics is a real risk in equine practice. If we do not use our dewormers in a smart, strategic, manner we could be left with nothing in the arsenal to effectively protect horses from internal parasites. Understanding the factors that increase risk for parasitism in horses will help in designing more rational deworming programs compared to the traditional interval rotational methods that have been used extensively over the past several decades."

The study, "Fecal egg counts after anthelmintic administration to aged horses and horses with pituitary pars intermedia dysfunction," was published in the February 1, 2010 edition of the Journal of the Veterinary Medical Association. The abstract is available for free on PubMed.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.