Published October, 2011

Regardless of the method of choice, most equestrians have deworming down to a science. For years, horse owners have been told to control the amount of worms in their horses' bodies to keep them feeling and performing their best. But what effect do worms really have on equine performance? A team of researchers recently found that high strongyle egg counts in a population of Standardbred trotters didn't have as much of an association with racing performance as once thought.

"Subclinical parasitic infections are often assumed, by horse owners as well as veterinarians, to affect horses in various ways," explained Martin K. Nielsen, DVM, PhD, assistant professor in equine parasitology at the University of Kentucky's M.H. Gluck Equine Research Center. "Retarded growth and ill-thrift can be observed in young horses with heavy parasite burdens, but no observational studies have systematically investigated this in the field setting.

"Similarly, it appears to be widely assumed that parasites can cause poor performance in competition horses, but this is not supported by any published study," he continued. "In the equine racing in industry in particular, there is a tradition for frequent prophylactic anthelmintic treatments as a part of a general health care plan."

In their study Nielsen and colleagues employed 213 Standardbred trotters, ranging in age from two to six years, at six different Danish tracks. The horses (mares, stallions, and geldings) were cared for by 21 different licensed trainers and had been in training for at least three months. All of the participating trainers dewormed their horses three to four times annually, but none of the study horses had received anthelmintic treatment in at least 12 weeks prior to fecal samples being collected from their stalls. The team also reviewed all study horses' race records.

Upon reviewing the results of the study, the team found that:

• Strongyle fecal egg counts ranged from 0-3,500 eggs per gram, with the average count being 319 eggs per gram;
• There were no significant effects of gender, age, or trainer;
• A statistically significant association was found between finishing position and fecal egg counts; and
• Surprisingly, the horses that finished in the top three positions were more likely to have a higher fecal egg count.

"A large proportion of Danish Standardbred trotters were included in the study, and their race results were not negatively affected by strongyle fecal egg count levels," Nielsen concluded.

He cautioned, however, that it's important to consider that the horses in the study were all in good health: "Horses showing clinical signs of any disease would probably not be racing, so it can be argued that horses affected by their parasite burdens never became part of the data set."

Additionally, Nielsen noted that further research is needed to identify the role strongyles play in athletic horses' performance so a suitable anthelmintic treatment plan can be developed.

It is advisable to discuss any adjustment in deworming protocol with a veterinarian before implementing the change.

The study, "Strongyle egg counts in Standardbred trotters: Are they associated with race performance?" was published in the August issue of the Equine Veterinary Journal. The abstract is available online.

Erica Larson, News Editor at The Horse