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.

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