Horses with allergic diseases such as recurrent airway obstruction (RAO) might also have fewer parasites, according to recent research. A new study suggests that genetics might be the primary explanation for this phenomenon.

By investigating RAO and parasite infestations in a family of horses sharing the same bloodline, scientists were able to determine that evolutionary genes which help ward off parasites might also be responsible for susceptibility to allergies. The study, led by Vincent Gerber, DVM, PhD, Dipl. ACVIM, ECEIM, head ad interim of the equine clinic of the department of veterinary clinical sciences at the University of Bern, focused on 73 direct descendants of a RAO-affected stallion and 73 unrelated pasture mates of these horses.

What they found was that the RAO-affected family members were six times more resistant naturally to intestinal parasites than RAO-negative family members, according to the study. Furthermore, the pasture mates, living in identical environmental situations as the family group, were 17 times more likely to harbor parasites than the RAO-affected family members.

"It seems to be some sort of evolutionary advantage that these horses developed genetically," Gerber said. "But with modern environments, stabling, hay-feeding, frequent deworming, and so forth, the downside of the genes–having hypersensitivities like RAO–may be outweighing the upside