Can Horses be Allergic to Stress?

Jack is a sensitive soul. The sweet Quarter Horse gelding belongs to our digital managing editor, Michelle Anderson, who has regaled our team over the years with tales about his idiosyncracies—many of which center around his allergies. When juniper tree pollen is airborne, he breaks out in hives. If his stablemates are being silly and aggravating him, it has the same result.

A few weeks ago Michelle posted a video of Jack and one of his pasturemates in the snow. Jack was intent on getting in a good roll, whereas the mare was doing gleeful wheelies around him. After rolling he retreated to the run-in shed, then hesitated quizzically before darting out playfully to drop and roll again. All the while the mare whirled around him. I commented that Jack didn’t know whether to join in on  the fun or break into hives. Michelle commented to me later that he did the latter.

This month freelance contributor Lucile Vigouroux delves into equine allergies, why they happen, and what we can do about them. I was curious about Jack’s seemingly allergic response to stress, so I reached out to one of Lucile’s sources, Dr. Samuel White of Nottingham Trent University, in the U.K.

He explained that in human and animal research models, stressed individuals’ bodies release a variety of hormones and chemicals, including histamine. We know—if not from biology class, from allergy medication commercials—that histamine is responsible for allergy symptoms. “So, although stress does not result in allergies,” he says, “it can heighten the response by increasing histamine levels systemically.”

Can Horses Be Allergic to Stress?

He suggested if Jack is very prone to hives (which he is), histamine levels released through stress could be enough to lead to this response. And if Jack tends to stress in general— which he does, says Michelle, if separated from friends or around someone new—his immune system might be mildly altered. White referred to studies in swine showing that housing space allowance significantly influences proteins called proinflammatory cytokines, resulting in inflammation.

“Similarly, secretory immunoglobulin A (sIgA) is secreted by all mucosal surfaces, playing a key role in disease prevention,” White added, and “reflects the functional status of the mucosal immune system.” Scientists have shown that physical and psychological stress highly influence IgA concentrations in a range of animals, a decrease he said would also make it more likely for an allergic reaction to occur.

Put simply, Jack is primed to release histamine at a moment’s notice.

I was lucky enough to meet Jack in 2018. As I approached his paddock, he eyeballed me skeptically, so I offered treats and scratches. Later we rode in the Oregon Badlands Wilderness Area and on trails at the foot of Mt. Bachelor. I’m happy to report we completed the rides juniper-pollen- and—despite some snowmelt-fueled mosquito hatches—hive-free.

Editor’s note: This column appears in the May 2021 issue of The Horse: Your Guide to Equine Health Care.