Question: My 6-year-old Paint mare has dime-size bumps on either side of her back and a marble-size one on the right girth area. These bumps have become more noticeable in the last several months but do not seem to bother her when gently pushed or squeezed. The only time my mare seems to experience discomfort is when I am saddling her. She reacts negatively when I approach with my tack—ears pinned and teeth bared—and always tenses up and turns to bite at me when I am tightening the cinch, even though I'm careful to do it incrementally to avoid pinching. The bumps also seem to be more tender after riding.
My veterinarian said that it is possible the bumps are old reactions to insect bites that may have flared up. It worries me to think that riding might be causing my horse pain. What can these bumps be and what can I do to treat them?
Answer: Your veterinarian is suggesting that your horse may have eosinophilic granuloma—a hypersensitivity disorder of the skin that is also called nodular collagenolytic granuloma, acute collagen necrosis or nodular necrobiosis. Variations of this disorder also occur in dogs and cats as well as people.
Eosinophilic granuloma is the most common nontumorous nodular skin disease in horses, and it is characterized by distinct bumps caused by the breakdown of collagen in the middle layer of the skin. Single or multiple nodules may occur, most commonly on the back, withers and neck. They are round, elevated, firm and well-defined, and the overlying skin and hair coat appear normal. Usually, they are not painful or itchy, unless they are irritated by tack or rubbing by the horse. The condition can occur in horses of any age, breed or gender, and diagnosis is based on history, physical examination and skin biopsy.
Why these nodules develop is unknown, but there are many theories. Because the lesions most often begin in spring and summer, insect hypersensitivity has been suspected—in other words, the horse may be having an allergic reaction to insect saliva. Other possible causes include any combination of trauma, injections with silicone-coated needles, body clipping and irritation from saddle pressure.
The most conservative treatment option is to leave the nodules alone since they generally don't bother the horse. I would suggest applying insect repellent and keeping your horse covered with a scrim sheet to deter biting insects. However, if the bumps continue to enlarge or if they get in the way of tack, they can be injected with a steroid, such as cortisone, to shrink them. These injections are typically repeated at two-week intervals. Lesions that are still present after three injections will probably require surgical removal.
These nodules may or may not be why your mare resents being saddled. Eosinophilic granuloma does not usually cause pain unless the bumps are irritated by, for example, a saddle or pad—which may be the case here, since you mention that the ones on your mare are tender after she is ridden. I might suggest trying different equipment to see if that makes a difference.
But overall, I'd recommend that you have your mare's lesions biopsied—especially the one on the girth area because it's in an atypical location and may be unrelated to those on her back. Only by determining exactly what the nodules are will your veterinarian be able to prescribe the most appropriate treatment. Then, if your mare continues to resent being saddled, I'd suggest you investigate other possible causes of back pain.
Steve Adair, MS, DVM, DACVS, DACVSMR
University of Tennessee-Knoxville