Baffling Bumps -- Collagen Granuloma

EQUUS Medical Editor Matthew Mackay-Smith addresses a reader's concerns about mysterious back bumps--Collagen Granuloma.
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EQUUS Medical Editor Matthew Mackay-Smith addresses a reader's concerns about mysterious back bumps--Collagen Granuloma.

Question:I purchased a 7-year-old Thoroughbred gelding about two months ago. I plan to event him to preliminary level. He was given a prepurchase exam by a veterinarian, but has a few bumps under the skin the saddle area. The veterinarian said that it was nothing to worry about. Since we've owned him, the bumps have become more numerous and blanketing seems to make them larger. Most range in size from a grain of rice to a pea, but the largest ones are about the size of a thumbnail. They have normal hair covering and do not seem to affect him adversely. Another veterinarian said they were something called nodular skin disease or collagen skin disease. What causes this condition? Will it get worse? Can it be cured?

Matthew Mackay-Smith

Matthew Mackay-Smith

Answer: Your horse's condition, collagen granuloma, is caused by insect bites. In some horses, the immune system's reaction to the saliva of particular insects (mosquitoes are most often the culprits) causes a small number of collagen-producing cells to die. Saddle or blanket pressure can then provoke a slow, sterile inflammatory reaction that generates small nodules of "granulation," hence the term collagen granuloma. The bumps are painless and harmless to the horse, but can be very annoying to the horse's owner.

Such bumps rarely go away on their own. In fact, they sometimes enlarge or multiply until local hair loss and tenderness results. The injection of tiny doses of steroids into newly formed nodules may reduce inflammation and flatten them out. In chronic cases, however, I have found that the best results are achieved by "decapitating" or "saucering" the nodules under local anesthesia. This is essentially the excision of the top of the bump to allow the body to reject the tiny nidus of dead collagen. The scab that forms over the area incorporates the offending particle and lifts it out as the skin heals. Talk to your veterinarian about this procedure if you decide the bumps need to go.