My horse had rain rot this summer, not terrible, mostly under his chest and near his legs only. I noticed a tiny little bump, hard, doesn’t seem to be attached to anything (no root) looks like a rain rot scar. Is this possible?
Rain rot actually tends to affect the back and sides of the horse, where moisture due to rain and sweat tend to keep the skin chronically damp and less resistant to fungal and bacterial invasion. Your horse probably has another form of skin infection, but either way it is unlikely that it is related to the nodule you have recently noticed. This probably a small collagen granuloma (often called nodular necrobiosis). Skin irritation from a bug bite is one cause, and they often also occur under pressure points- mainly in the saddle and girth area. The compression and friction that occurs from tack can rupture the hair follicle, releasing collagen and stimulating a local inflammatory response. I often compare it to an ingrown hair. This causes a palpable lump, which over time may calcify. If the bump is in an area that is becoming rubbed or irritated I usually have very good results by simply injecting a tiny bit of triamcinolone (a type of light steroid) into the lump, which then goes away after a few days. There are some horses that will develop these lumps repeatedly but they always respond very well to injection if you catch them before significant scar tissue has formed. If the nodule is not growing in size, not painful or ulcerated, and does not get rubber by tack, I would simply keep an eye on it for now.
Melissa McKee DVM