Rain rot shows up as clusters of raised scabs with tufts of matted hair sticking out from them, often described as looking like a paintbrush. These crusty lesions typically appear along the horse’s back, rump, and other areas where rain runs down the body. When you pull a scab away, it comes off with a clump of hair attached, leaving a bald, raw-looking patch of skin underneath.
What Rain Rot Looks Like Up Close
The earliest sign is usually small raised bumps scattered across the skin. These bumps quickly develop into crusty scabs that trap clusters of hair inside them. The scabs feel rough and lumpy under your hand when you run it along your horse’s coat, and the hair around them looks matted or stuck together. In horses with long winter coats, the matting is especially obvious, and you may notice yellow-green pus beneath the crusts. In horses with short summer coats, the matting is less dramatic, and the lesions look more like patchy, flaky spots.
The hallmark feature is what happens when a scab is removed. Each crust pulls away with a plug of hair embedded in it, leaving behind a small bald patch. The exposed skin is typically pink, moist, and irritated. These bald spots can range from the size of a dime to much larger areas where multiple lesions have merged together. In chronic or untreated cases, the scabs and crusts spread over large portions of the body, particularly along the topline.
Where It Typically Appears
Rain rot follows water. The lesions concentrate on the parts of the horse that stay wettest: the back, croup, and hindquarters where rain pools and runs off. You’ll also see it along the neck and shoulders. In chronic infections, it can spread across a large portion of the body. Horses that wear blankets or saddle pads are sometimes affected in areas where sweat gets trapped under equipment, since moisture against the skin is the key trigger regardless of its source.
How It Progresses Over Time
Rain rot doesn’t appear all at once. It starts with small, barely noticeable bumps that you’re more likely to feel than see. Within days, those bumps develop into the characteristic scabs with matted hair tufts. If conditions stay wet, new lesions continue to form while existing ones grow larger and merge. A case that starts as a few scattered bumps on the back can eventually cover the entire topline in a thick layer of crusts. The hair loss becomes more extensive as the infection spreads, and the horse may become sensitive to touch in the affected areas.
Once the skin dries out and treatment begins, the progression reverses. Scabs loosen and fall off, the pink skin underneath begins to heal, and new hair slowly grows in. The bald patches can take several weeks to fully fill back in with hair, so a horse in the recovery phase will have a patchy, uneven coat for a while even after the infection itself has cleared.
What Causes the Lesions
Rain rot is caused by a bacterium called Dermatophilus congolensis that lives only in the outermost living layer of the skin. Under wet conditions, rain and humidity break down the skin’s protective barrier, creating tiny openings where the bacteria can take hold. The bacterium produces an infectious form called zoospores that are released whenever the lesions get wet, which is why the condition spreads so easily during prolonged rainy weather.
Sweat under blankets or saddle pads and hosing off after exercise are common ways the skin stays moist enough to support the infection. A horse with a compromised immune system or pre-existing skin issues is more susceptible. The bacteria can also spread between horses through shared tack, blankets, and grooming tools, so an outbreak in one horse sometimes moves through an entire barn.
Rain Rot vs. Ringworm
These two conditions are easy to confuse because both cause crusty, hairless patches on the skin. The differences become clear when you look closely. Rain rot produces thick, lumpy scabs with hair tufts embedded in them. Ringworm produces thinner crusts over small bald spots that sometimes form a circular or ring-shaped pattern. Horses with ringworm tend to be itchy, rubbing the affected areas against fences and stall walls. Rain rot is more painful than itchy, and horses typically flinch or pull away when you touch the lesions rather than seeking out something to scratch against.
Location can also help distinguish them. Rain rot favors the topline and areas exposed to rain. Ringworm shows up anywhere, including the face, girth area, and legs, and it often appears where tack contacts the skin. If you’re unsure which you’re dealing with, the scab test is a quick clue: rain rot scabs pull away with a distinctive clump of hair, while ringworm crusts tend to flake off more cleanly.
Managing and Clearing It Up
The single most important step is getting the horse’s skin dry and keeping it that way. Move the horse to shelter during prolonged rain, remove wet blankets promptly, and make sure the coat has a chance to dry completely after bathing or sweating. Without moisture, the bacteria can’t survive or spread.
Gently removing the scabs helps the skin heal, though this can be uncomfortable for the horse. Soaking the affected areas with an antimicrobial shampoo softens the crusts and makes removal easier. After the scabs come off, the exposed skin needs to stay clean and dry. Most mild to moderate cases resolve within a few weeks once the environment changes and the skin gets proper care. Severe or widespread infections may need additional treatment from a veterinarian, especially if the horse’s immune system is struggling to fight the bacteria on its own.
Preventing reinfection means addressing the conditions that caused it. Clean and disinfect any shared grooming tools, blankets, and tack. During wet seasons, check your horse’s back and topline regularly by running your hands along the coat to feel for the telltale raised bumps before they develop into full-blown lesions. Catching it early, when there are just a few small bumps, makes it far easier to manage than waiting until the crusts have spread across the back.