Where mud fever shows up first
Mud fever — pastern dermatitis — is an inflammation of the skin of the lower leg, most often at the back of the pastern and the heels, and more commonly on the hind legs than the front. It begins where wet, muddy conditions soften the skin and bacteria get in through tiny breaks. The earliest sign is usually found by hand rather than eye: small, crusty scabs clustered in the heel and pastern, with the hair over them standing up in rough tufts. Pink or lightly pigmented skin under white socks shows the redness earliest and tends to suffer worst, which is why white-legged horses are the classic cases, but any leg standing in mud day after day can develop it.
What the affected leg looks and feels like
As it takes hold, the scabs thicken and multiply, the skin beneath them becomes sore, moist and weeping, and hair mats into the crusts or comes away with them, leaving raw patches. The area is tender — many horses flinch, stamp or snatch the leg away when the scabs are touched or the leg is washed — and in more established cases the whole pastern and fetlock swell, feel warm, and fill in a way that hides the normal contours of the leg. Cracks can open across the back of the pastern where the skin flexes, and those cracks weep and crust over again. Feathered breeds present a particular challenge because dense feather hides the early stages completely; with cobs and native types you have to part the feather and feel for scabs with your fingertips, not just look.
When it stops being a cosmetic problem
Mild mud fever is a nuisance; established mud fever is a welfare issue. The point at which it needs more than field management is when the horse is lame or walking stiffly on the leg, the swelling is marked or climbing, the skin smells infected, or the patches are spreading despite being kept clean and dry. Swelling that persists can become permanent thickening of the leg if the cycle repeats winter after winter, and deep cracks at the pastern are genuinely painful at every step. A vet should see legs with significant swelling, lameness, or sores that fail to improve, because other skin conditions imitate mud fever and some need quite different treatment.
The conditions behind it, noted for prevention
Mud fever is as much a field report as a skin problem: it tells you the horse's skin has been wet and mud-caked for too long. Note which fields, gateways and weeks of weather produced the case — legs that flare every February on the same wet paddock are a management pattern, not bad luck. Prevention runs through dry standing areas, rotating or resting the worst gateways, and resisting the urge to wash legs aggressively every day, since constant soaking itself softens and damages the skin. Keep dated photographs of the legs, your treatment notes and any vet or farrier comments in KinsLe's Pet Passport, so the record shows which legs flare, in which conditions, and what settled them, year after year.