Appearance: scabby crusts that form raised bumps with upright tufts of matted hair. The crusts form on parts of the body that are chronically damp—often along the topline and where rain runs off down the barrel, shoulders or hindquarters, but also on the lower legs or faces of horses who regularly stand in mud or graze tall, wet grass. Over time, the crusts peel off, leaving small, round bare spots; pus may also be seen under newly sloughed scabs.
Causes: Rainrot is a bacterial infection. The causative organism, Dermatophilus congolensis, can reside on the skin without causing trouble, but it multiplies rapidly in a moist environment. If the bacteria find a break in the skin, whether a small wound or insect bite, an active infection can develop. Anything that compromises a horse’s immunity—advanced age, malnutrition, illness—can make him more susceptible to the infection, as can having a heavy winter coat, which tends to trap moisture against the skin.
