Mud fever, also known as scratches or pastern dermatitis, is a group of diseases of horses causing irritation and dermatitis in the lower limbs of horses. Often caused by a mixture of bacteria, mud fever can also be caused by fungal organisms. Photosensitization, chorioptic mange mites, contact dermatitis and other conditions also contribute to some cases. This condition is also known as dew poisoning, grease heel, or greasy heel.
Mud fever affects most horses and ponies during winter and early spring, resulting in painful sores and scabs, which in severe cases can make a horse lame. Mud fever most commonly affects the pastern and heel area.