Commonly affecting the lower limbs (especially pasterns and fetlock areas) of a horse, pony or donkey, it can be an extremely painful condition. 

Skin will become cracked and sore, with scabby lesions.  Any equine can be affected, whether they are particularly "feathered" or not.  Horses who have white socks can be more prone, which suggests that in some instances photosensitation may have an effect.  An organism called "Dermatophilus congolensis" is often linked to mud fever.   

Prevention can definitely be better than cure, - checking your horses legs at least once a day, and taking action at the first sign of mud fever can often help the problem deteriorating and leading to lameness.