Mud Fever, also known as the Scratches, dew poisoning, pastern dermatitis, grease heel, or greasy heel, rain rot, is conflicting but putting them all together concludes that Scratches is dermatitis, or inflammation of the skin, and the most common cause seems to be fungus which can be complicated by a bacterial infection. The fungus lives in organic matter and enters through breaks in the skin when the horse walks through contaminated pastures or muddy, swampy areas. I know that if not treated it can become serious and require a veterinarian and several treatments including antibiotic therapy. I have seen horses have trouble maintaining weight, with swollen legs, and basically looking like death warmed over from severe cases. Sometimes it is contagious as in all the horses get it, and other times it is not.