Rain Rot and Rain Scald: Dermatophilus

Introduction

If your horse is developing scabs that are painful and when removed have hair embedded in them you probably have Dermatophilus. Perhaps the most common cause of infectious skin disease in horses is a bacterial infection caused by Dermatophilus congolensis. Horse men have called this rain rot or rain scald and even sometimes call it skin fungus. It is also among a complex number of infections responsible for scurf, grease heel, and mud fever. This article contains the clinical signs, diagnosis, and treatment of this problem.

Clinical Signs

The seasonal forms are not strictly confined to their time of year. Environmental conditions and condition of the horse will affect the variety seen.

Summer Form

This disease is not always associated with wet conditions and has several forms. At its mildest sometimes called the "summer form", the horse develops dozens of tiny little scabs maybe an eighth of an inch across. They can be felt as small bumps as you run your hand over the affected areas. They are easily scrapped off with your finger nail and will have hair embedded in the scabs. Usually the horse will show obvious discomfort when they are removed. This miliary form develops anywhere on the body and is a cause of cannon scurf so common among show and racing horses.

Winter Form

At its worse, the winter form, Dermatophilus can cause large scabs several inches across that when removed (again the hair over the scab will come off and remain embedded in the scab) leave behind a moist or purulent irritated shallow erosion. These areas are very painful. This is the form written about in books that usually is seen in wet weather on horses receiving little attention.

Other Forms

Other forms include grease heel and cannon scurf. Grease heel may also be mixed infections that include staph and strep. When chronic there may just be open sore or scabs with the hair embedded scabbing around the periphery.

Diagnosis

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