Question of the Week: Rain Rot
How can I treat my horse's skin condition when it's too cold to give her a bath?
Q: I have two horses that live out in a pasture together and one has recently developed bald patches in her winter coat. I think this may be rain rot as she often doesn't go in the barn when it rains. It is much too cold to bathe her right now. Is there another option to treat this?
A: Rain rot, also known as rain scald or dermatophilosis, is a bacterial skin disease caused by Dermatophilus congolensis. Chronic moisture is the key ingredient for this bacteria to proliferate, causing a localized mild inflammatory reaction just underneath the horse’s skin, which is not usually enough to be painful, but enough to disrupt hair growth. The classic sign of rain rot is a “paintbrush” appearance of the coat, caused by small clumps of hair that are slightly raised from the skin. This manifestation of this dermatologic condition is most commonly seen over the top of the rump and along the back of the horse, but can also be seen along the face and neck and down the legs. As the disease progresses, these tufts of hair fall out, leaving bald patches on the horse’s skin.
Treatment for mild rain rot is usually accomplished with topical therapy alone. To treat this condition, you actually would not want to bathe the horse, but rather do spot treatments on only the affected areas. Using an antibacterial shampoo such as a dilute chlorhexidine, iodine, or benzoyl peroxide solution on a sponge, gently soak the affected areas and remove the crusts. Then dry these areas completely with a clean towel. This should be done once or twice daily for about a week. This treatment accomplishes two important things: firstly killing the proliferating bacteria that are invading the skin, and secondly creating a dry environment to prevent bacteria from re-populating the area. If your horse has a particularly thick winter coat, the treatment will work best if you first clip the excessive coat away and then apply the soak. This will help when drying the affected areas as well.
In severely affected horses, systemic antibiotics may be warranted, as secondary bacterial infections may erupt. In cases such as these, or in horses with particularly sensitive skin, sedation may be required to thoroughly scrub and cleanse the affected areas.
As far as keeping your mare dry, if she won’t go into the barn, this may prove difficult. You may need to investigate reasons why she won’t go in the barn, taking into consideration situations where the other horse is bullying her, or if she’s fearful of something inside the barn, etc. You might consider body clipping her once or twice over the winter and putting a coat on her to keep her dry. If body clipping is not an option, then it’s not recommended to put a coat on her since a heavy winter hair coat under a man-made coat will create more sweat and heat - prime conditions for bacteria to proliferate.
Other causes of patchy bald spots, or alopecia, on your horse include bacterial folliculitis and ringworm. Folliculitis, or inflammation of the hair follicle, is most commonly caused by bacteria of the Staphylococcus genus, which usually act as opportunistic pathogens that take advantage of a weakened immune system, skin trauma, or generally compromised, unhealthy skin tissue. Ringworm, or dermatophytosis, is a superficial fungal infection of the skin due to Trichophyton or Microsporum species of dermatophytes. Highly contagious between horses and people and very annoying but not life threatening, this skin condition usually manifests as irregularly round, hairless lesions typically around the face, legs, girth, and shoulders. Veterinary evaluation of your horse will confirm your suspicions of rain rot or perhaps turn up something else. Many times, the common equine skin conditions mentioned above are diagnosed merely by their clinical presentations. Occasionally, however, further diagnostics such as fungal and bacterial cultures, cytology, and skin biopsies are warranted.
-- Anna O'Brien, DVM
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Question of the Week: Rain Rot