Karla S. Rugh, DVM, Ph.D.
Rain rot (dermatophilosis)
What it is: A skin infection caused by Dermatophilus congolensis, a fungus-like bacteria present on the skin of many horses. The organism only causes problems under certain conditions, such as excessive moisture (from rain or high humidity), a heavy coat, high temperatures and damage to the skin, usually by flies and ticks. Some horses seem to be more susceptible to Dermatophilosis than others.
Symptoms: Patches of crusty sores with matted hair and hair loss, often on the back but also on the head, girth and other areas. Detached crusts have small tufts of hair attached; the sores may ooze serum or pus. If left undisturbed, the lesions aren’t itchy or painful, but crust removal or friction from tack and riding can cause discomfort.
What to do: This is not an emergency. Many cases are somewhat self-limiting, clearing up spontaneously in about three weeks, especially if the weather becomes drier and the horse sheds his winter coat. To speed healing, lather the area with Betadine scrub and gently remove the crusts. If your horse protests too much because it hurts, you may have to do this over the course of a few days. Once you’ve removed all the crusts, apply an antiseptic solution, such as Betadine or Novalsan, once daily for five days. Isolating an infected horse will help prevent the spread of rain rot. Don’t share the horse’s tack or grooming tools with other horses, and regularly disinfect or wash brushes, blankets and other items used on the infected horse.
Outlook: Rain rot usually responds well to treatment, but may return if the conditions described above recur.
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This article originally appeared in the 2012 issue of Horses USA. Click here to purchase the most recent issue.