A Rash of Questions

Q. We have a Paint mare that is mostly white. In the spring and summer she has a rash-type skin problem around her nose and moving to her shoulders. It starts out looking like what we were told was dew poisoning, but it has spread to the point of bumps, scabs, raw sores, and bleeding. What could be causing this? Are there any preventive measures we can take? What are our treatment options?

Charles, via e-mail

A. It is difficult to answer this question without some more information:

  • How old is the horse?
  • Where in the USA is this horse?
  • Does the "rash" only affect the white areas? Or both the pigmented and the white areas?
  • How many years has this occurred?
  • How long have you owned the horse?
  • If this rash occurs in the spring and summer, by what time of year does it resolve?
  • Does this mare have a different feed or pasture access in the spring and summer versus the fall and winter?
  • What treatments have you tried in the past? Did any of them work? Did any of them make the problem worse?
  • When this occurs, how much does it bother the horse--does she rub affected areas against posts or barn walls, or does she shy away from people touching the affected areas?

In general, spring/summer seasonal problems might be:

  1. Allergies (either to insect bites or pollens)--these are usually quite itchy (the scientific word is pruritic) for the horse.
  2. Photosensitization (brought on by increased exposure to sunlight, as well as the possibility of ingesting photosensitizing agents in the feed--for example, buckwheat, wild carrot, etc.).
  3. Dermatophilosis--a bacterial infection (one of its nicknames is dew poisoning) caused by the organism Dermatophilus congolensis. The bacteria only gain access to the skin if the skin is wet and damaged in some way, such as with mild abrasions. However, this would only be likely if the majority of the rainfall in your region falls in the spring and early summer, and if the mare was kept inside when it did rain (otherwise the infection would be more noticeable on her back). Therefore, she would only be getting her lips wet (and potentially infected) when the rain let up and she was eating wet grass. The bacteria are found worldwide, but they are difficult to culture from the environment.

Depending on the answers, diagnostic tests include a skin biopsy, bacterial or fungal culture of the affected areas, or even a test for allergies. I think a skin biopsy would be the starting point.

Treatments for insect allergies involve using repellents, keeping the horse's area clear of manure, and draining standing water, if possible. Fly predators, which are small wasps that feed on various flies, are also helpful--several companies sell these.

Options for treating pollen allergies are antihistamines, hyposensitization injections (allergy "shots" based on either serum or intradermal allergy testing), or, in severe cases, corticosteroids.

Preventing photosensitization involves keeping the horse out of direct sunlight (at least from 10 a.m. to 4 p.m.) and evaluating pasture for the presence of photosensitizing plants. Antibiotics and topical washes are the treatments for dermatophilosis.

Of course, all my thoughts are based on the rather small amount of information provided. It is entirely possible that with the answers to my questions, other potential diseases might be more likely.

About the Author

Stephen D. White, DVM, Dipl. ACVD

Stephen D. White, DVM, Dipl. ACVD, is a professor in the Department of Medicine and Epidemiology at the University of California, Davis.

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