Understanding Pastern Dermatitis, aka Dew Poisoning

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We know it by many names, including dew poisoning, scratches and greasy heel. But whatever you call it, pastern dermatitis is a problem to horses in some parts of the country. Dermatitis is simply an irritation to the skin caused by an allergic reaction or some external contaminant. Understanding why dermatitis happens in the pastern region and how a veterinarian might go about diagnosing the cause in order to treat it more effectively will help you resolve the issue quicker and more effectively. The following information from the University of Kentucky College of Agriculture's Livestock Disease Diagnostic Center comes courtesy of Lynne Cassone, DVM, a veterinary pathologist.

Inflammatory conditions of the pastern will be appearing with increasing frequency during the summer season. But as a diagnosis, “pastern dermatitis” leaves much to be desired. When this alone is noted on a biopsy report, it leaves the clinician and client with little useful knowledge.

Other names for this condition, with slightly varying clinical appearances, include scratches, mud fever, grease heel, dew poisoning, grapes, canker, and verrucous pododermatitis. But again, these colorful descriptors do not indicate the root of the problem. For that, pathologists rely on a series of modifiers describing the more subtle histologic (microscopic) variations on the theme of “dermatitis,” and, just as importantly, a detailed history.

The skin overlying the pastern is not inherently different from anywhere else on the body, (with the exception of having longer hair, if not clipped). As such, the way it reacts to insult is rather ordinary: erythema (redness), erosions, ulcerations and/or serous exudates in the acute phase of inflammation. Purulent (pus-like) exudate and granulation tissue form with a more prolonged insult, and fibrosis (scarring), epidermal thickening, and hyperkeratosis (scale, cornification) in chronic cases. The pastern, however, is uniquely qualified to develop severe, persistent, refractory skin disease because of its location and exposure to dirt, fecal material, persistent moisture, chemical irritants, ultraviolet rays and direct trauma (plant stubble, rocks, over­reaching/interfering). Draft breeds are more susceptible to pastern skin disease due to heavy feathering and possibly genetic factors.

Veterinarians routinely utilize in-house cytology, skin scrapings, direct microscopic examination of hairs, fungal cultures and complete blood counts in their diagnostic work-ups for complex skin cases such as pastern dermatitis. In acutely severe cases or cases that are unresponsive to treatment, biopsies offer a direct view of the disease process and can lead to a definitive diagnosis. However, histopathology of inflammatory lesions is only useful when interpreted in light of the clinical history and gross appearance of the lesions. In order to maximize the utility of a biopsy, it is extremely important to take adequately large (6 mm punch) and multiple specimens of active lesions. Providing a detailed description, clinical history, overview of the animal’s environment, diet, vaccination history and overall herd health are also critical as well as good-quality digital photographs.

While the treatment will obviously largely de­pend on the diagnosis, some basic principles are:

  • Keep the area clean and dry.
  • Monitor for and manage any sign of exuberant granulation tissue (proud flesh).
  • Check the other feet and legs regularly to moni­tor any disease spread.
  • If the skin appears to be dry and cracking, oil-based emollients or antimicrobial ointments can help reduce fissuring and secondary infections.

Pastern dermatitis can be a debilitating condition for horses. Correct diagnosis early in the course of disease can greatly expedite its resolution, and most horses will return to function.