Horse Health

Pastern Dermatitis

September 12, 2013

This horse-health issue goes by many names and has many causes.

Scratches, also known as mud fever or mud rash, is the mildest and most prevalent form of pastern dermatitis. Journal photo.

Scratches, also known as mud fever or mud rash, is the mildest and most prevalent form of pastern dermatitis. Journal photo.

By Dr. Thomas R. Lenz in The American Quarter Horse Journal

Wet weather during the winter and spring combined with longer hair that traps moisture and dirt on most horses’ lower legs are ideal conditions for pastern dermatitis.

Pastern dermatitis is not a single disease, but a skin reaction with a variety of causes. Most cases are due to bacterial infection, but other causes include irritation from caustic substances, mites, fungal infection, allergies and photosensitization related to exposure to clover pastures or toxic weeds. Dermatitis due to photosensitization only affects the white hair and pink skin areas on the horse’s leg and is easily differentiated from other causes of the disease. Some draft breeds (Clydesdales and Shires) suffer from immune-mediated problems that have a genetic component that predisposes them to the condition, but this form of the disease is extremely rare in light breeds such as the American Quarter Horse.

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There are three different presentations for the condition. The mildest and most prevalent form (commonly known as scratches, mud fever or mud rash) begins as redness, hair loss and scaling along the back of the pastern and heels. The lesions can itch or be painful, so affected horses are often seen stomping their feet or chewing on the back of their pasterns or heels.

A more severe form of pastern dermatitis (commonly called grease heel or dew poisoning) is characterized by redness, hair loss and oozing of serum that produces thick crusts that are extremely painful to the touch and may cause lameness.

The third form of pastern dermatitis is the chronic form, aka “grapes,” which is characterized by the development of hard, cornified granulation tissue on the back of the pastern or heels and may eventually progress up the leg.

If your horse develops pastern dermatitis, a detailed history is important in aiding your veterinarian in determining the specific cause: horse’s age at onset, month the problem was first noted, housing conditions and whether the condition has been seasonal or present in other horses on the property.

Treatment begins with clipping the hair from the affected area to decrease moisture retention, removing crusts and treatment with topical antibacterial and antifungal shampoos. Because the lesions are extremely painful, the affected legs should be soaked to facilitate removal of the crusts. Soaking and medicated shampoo treatment is usually done daily for seven to 10 days or until all crusts as well as pain and swelling are gone. Providing a dry, stable environment is extremely important. Some dermatologists recommend using a padded, water-repellent bandage, changed every one to two days, to keep medications in place and the leg dry. In severe cases, antibiotics may be indicated, as well as steroids to control infection and reduce inflammation.

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In the rare cases of pastern dermatitis due to mange mites or parasite larvae embedded within the horse’s skin, topical organophosphates and/or the appropriate anthelmintic should be administered.

Environmental modifications can prevent recurrence of the condition:

•    Avoid chronic exposure to deep mud and/or standing water
•    Keep horses in clean, dry stalls during wet weather
•    Avoid turnout before the morning dew has dried
•    Find an alternative source of bedding if a skin allergy is suspected
•    Clip excessive hair over the pasterns to decrease moisture retention

The prognosis for pastern dermatitis depends on identification of the underlying cause and early treatment. Recurrence is common, especially if the underlying cause is not addressed.