Scratches, or equine pastern dermatitis, is not a disease but, rather, a cutaneous reaction pattern. | Photo: Alexandra Beckstett/The Horse

Scratches, or equine pastern dermatitis (EPD), is not a disease but, rather, a cutaneous reaction pattern. Veterinarians and owners must address the primary, predisposing, and perpetuating causative factors for a successful outcome. Note that treating the predisposing and perpetuating factors is just as important as addressing the primary cause.

Clinical Signs and Pathogenesis

Scratches can affect any breed, but is most prevalent in draft horses due to long pastern hair (“feathers”). It most commonly affects the rear aspect of the hind pasterns and especially nonpigmented skin. Without treatment the lesions can spread to the front of the pastern and fetlock. Clinical signs vary, but initially owners might notice edema (fluid swelling), redness, and scaling, rapidly progressing to oozing, hair matting, and crusting. If the cause is vasculitis (blood vessel wall inflammation), ulcers might form on the skin. Secondary bacterial infection is a common complication and can perpetuate the signs. In chronic cases skin can thicken and fissure due to constant movement and flexion in this area. The lesions are often painful.

Diagnosis

In a veterinarian’s detailed history, he or she should include the horse’s age, month of onset, whether the scratches is seasonal and/or pruritic (itchy), if there has been overzealous use of topical medications or home remedies, and response to prior treatment. He or she should also inspect the environment because wet or chemically treated bedding, along with muddy pastures and insects, can cause allergic contact dermatitis (from allergens touching the skin). If in-contact animals or humans are also affected it might indicate an infectious or zoonotic condition such as dermatophytosis (fungal infection). Another diagnostic rule-out is parasites—chorioptic mange is a common cause of pastern dermatitis in draft horses. But more commonly the cause of pastern folliculitis (a pus-forming skin infection) is bacterial, with Staphylococcus aureus and Dermatophilus congolensis as the culprits. Horses with white extremities can suffer from photosensitization  or immune-mediated pastern leukocytoclastic vasculitis (PLV), both exacerbated by UV light. Based on the information provided, your veterinarian might pursue specific diagnostics such as skin scrapings, fungal cultures, tape impression, skin cytology, skin biopsies, allergy testing, or