Written by: Anthony Yu, DVM, MS, DACVD
Scratches, or equine pastern dermatitis (EPD), is not a disease but, rather, a cutaneous reaction pattern. Veterinarians and owners must consider the primary, predisposing and perpetuating causative factors in a case, recognizing that treating the predisposing and perpetuating factors are just as important as addressing the primary cause.
Clinical Signs and Pathogenesis
Scratches can affect any breed, but is 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.
In a veterinarian's detailed history, he or she should include the horse's age, month of onset, whether the scratches are 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 inspect the environment because wet or chemically treated bedding, along with muddy pastures and insects, can cause allergic contact dermatitis (from the allergens touching the skin). If in-contact animals or humans are also affected, it might indicate an infectious or zoonotic condition such as dermatophyte fungus. 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 folluculitis (a pus-forming skin infection) is bacterial, with Staphylococcus aureus and Dermatophilus congolensis as the culprits. Horses with white extremities can suffer from systemic and contact forms of photosensitization (eg., pastern leukocytoclastic vasculitis, PLV) in the presence of UV light.
After your veterinarian identifies the causative factors, it's time to pursue appropriate therapy. Here are some environmental changes you can make:
1. Avoid turning affected horses out in pastures with mud, water or sand, which can worsen the condition.
2. Keep horses in clean, dry stalls during wet weather.
3. Do not turn horses out until the morning dew has dried.
4. If you suspect contact allergic dermatitis, try an alternate source of bedding that isn't treated or aromatic.
5. Clip heavy feathers over the pasterns to reduce moisture retention.
6. If you suspect PLV, avoid UV light exposure by stabling the horse during the day and/or wrapping the affected legs.
7. Clean affected skin immediately after exercise using an antiseptic shampoo.
Clinical treatments include topical antibacterials and systemic therapeutics. Secondary infections with Staphylococcus spp are common and can complicate diagnosis. Available antibacterial shampoos typically contain ethyl lactate, accelerated hydrogen peroxide, 2% benzoyl peroxide, or 2% chlorhexidine. When using these, shampoo the area once or twice daily for seven to 10 days, lather, leave on 10 minutes, rinse and dry well. Then decrease the frequency to two to three times weekly. If lesions ooze exudate, apply astringent solutions, such as lime sulfur or aluminum acetate.
Ointments such as silver sulfadiazine and 2% mupirocin ointment are available for treating localized bacterial infections. To treat dermatophytes and mites, owners can apply lime sulfur dips and spray. Enilconazole labeled for equine use in conjunction with miconazole, with or without chlorhexidine shampoo, can be used to treat fungal infections.
Sprays such as hydrocortisone aceponate or 0.015% triamcinolone can be used in conjunction with systemic immunomodulators to treat allergic and immune-mediated conditions such as PLV. In additions, veterinarians have noted success applying 0.1% topical mometasone, 1% betamethasone or 0.05% aclometasone creams or ointments to lesions. Horses with immune-mediated conditions might also require immunosuppressive doses of dexamethasone or prednisolone.
Antiparasitic approaches for Chorioptes include ivermectin, topical eprinomectin solution, selenuim sulfide shampoo followed by lime sulfur, and fipronil spray.
Article provided by AAEP Media Partner, The Horse.
About the author: Anthony Yu, DVM, MS, DACVD operates Yu of Guelph Veterinary Dermatology practice out of Guelph Veterinary Specialty Hospital in Ontario, Canada.