From Equine Disease Quarterly, funded by Lloyd's of London Underwriters, Brokers, and their Kentucky Agents

The skin is one of the largest organs of the body; however it rarely receives the attention given to many of the other organ systems. Examination of the skin is much like that of many organs, requiring a detailed history of the problem. This is followed by visual inspection and direct palpation by region: face, neck, chest, abdomen, legs, mane, and tail.

It is important point to define the types of lesions observed, with crust or scaling, papule, pustule, vesicle, bulla, wheal, macule, and nodule being among the most common.

Skin diseases are often grouped into categories that cause these specific lesions and a differential diagnosis is pursued from that category. For example, crusting skin diseases may be due to fungal agents (dermatophytes or ringworm), bacterial infections (dermatophilus or "rain scald"), or immune-mediated disorders (pemphigus foliaceous). Nodular skin diseases may lead the examiner down a different track such as sarcoid, allergic collagenolytic granulomas, or other tumors involving the skin.

The pattern or distribution of skin lesions is also helpful in determining a cause. Certain hypersensitivity reactions to insects might involve the mane and tail region, whereas nodules resulting from allergic collagenolytic granulomas are most commonly found on the chest wall just behind the elbow.

Once the type of lesion and pattern of distribution have been determined, the next step is to decide if ancillary testing is required. Common skin disorders such as ringworm or rain scald do not require additional testing. Alternatively, if a skin problem fails to respond to therapy, additional testing may be necessary to rule out an underlying problem or another diagnosis.

The most common ancillary diagnostic tests include skin scrapings for cytology and culture, skin biopsy and allergen testing. Culture and cytology are generally used to determine a specific bacterial or fungal pathogen and its sensitivity to specific antimicrobial drugs. Cytology may be useful in pointing to an immunological cause (acantholytic squamous epithelial cells and non-degenerate neutrophils in cases of pemphigus foliaceous).

Skin biopsy can be used to determine the histological distribution of the skin problem as well as any structural alterations of specific components of the skin (epidermis, dermis, hair follicles, adnexa, sweat glands, dermal connective tissue, and dermal blood vessels).

The skin biopsy may point to a particular mechanism, but not a particular etiology. In other situations, the skin biopsy may provide a definitive diagnosis.

Congenital skin disorders are occasionally seen. Some are common to many breeds, while others are breed specific. Epitheliogenesis imperfecta is a fatal condition seen in several breeds, but American Saddlebreds may be over-represented. Large regions of skin may be completely missing with exposure of the underlying soft tissue.

Another congenital condition is mechanobullous disease (also known as epidermolysis bullosa), a disorder of the connection between the epidermis and underlying dermis. It is most commonly inherited in Belgian foals and is fatal. The skin is easily torn loose from the underlying tissues and vesicles or bulla may be observed over much of the body, including the oral cavity, and hooves may separate from the underlying laminae.

A condition seen primarily in Quarter Horse foals involves a congenital defect of the dermal collagen (known as hyperelastosis cutis, cutaneous asthenia, or Ehlers-Danlos syndrome after the human counterpart). It may not be recognized until the foal is older, but presents as hyperextensible skin that is easily torn or as separation of large areas from the underlying dermal connective tissue forming seromas. All these conditions would be confirmed by skin biopsy.

Infectious conditions of the skin include the commonly recognized ringworm (dermatophyte) infections and "rain scald" (dermatophilus congolensis). Routine therapy usually resolves these problems, but severe infections may require prolonged treatment or reassessment with skin culture, cytology, or biopsy to make sure there is no other underlying problem.

Often included in the infectious category is lymphangitis, which is presented as an acute inflammation of the soft tissues of a lower limb. Occasionally, it becomes a chronic problem, with extensive scarring of the sub-dermal tissues resulting in an enlarged, stiff limb. The prognosis becomes poor as the condition becomes more chronic.

Papilloma, a contagious skin problem in young horses, is often self-limiting, as the animal's immune system will attack and destroy it. Surgical removal may speed healing and provide a more rapid cosmetic result.

Urticaria or hives is probably the most common immunological skin disorder and represents systemic hypersensitivity to drugs, feed components, or insect allergy, or following equine arteritis virus infection. Pemphigus foliaceous is occasionally seen as a severe, diffuse, crusting skin disorder that can be confirmed by skin biopsy but often requires prolonged immunosuppressive medication. It can occur in foals, which seem to have a better prognosis than adults.

In summary, an organized approach to skin disorders can bring about a specific diagnosis with a subsequent treatment plan and prognosis.

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About the Author

Fairfield Bain, DVM, MS, Dipl. ACVIM, Dipl. ACVP

Fairfield T. Bain, DVM, MS, Dipl. ACVIM, Dipl. ACVP, specializes in internal medicine and pathology. He has worked at Equine Sports Medicine & Surgery in Weatherford, Texas since December 2008.

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