By Frank G. Vice, DVM, BS Pharm
A brief review of skin anatomy will help the reader focus on areas of dermatological concern. The skin, hair, pads, toenails and surrounding tissues are developing while the pup is still an embryo. These tissues are formed in the ectodermal embryonic layer and are part of an organ system called the Integument. The complex nature of skin is further illustrated through basic skin structures which are broadly differentiated into layers, (epidermis, basal layer, dermis, and hypodermis or subcutaneous). As embryonic tissue continues differentiation, specific anatomical structures emerge enabling dogs to have cushioned feet, teeth for defense, and hair to cover all their skin. The developing skin extends over the dog’s body as a flexible covering securely attached, but thoroughly continuous as it transitions to the mucus membranes of each body opening. As a general rule, a dog’s skin is thicker along the back (dorsum) and thinner as it approaches the inguinal and abdominal sites. Skin is functional tissue, the largest organ of the body, representing about 20% of the dog’s body weight.
These integument tissues serve as anatomical and physiological protective skin barriers. They attempt to protect the body from physical, chemical, parasitic, microbiological and immunological damage. Deeper skin tissues have additional protective responsibilities which include helping to protect internal organs, serving as a reservoir for fat, protein, carbohydrates, and helping to maintain fluid and electrolyte control. Skin also has temperature regulation and sensory perception which allows the dog’s body to perceive hot, cold, touch, pressure and pain. These functional skin characteristics are influenced by a variety of factors including diet, specific gender hormones, external and internal environmental exposures, and genetics. Each skin layer (epidermis, basal layer, dermis, and hypodermis) has the potential for inflammatory reaction, when exposed to the variety of skin insults. Skin layers interact through an interwoven matrix of cells, glands, nerves, and blood vessels all contributing to the dog’s over-all health. The protective barrier of the basal membrane, located between the epidermal cells and dermis, is subject to inflammatory skin diseases. The dermis has collagen fibers, blood vessels, nerves and many cell types which include fibroblast, mast cells, neutrophils, lymphocytes, and plasma cells. These skin layers are very sensitive to inflammatory stimulation and will react when exposed to flea bites, food varieties and environmental chemicals. When a dog’s skin (toes, ears, anal glands) develops an inflamed unhealthy appearance, a physical examination is needed, probably requiring medical attention.
The specific cells, keratinocytes and melanocytes, located in the basal layer, go through mitosis (cell division) constantly, reproducing and replacing the epidermal cells. The newly developed keratinocytes migrate to the epidermal surface forming a compact layer of dead epidermal cells for skin protection. These dead cells will flake off only to be replaced. Keratinocyte’s function is to produce keratin, a fibrous protein material that remains soft and flexible enough to form the outer layer of the skin’s epidermis and hard keratin sufficient to support rigidity to claws, toenails and hooves. In addition, melanocytes located in the basal layer produce skin pigment. If the epidermis is exposed to trauma, irritants, inflammatory stimuli or metabolic changes, the basal layer production of these cells is altered, shortening the cell cycle, causing increased deposition of pigment, and darkening and scaling the skin According to reported research, the normal epidermal cell replacement cycle in beagles is approximately 22 days. If this normal cell cycle is shortened in response to some type of skin inflammation, the epidermal layer will suffer. Support and nourishment of the epidermis is regulated through the dermal skin layer resulting from blood vessels and sensory nerves distributed throughout this layer.
A review of the multiple skin diseases affecting dogs is beyond the scope of this brief article, however, a description of the common dermatological signs and symptoms seems appropriate. Many pathological skin problems look alike. During an examination, a review of the dog’s history and the skin changes that have developed over time will help guide the diagnosis. The presenting signs and symptoms of a dog experiencing a dermatitis episode usually include skin eruptions with crust and scabs, flaking skin with hair loss (alopecia), and intense itch (pruritus). These symptoms result from exposure to a variety of allergic stimulating products (allergens), fleas, food or inhalants which are present in the dog’s environment. These allergens create an inflammatory cascade throughout the skin layers.
The dogs presented to our office for dermatological examination usually have all the above inflammatory symptoms. Inflammation in a dog’s skin normally begins with exposure to some allergen (allergic stimulating product) which activates the dog’s immune system. This initial inflammatory response, oily skin with flakes, are defense mechanisms designed to protect surrounding tissues. Inflammation due to an activated immune response results in tissue layer changes. Initially, the inflamed tissue responds with mild swelling, blood flow alterations, and increased blood vessel perme ability (leakage into surrounding tissues) along with local changes in protein and electrolytes concentrations. The developing inflammation stimulates the release of a number of cell types which gather at the site of the injury. Although skin inflammation is initially beneficial, it often results in unpleasant skin sensations (pain and itch) and tissue layer damage (alopecia with infected and erupted skin) .These hypersensitive inflamed tissue sites are due to chronic inflammation and are usually found in specific areas such as the feet, perianal area, dorsum near the tail and ears. These are areas that seem to react in a more dramatic manner in some dog breeds.
Dermatitis is the general diagnosis but the cause may remain nonspecific. The dog’s exposure history and physical examination serve as a diagnostic guide, however blood testing, skin scraping and allergy testing are all available when needed. Canine dermatitis has multiple causes which includes flea bite allergy, food allergy and environmental inhalant allergies, but exact causes may never be completely understood. General atopic dermatitis treatment should start with owner education to identify possible allergic stimuli. Owners can help by avoiding the most obvious causes. Bathing and grooming with appropriate shampoo and flea control, along with diet restriction, requires strict supervision. Restriction of inhalant chemicals are a part of the allergic avoidance program. It should be understood that no single drug or management tool works the same in each dog. Owners may elect to treat the symptoms simply to improve the quality of life for their pet. Many owners will say, “I don’t care what causes the problem, just fix it. Don’t you have a shot, Doc?” “Yes, shots are available, but determining the cause will all allow us to avoid the problem, as well as the side effects of the shot.”
The mainstay of canine dermatitis therapy has been corticosteroids (prednisone). They quickly reduce atopic dermatitis inflammation. These drugs are non-specific in their mechanism of action and have limited long term therapeutic benefits. Many times, long term steroid use will create long term side effects, which results from adrenal gland suppression. These side effects include increased thirst, increased hunger, and increased urination along with panting and possibly immune suppression. The choice of steroid management is excellent, initially, but proves to be disappointing in long term dermatitis management. Additional medications used for atopy include Atopica (cyclosporine), Apoquel (oclacitinib), and Cytopoint (monoclonal antibody). Each of these products reduces the skin’s immune response in an attempt to reduce the inflammatory symptoms of atopic dermatitis. Their use will require very close monitoring.
The author would like to thank the following reference sources for literary information. The Merck Manual “Overview of Integumentary System” Karen Moriello DVM, Muller, Kirk and Scott: “Small Animal Dermatology” 3rd Edition and Kirk 8th edition “Current Veterinary Therapy”.