Dermatology

Dermatology is the study of the skin. We will be discussing skin disease and therapy as it applies to small animal veterinary practice during this lesson. Skin disease is the most common reason for a pet to be presented to the animal hospital for examination. The skin is the largest organ of the body and happens to be on the outside. It is a mirror reflection of internal function and can be seen and examined easily by the client. Therefore, when hair loss, flakes, bumps, itching or any other change occurs, the client will bring the pet in for examination.

It is important to understand and appreciate the relationship that clients have with their pets regarding skin disease. Pets have become members of our family, and because we interact with them every day by touching and petting them, abnormalities on the skin become self-magnifying concerns. We need to guard against minimizing or belittling the concerns of our clients. For example: A client presents their pet for vaccinations and then, almost in passing, mentions the bump on his head. We should examine the bump, write it in the record, have the doctor examine it and perhaps biopsy it. Even if this bump looks like a “wart” that you have seen on many other dogs before. The client does not know what the bump is, and feels it every time she pets him on the head. It would be a great disservice to the appreciation of the human/companion animal bond if we did not appreciate the concerns of the client – no matter how minor they may appear.

Skin disease can be frustrating to manage because many skin diseases cannot be cured. This is not to say that they cannot be managed, just that, when the management stops, the skin disease will reoccur. As a part of the animal health care team, it is your responsibility to help the client understand the chronic and ongoing nature of skin disease. And it is your responsibility to help maintain the continued investment of time and energy that skin problems often require.

The following material will give you the foundation to carry on conversations and communications with the doctors in the clinic as well as the clients on matters that involve the skin. Your vocabulary will be expanded to include terms that describe the way the skin has responded to the disease. This will empower you, in turn, to communicate to the client more effectively the recommendations of the veterinarians in your hospital.

Diagnose before you treat” should truly be a guiding principle with skin disease. Minor diagnostic procedures such as magnification glass examination, skin scrapings and fungal cultures are often overlooked. More invasive tests for skin disease include biopsies. Blood tests and allergy tests can be done in cases that are more difficult. By understanding the value of these tests, you can help communicate the necessity for them to the client, thereby, helping the pet receive a more accurate therapy. You serve as a vital link between the veterinary client and the doctor. Your opinion will be asked many times in the absence of the doctor, so it is very important that you understand all the recommendations made by your doctor in order to give accurate advice to the client. History taking is an important part of diagnosing skin disease, and a brief primer in how to collect all the information is in order. The first question that you should ask the client is, “Does the pet itch?” If the answer is,“Yes”, then an examination of the skin for lesions is in order. If there are lesions, then they are described in the record. The client is then asked how the lesions first appeared and how long they have been present. In addition, it is important to know previous therapies that have been given; most clients will have used or tried something from the grocery store prior to coming to the veterinarian. If the pet is itching, and the lesions are mostly secondary to biting and scratching, then we most likely have an allergic or parasitic problem. If the pet is not itching, then we should consider Demodex mange, ringworm or hormonal diseases. If the pet is young, you would consider mange or ringworm higher on the list than hormonal disease. An older or adult pet is more likely to have hormonal dysfunction than a young pet. The most common hormonal problem that is associated with skin disease is low thyroid hormone – hypothyroidism. After you have asked the client if any testing has been done, you can proceed to tell them that we like to “diagnose before we treat”. Most pets with skin disease require a skin scraping and this should be explained to the client as a first step in the diagnosis of the problem.

Once a diagnosis is made, the veterinarian will recommend therapy. Skin disease often requires some form of topical therapy, either as a primary therapy, or as an adjunctive therapy to oral medications. It will be your responsibility to understand all the commonly prescribed topical formulations in our hospital so that you can explain their use to the client. It is very helpful for you to understand how the skin responds to disease and how this topical therapy counteracts that response to normalize the skin.

The skin can respond to disease only by making new skin. The skin can respond to disease only by making new skin – Again – the skin can respond to disease only by making new skin. The previous three statements were not typos – I want to emphasize this point. The outer cells of the skin reproduce in an effort to protect the skin from disease. If the skin is abraded – more skin will replace what is lost in the abrasion. If the skin is inflamed from a fleabite, the skin will thicken where the flea has bitten. If the animal has chronic irritation, the skin will produce excessive skin in the form of flakes and scales. If the flakes and scales are mixed with excessive oil, then the pet has seborrhea oleosa. Regardless, the excessive skin production is treated topically in most cases. This topical therapy is directed at removal of the excessive accumulation of skin, and is called “keratolytics”. The topical medications that contain coal tar actually reduce the rate of production of the skin. Topical medications have many functions; some kill bacteria, some moisten dry skin, some provide agents to stop itching, and some have antihistamines, corticosteroids or oatmeal to reduce inflammation. Shampoos are for cleansing, and medicated shampoos must stay on the pet long enough for the medication to be effective. By studying the following materials, you will gain insight on how to communicate the doctor’s recommendations to the clients.

Structure and Function of the Skin

The Skin

The skin is the largest organ of the body and the anatomic and physiologic barrier between animal and environment. It provides protection from physical, chemical, and microbial injury, and its sensory components perceive heat, cold, pain, touch, itch and pressure. In addition, the skin is synergistic with the internal organ systems, and thus reflects pathological processes that are either primary elsewhere or are shared with other tissues. The skin is a mirror reflecting not only the internal conditions but also the outside world to which it is exposed.

Anatomy

Physical Characteristics

At each body opening, the skin is continuous with the mucous membrane that lines the internal surfaces – digestive, respiratory, ocular, and urogenital. The skin is thicker on the top of the body than the belly and thicker on the feet than on the rest of the leg. The skin is thickest on the forehead, dorsal neck, dorsal thorax, rump, and base of the tail. It is thinnest on the pinnae, under the arms and legs, inguinal and around the anus. The hair surface is acidic, with a pH that ranges in dogs and cats from 5.5 to 7.5.

Hair Follicle/Epidermis

The hair follicle and its associated structures are important to the study of dermatology of our pets. The follicle extends from the hypodermis, through the dermis, to the epidermis layer. These three layers are important. The Epidermis, which contains the Stratum Corneum, is the outer, thin layer composed of many layers of cells. In general, the epidermis is quite thin although there are many layers. The five layers are (from outer to inner): horny layer, clear layer, granular layer, spinous layer and basal layer.

Horny Layer – Thin outer layer of the skin composed completely of keratinized (filled with keratin which is a tough flexible substance like what calluses are made of), dead cells which are constantly being shed. Seborrhea sicca (dry skin) can cause a build up of excess keratin making the horny layer thicker. As the horny layer becomes thicker, it cracks and increases the amount of shedding. Thickening of the horny layer is a normal response of the body to damage or trauma. This is evidenced as a callus, or thickening of the horny skin in the area of trauma. The function of the horny layer is to provide a barrier from external influences and preclude outward losses of moisture and other elements. The horny layer makes up the normal surface in health, as well as the most prominent signs of disease. These signs are scales, crusts, flakes, dandruff, seborrhea, etc.

NOTE: The production of the horny layer is the most important function of the epidermis. The skin can respond to damage only by making new skin – the horny layer is the result of this new skin production.

Clear Layer – One cell thick layer of dead, keratinized cells. In dogs and cats, this layer is found only in the footpads.

Granular Layer – This layer is not present in all species of dogs. If present, it is only one cell thick and the cells are in the process of dying and eventually become a part of the horny layer.

Spinous Layer – This layer is two to three cells thick and does not reproduce. These cells will eventually die and become a part of the horny layer.

Basal Layer – This is the only layer that produces the cells of the epidermis. There are two types of cells. One produces keratin, the protein substance that gives strength to the horny cells. The other is the melanocyte that produces melanin pigment (skin color). This layer separates the epidermis from the dermis.

The Dermis

The dermis accounts for most of the thickness of the skin. The dermis is composed of cells and fibers that contain the arrector pili muscles, blood vessels, lymph vessels, glands and nerves. The dermis is responsible for the elastic nature of the skin. The dermis of the scrotum is unique in that it contains many muscle bundles that allow it to contract in cold conditions. The dermis has fibers that are responsible for most of the strength of the skin. The following is a list of some of the structures contained in the dermis.

Arrector Pili Muscles – Muscles that are attached to the hair follicle located in the dermis that allow the hair to “stand up”. This reaction will be seen in times of danger or excitement to show aggression or to appear bigger to an opponent. It may also be used for temperature regulation. These muscles may help in the expression of sebaceous glands secretions.

Sebaceous Glands – Glands that open into the hair follicle. Their oily secretion keeps the skin soft and pliable by forming a surface coating that retains moisture and maintains proper hydration. The oil film also spreads over the hair shaft giving it a glossy sheen. The secretion is also antimicrobial. In periods of malnutrition or stress, these glands do not function giving the coat a dull and dry appearance.

Sweat Glands – Sweat glands are of two types in dogs and cats. One type is the same as those in humans and causes the typical sweating. This type is called the eccrine sweat gland. Eccrine glands are located only on the footpads and nose of dogs and cats. Sweating is seen in agitated or excited dogs and cats. The other type of sweat gland is called the apocrine sweat gland and is described below.

Apocrine Sweat Glands – Apocrine glands are located in close association to most hair follicles all over the body. These glands open into the hair follicle. There are usually more of them where the hair is thinner. Sweating from these glands is possible especially in German shepherds and Labradors and these breeds may show sweating under the arms, in the inguinal areas, or around the groin.

Specialized Glands

Perianal Glands – These glands surround the anus and may be the source of perianal gland infections that occur in German shepherds and other breeds. Perianal gland tumors are also common around the rectum of male dogs. These tumors may respond to surgical removal and castration.

Anal Sacs – The anal sacs are located caudal to the rectum and produce a foul smelling, thick fluid. This fluid should be expressed each time a pet defecates. Its primary purpose is for scenting and marking. There is no other known function of the anal sacs. When the anal sacs do not empty properly, anal sac impaction can occur. The pet will usually lick the anal area excessively and scoot the rump on the ground. If this action does not relieve the impaction, anal gland abscess may occur. Manual expression of the anal glands is a common grooming procedure and is done as a part of the annual examination of dogs. This procedure is accomplished by the following steps: raising the tail firmly, then with a gloved hand, grasp anterior to the glands with the thumb and forefinger. Each gland should be palpable between the respective thumb and forefinger. Squeeze firmly and apply caudal pressure to empty the glands. If the glands cannot be expressed in this manner, insert the gloved forefinger into the rectum and grasp the gland between the thumb and forefinger – squeeze and apply pressure caudal to empty the gland. Repeat on the opposite side. Chronic impaction or infection is a reason to have the glands surgically removed.

Ear Canal Glands – These glands are responsible for producing earwax and can become a major problem if they become hypertrophic (enlarged). This most often occurs in cocker spaniels.

Caudal Tail Gland of Dogs – A gland that is on the dorsal surface of the tail and is present in only five percent of the dogs. The “guard hairs” are found in this area and a very thick amount of sebaceous secretion is associated with them.

Caudal Tail Gland of Cats – A gland that is present in non-neutered male cats and secrets waxy, sebaceous secretion that causes matting of the hair in this area. This is called “Stud Tail” in cats. The caudal tail gland is responsive to testosterone and other hormones. This oval area on the tail is subject to infection, tumors and seborrhea.
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Figure 1. Anatomy of Skin and Hair Follicle