Preface
About the Author
Understanding Dermatology
Anatomy of the Skin
The Immune System and the Skin
Nutrition and the Skin
Allergies
Inhalant Allergies
Flea Allergy
Food Allergies and Food Intolerance
Contact Dermatitis
Drug Eruption
Bacterial and Fungal Infections
Bacterial Infections
Fungal Infections
Parasites Affecting the Skin
Fleas
Ticks
Mites
Other Parasites
Insecticide Alert
Keratinization Disorders (Seborrhea)
Underlying Problems Causing
Congenital Keratinization Disorders
Nutritional Related Disorders
Metabolic Dematosis
Sebaceous Adenitis
Schnauzer Comedo Syndrome
Follicular Dysplasia
Callus
Keratoses
Lichenoid Dermatoses
Immune-Mediated Skin Diseases
Autoimmune Diseases
Other Immune-Mediated Skin Diseases
EXCERPT
Chapter One
UNDERSTANDING DERMATOLOGY
Dermatology is the medical study of the skin and the diseases that affect it. In veterinary medicine, it looms large: approximately 40% of all visits to veterinarians are prompted by skin problems. There are hundreds of different diseases that affect the skin, and this often surprises people. The average person may be familiar with acne and psoriasis and a few other conditions that affect humans but not be aware of the multitude of different skin diseases that affect animals. Some are similar to diseases seen in people, and others are unique to the animal world, sometimes unique to a single species.
Not long ago most dog owners thought dogs only got fleas, mange, and seborrhea, but thanks to modern technology and diagnostic testing, those days seem like the dark ages. Today, veterinary dermatology is recognized as an important medical science, often interrelating with other disciplines such as immunology, parasitology, microbiology, endocrinology, oncology, and nutrition.
Like every specialty, dermatology has a language of its own, and the vocabulary is expanding on a daily basis. Older, more generalized terms such as seborrhea, mange, and dermatitis are being replaced by names that are more descriptive, or more accurate. The terms used in this book are very current, but be aware that dermatology is a progressive science and things do change as we learn more. We welcome these changes, because they allow us to better diagnose and treat the dogs that need our help.
ANATOMY OF THE SKIN
In dogs as well as humans, the skin is the largest organ of the body. It protects us from the environment, provides a way to conserve and discharge heat, and also acts as a reservoir to maintain water balance in the body. It is the first line of defense in protecting our bodies from a harsh and unforgiving world.
The skin is well adapted to performing its specialized functions. The surface is a tough layer of keratinized (shingle-like) cells that provide a waterproof barrier to the environment. Millions of dead skin cells are shed from this layer each day, and the skin continues to renew itself in an orderly fashion throughout our lifetime. Beneath this layer of dead skin cells are the living cells, the keratinocytes, that live solely to eventually die and provide us with a barrier to the outside world. These cells originate from a basal cell layer, the bottom layer that constantly pushes its offspring toward the surface. The journey from the bottom layer to the top takes about twenty-one days; most cells spend another twenty-one days as dead surface scale before being shed. It is the collection of living and dying cells, that rise from the bottom layers to the top with such regularity that is referred to as the epidermis.
The epidermis has no blood supply of its own and receives all of its nutrition from the underlying tissue, called the dermis. The dermis is composed of connective tissue fibers, blood vessels, nerves, and a ground substance, which is gel-like and enables the dermis to be firm and supportive yet still flexible. The dermis is also home to several of the white blood cells that guard this territory and provide an immune response when the skin is threatened or attacked.
The boundary between the dermis and the epidermis is called the dermal-epidermal junction, or the basement membrane zone. This zone is very thin, but it does perform a number of critical tasks, including important stabilizing, barrier, and filtering functions.
Beneath the dermis is a large network of fat cells. This area is called the hypodermis, the panniculus, or the subcutis. The collection of fat cushions everyday wear and tear on the skin and underlying tissues.
Hair follicles and the glands that supply them are collectively termed pilosebaceous units. The root word pilus means hair, and the sebaceous part refers to the large grape-like glands that empty their products into the follicles by way of ducts. Production of a hair is a joint venture between the dermis and epidermis. The base of the hair follicle (hair matrix) rests in the deep dermis or subcutis and provides the raw materials for hair growth. In dogs, specialized cells from the epidermis dip down into the dermis before pups are born to form the actual follicle and the “pore” on the skin surface. This is a critical stage, because following birth, no new hair follicles are produced. Since follicles begin as epidermal cells, and since the epidermis does not have its own blood supply, nourishment for the hair must come from the dermis. A specialized structure, the dermal papilla, forms there in intimate contact with the hair matrix to supply it with blood and nutrition.
There are two main types of hair in dogs: the long, bristly guard hairs, and the downy vellus hairs. Whiskers (vibrissae) are specialized sensory hairs that are present in a few choice locations in the body but are not widespread. Guard hairs are sometimes referred to as primary hairs because they originate from the largest and deepest follicles. VeIlus hairs, the secondary hairs, are the first hairs evident (puppy fur). They grow in smaller follicles grouped around a primary follicle and share the same opening to the skin surface. Dog fur is therefore described as originating from compound follicles, since both guard hairs and vellus hairs may emerge from a common pore.
There are three types of glands found in skin, and all are located in the dermis. The sebaceous glands are often the most conspicuous; they look like large bunches of grapes located on either side of a follicle. Their job is to secrete sebum into the hair follicles and then onto the skin surface as the hair grows outward. Sebum is composed of cholesterol and waxes. It makes the skin surface water resistant and the haircoat shiny.
The sweat glands of animals are similar but not identical to those of people. Apocrine glands are the major sweat glands in animals. They secrete a form of sweat via ducts into the hair follicle. In people, these glands are present only in the armpits, anogenital regions, abdomen, face, and scalp, but in animals they are present everywhere. The eccrine sweat glands are similar in structure to the apocrine glands, and in people are important in temperature regulation. In humans they are distributed over the entire body surface, with different regional densities, but since they are not used for heat dissipation in animals, they are prevalent only on the footpads and nose of dogs. Thus, dogs do not sweat in the same manner as people and must dissipate heat differently.
THE IMMUNE SYSTEM AND THE SKIN
Part of the barrier function of the skin has to do with the role it plays in the immune system. A special type of white blood cell, the T-lymphocyte, patrols the perimeter of the body, looking for things that don’t belong. These T-cells are programmed by the thymus to recognize the difference between the body (self) and everything else (nonself). The thymus (not to be confused with the thyroid gland) is located in the neck and is very large in pups, getting progressively smaller as dogs age.
When T-cells encounter a real or perceived invader, they trigger responses by other components of the immune system to destroy it. An immune system that is perfectly coordinated wards off microorganisms (bacteria, parasites, and fungi) and produces antibodies against many other invaders, especially viruses. However, animals with defective T-cell systems may have problems ranging from recurrent infections to demodectic mange.
In addition to the important lymphocytes, there are other cells in the skin that act in conjunction with the immune system to protect the body. The mononuclear-phagocyte, or reticuloendothelial, system forms a net to catch things that pass through the skin and make sure they get introduced to the lymphocytes. The cells in this system also secrete a number of important enzymes and proteins as well as acting as scavengers by swallowing and digesting things that don’t belong. The proteins they produce provide important information to the lymphocytes, giving directions and keeping them updated on what’s going on in the body.
Other white blood cells, such as the mast cell, eosinophil, and neutrophil (pus cell) may be located in the skin, and all have a part to play in the immune system’s complex balance.
NUTRITION AND THE SKIN
Of all the factors involved with skin problems, dog owners tend to concentrate most on nutrition. People are always asking which dog food is best for skin problems, which supplements should be added to the diet, and which preservatives in dog foods are to be avoided. Clearly, diet does play a major role in health, including that of the skin, but most diet-related skin problems are not what owners expect.
The most important concern regarding diet is that the foods be tested in accordance with procedures established by the Association of American Feed Control Officials (AAFCO) or, in Canada, as certified by the Canadian Veterinary Medical Association. Foods that meet the guidelines established by these bodies are far superior to diets based on National Research Council (NRC) guidelines, which only provide nutrient values that prevent deficiencies. The NRC is neither a government agency nor a research laboratory, and it does not police the pet food industry at all. Since the manufacture and sale of pet foods is a multi-billion-dollar industry, it should not be surprising that many dog foods on the market are there to claim their piece of the action, not necessarily to provide optimal nutrition. To provide optimal nutrition, the food should contain wholesome ingredients and in the proper balance, and be demonstrated by feeding trials to do what it is supposed to do.
Beware of judging a dog food simply by reading the analysis values on the side of the package. You just cannot reliably compare two dog foods by looking at their protein content or by the amount of fat in the diet. Repeated studies have shown that these values are no assurance that the nutrients are of good quality, can be absorbed, or are digestible. It is important to only buy foods that have been tested in feeding trials to prove that several generations of dogs have eaten the food, digested it adequately, and remained predictably healthy throughout their lives. Studies have also shown that some generic and regional brands of dog food (with label guarantees that claim they met NRC recommendations) are not nutritionally adequate for dogs and result in clinical signs suggestive of zinc deficiency or copper deficiency. Not all foods are created equal; buy the ones whose manufacturers are prepared to provide evidence that they are nutritionally adequate.
As to nutritional supplements, the marketplace is full of them. The fact is, however, that if you are feeding an adequate, balanced diet (tested by feeding trials), these supplements are not needed. In fact, some nutritional supplements do more harm than good. Calcium supplements are notorious for making zinc less available and also for causing bone and joint abnormalities in rapidly growing dogs. Fatty acid supplements are generally sold to add luster to the coat, and although they may contribute to a healthy coat, they may also cause problems. High-fat diets or supplements may actually render vitamin E and other fat-soluble vitamins less available to the body, and they may contribute to digestive problems, including pancreatitis, gall bladder disease, and diarrhea. Feeding a well-balanced and well-tested diet is much more important to a healthy coat than the use of supplements.
Another concern that has surfaced recently is that preservatives in the diet may be causing certain health problems. The preservative singled out most often is ethoxyquin. Ethoxyquin and other antioxidants such as BHT and BHA are added to foods to stop the fats from going rancid. Canned foods have the least preservatives and dry foods the most. The antioxidants allow the food to be stored and maintain its fat content for many months. Without antioxidants, these diets have a very short shelf life indeed. Just imagine how long a bag of hamburger meat would last in your pantry without refrigeration and you can appreciate the need for preservatives.
Preservatives offer convenience, and in exchange, we accept some degree of risk. When you go into a restaurant with a salad bar, there is a very good chance that those fruits and vegetables look fresh (for hours) because they have been preserved with sulfites. Sulfites are generally very safe, but they are not without risks, especially to people with asthma. The flavor of Chinese food is often enhanced with monosodium glutamate (MSG). It is also relatively safe, but it does affect some people adversely. Ethoxyquin is no different—it is generally safe, but some animals may not tolerate it well. How can you tell? If you’re really concerned, feed a home-cooked meal to your dog for several weeks using ingredients that are fresh, not preserved. If there is substantial improvement, there may indeed be a diet-related problem that warrants further investigation.
Over the past few years, many diets for dogs have been manufactured and touted as being good for the skin. Most of these diets contain lamb or rabbit, or don’t contain soy. They market these diets as though lamb were, in some way, better than other ingredients such as beef or corn or soy. The fact is that these diets are no better for the skin than any other diet. This fallacy no doubt arose because dermatologists often feed dogs suspected of having a food allergy a diet of lamb and rice for a period of time (see chapter on allergies). The diet is fed not because lamb is good for the skin, but because most North American dogs have not eaten it as part of their regular diet. Prior to 1985, very few commercial diets contained lamb. Most were based on beef, soy, chicken, corn, and pork. The important premise here is that you can’t be allergic to something you’ve never eaten before, not that some foods are less allergy-causing than others. If you’ve never eaten strawberries before, you can’t be allergic to them. However, if you are prone, after having eaten them for a while, you definitely can develop allergies. Dogs (and people) can become allergic to foods they have eaten repeatedly, whether it be beef, corn, soy, lamb—or strawberries. Therefore, dogs can become just as allergic to lamb as to beef if they are fed it regularly. This is an important point if food allergy is ever suspected and a hypoallergenic food trial needs to be performed. If a dog has been fed lamb in the past, and many new diets contain it, it is no longer a suitable test diet. In fact, it is not advisable to feed dogs lamb-based diets for that very reason. If a food allergy is suspected, it gets difficult to find something readily available for the diet trial that they haven’t eaten before. Things like venison, rabbit, and moose are suitable in these cases, but much more difficult and expensive to find and prepare.
There is also much concern about the soy content of diets and its effects on the skin. Soy is an excellent source of protein and is often substituted for the more expensive meats. There is very little evidence to suspect that soy causes any problems in dogs. Some dogs may not absorb it as well as others; the Chinese Shar-pei has often been noted as a breed with intolerance to soy products. Animals with soy intolerance should not be fed soy-containing products, just as animals allergic to beef should not be fed diets including beef. This represents a relatively small, albeit real, segment of the dog population.
