The skin is the largest organ of the body and, depending on the species and age, may represent 12%–24% of an animal's body weight. The skin has many functions, including serving as an enclosing barrier and providing environmental protection, regulating temperature, producing pigment and vitamin D, and sensory perception.
Integumentary System Sections (A-Z)
Atopic dermatitis (AD) is a genetically predisposed inflammatory and pruritic allergic skin disease with characteristic clinical features. It is most commonly associated with IgE antibodies to environmental allergens. The atopic phenotype (see clinical signs, below) can be seen in animals with IgE-mediated skin disease, food allergy, or a condition called "atopic-like dermatitis" (ALD). ALD is defined as a pruritic skin disease in dogs with characteristic features of AD but negative tests for IgE antibodies. Feline atopic dermatitis has many similarities to canine atopic dermatitis.
Hypodermosis of cattle in the northern hemisphere is caused by bot fly larvae (cattle grubs or ox warbles) of Hypoderma spp (order Diptera, family Oestridae). In Central and South America, larvae (tropical warbles) of Dermatobia hominis (order Diptera, family Cuterebridae) are important pests of cattle.
Congenital and Inherited Anomalies of the Integumentary System
Congenital dermatoses of the skin may be genetic or arise during embryogenesis because of nongenetic factors. Genetic mutations that cause skin anomalies may be present at birth or become apparent weeks to months later. These late-onset manifestations are referred to as tardive developmental defects. Both congenital and tardive developmental dermatoses are fairly common in domestic animals of all species, with the greatest number of well-defined defects described in cattle and dogs.
Cuterebra Infestation in Small Animals
This opportunistic, parasitic infestation of dogs, cats, and ferrets is caused by the rodent or rabbit bot fly, Cuterebra spp (order Diptera, family Cuterebridae). Flies are usually host- and site-specific relative to their life cycle. However, rabbit Cuterebra are less host-specific and are usually associated with dog and cat infestations. Rarely, cats and dogs may be infested with Hypoderma spp or Dermatobia hominis. Ferrets housed outside may be infested by Hypoderma or Cuterebra spp.
This infection of the epidermis, seen worldwide but more prevalent in the tropics, is also erroneously called mycotic dermatitis. The lesions are characterized by exudative dermatitis with scab formation. Dermatophilus congolensis has a wide host range. Among domestic animals, cattle, sheep, goats, and horses are affected most frequently, and pigs, dogs, and cats rarely. It is commonly called cutaneous streptothricosis in cattle, goats, and horses; in sheep, it is termed lumpy wool when the wooled areas of the body are affected. Infection in camel herds has been related to drought and poverty. Recent isolates from chelonids may represent a new species of Dermatophilus. It is also a common disease in farmed crocodiles. The few human cases reported usually have been associated with handling diseased animals.
Dermatophytosis is an infection of keratinized tissue (skin, hair, and claws) by one of the three genera of fungi collectively called dermatophytes—Epidermophyton, Microsporum, and Trichophyton. (Also see Fungal Infections.) These pathogenic fungi are found worldwide, and all domestic animals are susceptible. In developed countries, the greatest economic and human health consequences come from dermatophytosis of domestic cats and cattle. A few dermatophyte species are soil inhabitants (geophilic), eg, M gypseum and T terrestre, and cause disease in animals that are exposed while digging or rooting. Other species are host-adapted to people (anthropophilic), eg, M audouinii and T rubrum, and infect other animals rarely. The most important animal pathogens worldwide are M canis, M gypseum, T mentagrophytes, T equinum, T verrucosum, and M nanum. These species are zoonotic, especially M canis infections of domestic cats and T verrucosum of cattle and lambs. The zoophilic species are transmitted primarily by contact with infected individuals and contaminated fomites such as furniture, grooming tools, or tack. Exposure to a dermatophyte does not always result in infection. The likelihood of infection depends on several factors, including the fungal species, host age, immunocompetence, condition of exposed skin surfaces, host grooming behavior, and nutritional status. Infection elicits specific immunity, both humoral and cellular, that confers incomplete and short-lived resistance to subsequent infection or disease. New information concerning dermatophytic virulence factors, notably secreted proteases involved in the invasion of keratin, aspects of host immune response against dermatophytes, and new molecular tools available for studying dermatophytes should hasten development of safe and effective vaccines against dermatophytosis in species without vaccination options.
Eosinophilic Granuloma Complex
The cause of this group of diseases that affects cats, dogs, and horses is primarily an underlying hypersensitivity reaction. This is particularly true in cats and horses. Insect, environmental, and dietary hypersensitivities have been documented in cats, while insect hypersensitivity has been seen in some equine cases and in a smaller number of canine cases. Genetic predisposition and bacterial infections have also been noted as causes in cats. In all species, idiopathic cases exist.
Exudative epidermitis is a generalized dermatitis that occurs in 5- to 60-day-old pigs and is characterized by sudden onset, with morbidity of 10%–90% and mortality of 5%–90%. The acute form usually affects suckling piglets, whereas a chronic form is more commonly seen in weaned pigs. It has been reported from most swine-producing areas of the world.
Fleas and Flea Allergy Dermatitis
There are >2,200 species of fleas recognized worldwide. In North America, only a few species commonly infest dogs and cats: Ctenocephalides felis (the cat flea), Ctenocephalides canis (the dog flea), Pulex simulans (a flea of small mammals), and Echidnophaga gallinacea (the poultry sticktight flea). However, by far the most prevalent flea on dogs and cats is C felis. Cat fleas cause severe irritation in animals and people and are responsible for flea allergy dermatitis. They also serve as the vector of typhus-like rickettsiae and Bartonella sp and are the intermediate host for filarid and cestode parasites. Cat fleas have been found to infest >50 different mammalian and avian hosts throughout the world. In North America, the most commonly infested hosts are domestic and wild canids, domestic and wild felids, raccoons, opossums, ferrets, and domestic rabbits.
Flies belong to the order Diptera, a large, complex order of insects. Most members of this order have two wings (one pair) as adults. However, there are a few wingless dipterans. Dipterans vary greatly in size, food source preference, and in the developmental stage that parasitizes the animal or produces pathology. As adults, dipterans may intermittently feed on vertebrate blood or on saliva, tears, or mucus. These dipterans are referred to as periodic parasites and may serve as intermediate hosts for helminth parasites or for protozoan parasites. They may also alternately feed both on feces and on food and may possibly serve as vectors for bacteria, viruses, spirochetes, chlamydiae, etc. As larvae (maggots), dipterans may develop in the subcutaneous tissues of the skin, respiratory passages, or GI tract of vertebrate hosts and produce a condition known as myiasis.
Adverse food reactions comprise allergic reactions termed food allergy as well as nonallergic reactions termed food intolerance. On a practical level, these terms are frequently interchanged, because the precise immunologic processes of most adverse food reactions are usually not known. Immunologic reactions types I, III, and IV are thought to be the most likely causes, but this is conjectural for most reported cases in small animals. Food allergies are very rare in herbivores. In this chapter, the more commonly used term food allergy will be used for all adverse food reactions.
Helminths of the Skin
Cutaneous habronemiasis is a skin disease of Equidae caused in part by the larvae of the spirurid stomach worms (see Gastrointestinal Parasites of Horses). When the larvae emerge from flies feeding on preexisting wounds or on moisture of the genitalia or eyes, they migrate into and irritate the tissue, which causes a granulomatous reaction. The lesion becomes chronic, and healing is protracted. Diagnosis is based on finding nonhealing, reddish brown, greasy skin granulomas that contain yellow, calcified material the size of rice grains. Larvae, recognized by spiny knobs on their tails, can sometimes be demonstrated in scrapings of the lesions. Many different treatments have been tried, most with poor results. Symptomatic treatment, including use of insect repellents, may be of benefit, and organophosphates applied topically to the abraded surface may kill the larvae. Surgical removal or cauterization of the excessive granulation tissue may be necessary. Treatment with ivermectin (200 mcg/kg) has been effective, and although there may be temporary exacerbation of the lesions (presumably in reaction to the dying larvae), spontaneous healing may be expected. Moxidectin at 400 mcg/kg also appears to be active against Habronema spp in the stomach. Control of the fly hosts and regular collection and stacking of manure, together with anthelmintic therapy, may reduce the incidence.
A hygroma is a false bursa that develops over bony prominences and pressure points, especially in large breeds of dogs. Repeated trauma from lying on hard surfaces produces an inflammatory response, which results in a dense-walled, fluid-filled cavity. A soft, fluctuant, fluid-filled, painless swelling develops over pressure points, especially the olecranon. If longstanding, severe inflammation may develop, and ulceration, infection, abscesses, granulomas, and fistulas may occur. The bursa contains a clear, yellow to red fluid.
Integumentary System Introduction
The skin is the largest organ of the body and, depending on the species and age, may represent 12%–24% of an animal’s body weight. The skin has many functions, including serving as an enclosing barrier and providing environmental protection, regulating temperature, producing pigment and vitamin D, and sensory perception. Anatomically, the skin consists of the following structures: epidermis, basement membrane zone, dermis, appendageal system, and subcutaneous muscles and fat.
Interdigital furuncles, often incorrectly referred to as interdigital cysts, are painful nodular lesions located in the interdigital webs of dogs. Histologically, these lesions represent areas of nodular pyogranulomatous inflammation—they are almost never cystic. Canine interdigital palmar and plantar comedones and follicular cysts is a recognized syndrome that may be a subtype of interdigital furuncles or a separate disease.
Numerous species of lice parasitize domestic animals. Lice are largely host specific, living on one species or several closely related species. Lice are obligate ectoparasites and depend on the host to complete their life cycle. Recent taxonomic changes have complicated the orders and suborders of lice. In general, lice are divided into two categories: bloodsucking (or sucking) lice (order Anoplura) and chewing (or biting) lice (formerly the order Mallophaga, now composed of three suborders). Bloodsucking lice are parasites of mammals, whereas chewing lice infest both mammals and birds. Lice live within the microenvironment provided by the skin and its hair or feathers, and are transmitted primarily by contact between hosts. All life stages occur on the host, although lice may survive off the host for a period of time. In temperate regions, lice are most abundant during the colder months and often are difficult to find in the summer. Infestations are most often seen on stressed animals, and husbandry and individual health are important in treatment and management of these parasites. (Also see Ectoparasites.)
Mange is a contagious disease characterized by crusty or scaly skin, pruritus, and alopecia. Mange is a general term for cutaneous acariasis and is the result of infestation with one of several genera of parasitic mites, including Chorioptes, Demodex, Psorobia (formerly Psorergates), Psoroptes, Sarcoptes, and others. The term "scabies" most appropriately refers to infestation with Sarcoptes sp mites (ie, sarcoptic mange); however, this term is commonly misused to refer to any type of mange.
Miscellaneous Systemic Dermatoses
A number of systemic diseases produce various lesions in the skin. Usually, the lesions are noninflammatory, and alopecia is common. In some instances, the cutaneous changes are characteristic of the particular disease. Often, however, the dermatosis is not obviously associated with the underlying condition and must be carefully differentiated from primary skin disorders. Some of these secondary dermatoses are mentioned briefly below and are also described in the chapters on the specific disorders.
Nasal Dermatoses of Dogs
Nasal dermatoses of dogs may be caused by many diseases. Lesions may affect the haired bridge of the muzzle, the planum nasale, or both. In pyoderma, dermatophytosis, and demodicosis, the haired portions of the muzzle are affected. In systemic lupus erythematosus or pemphigus, the whole muzzle is often crusted (with occasional exudation of serum) or ulcerated. In systemic and discoid lupus, and occasionally in pemphigus and cutaneous lymphoma, the planum nasale is depigmented, erythematous, and eventually may ulcerate. The normal “cobblestone” appearance of the nasal planum is effaced.
Parakeratosis is a nutritional deficiency disease of 6- to 16-wk-old pigs characterized by lesions of the superficial layers of the epidermis. It is a metabolic disturbance resulting from a deficiency of zinc (also see Zinc) or inadequate absorption of zinc due to an excess of calcium, phytates, or other chelating agents in the diet. Predisposing factors include rapid growth, deficiency of essential fatty acids, or malabsorption due to GI diseases. Parakeratosis is unlikely in commercial swine unless errors have been made in diet formulation, but it may be seen in backyard pigs. The widespread use of high zinc levels in feed to prevent enteric disease in weaned pigs has further reduced the likelihood of the disease.
Photosensitization occurs when skin (especially areas exposed to light and lacking significant protective hair, wool, or pigmentation) becomes more susceptible to ultraviolet light because of the presence of photodynamic agents. Photosensitization differs from sunburn and photodermatitis, because both of these conditions result in pathologic skin changes without the presence of a photodynamic agent.
Pityriasis Rosea in Pigs (Porcine juvenile pustular psoriaform dermatitis)
Pityriasis rosea is a sporadic disease of unknown etiology of pigs, usually 8–14 wk old, but occasionally as young as 2 wk and very rarely in pigs as old as 10 mo. One or more pigs in a litter may be affected. The disease is mild, but transient anorexia and diarrhea have been reported. The initial skin lesions are characterized by small erythematous papules, which rapidly expand to form a ring (collarette) with distinct raised and reddened borders. The lesions enlarge at their periphery, and adjacent lesions may coalesce. The center of the lesion is flat and covered with a bran-like scale overlaying normal skin. The lesions are found predominantly on the ventral abdomen and inner thighs but occasionally may be seen over the back, neck, and legs. Characteristically, there is no pruritus, and recovery is spontaneous in 6–8 wk. Treatment is generally considered unnecessary. Diagnosis can usually be made from the characteristic lesions, but laboratory tests, culture, and biopsy may be used to differentiate it from dermatomycosis, exudative epidermitis, dermatosis vegetans, and swinepox.
Pox diseases are acute viral diseases that affect many animals, including people and birds. Some poxviruses also cause zoonoses. Typically, lesions of the skin and mucosae are widespread and progress from macules to papules, vesicles, and pustules before encrusting and healing. Most lesions contain multiple intracytoplasmic inclusions, which represent sites of virus replication in infected cells. In some poxvirus infections, vesiculation is not clinically evident, but microvesicles can be seen on histologic examination and, in some, proliferative lesions are characteristic.
Pyoderma literally means “pus in the skin” and can be caused by infectious, inflammatory, and/or neoplastic etiologies; any condition that results in the accumulation of neutrophilic exudate can be termed a pyoderma. Most commonly, however, pyoderma refers to bacterial infections of the skin. Pyodermas are common in dogs and less common in cats.
The area of riding horses that is under saddle, or the shoulder area of those driven in harness, is frequently the site of injuries to the skin and deeper soft and bony tissues. Clinical signs vary according to the depth of injury and the complications caused by secondary infection. Emaciated horses are at increased risk. Sores affecting only the skin are characterized by inflammatory changes that range from erythematous to papular, vesicular, pustular, and finally necrotic. Frequently, the condition starts as an acute inflammation of the hair follicles and progresses to a purulent folliculitis. Affected areas show hair loss and are swollen, warm, and painful. The serous or purulent exudate dries and forms crusts. Advanced lesions are termed “galls.” When the skin and underlying tissues are more severely damaged, abscesses may develop. These are characterized as warm, fluctuating, painful swellings from which purulent and serosanguineous fluid can be aspirated. Severe damage to the skin and subcutis or deeper tissues results in dry or moist necrosis. Chronic saddle sores are characterized by a deep folliculitis/furunculosis (boils) with fibrosis or a localized indurative and proliferative dermatitis. Lesions are usually caused by poorly fitting tack.
Primary idiopathic seborrhea is a skin disease seen in dogs and rarely in cats. It is characterized by a defect in keratinization or cornification that results in increased scale formation, occasionally excessive greasiness of the skin and hair coat, and often secondary inflammation and infection. Secondary seborrhea, in which a primary underlying disease causes similar clinical signs, is more common than primary seborrhea. Seborrhea in horses is usually secondary to either pemphigus foliaceus or equine sarcoidosis (chronic granulomatous disease).
Ticks are obligate ectoparasites of most types of terrestrial vertebrates virtually wherever these animals are found. Ticks are large mites and thus are arachnids, members of the subclass Acari. They are more closely related to spiders than to insects. The ~850 described species are exclusively bloodsucking in all feeding stages. Ticks transmit a greater variety of infectious organisms than any other group of arthropods and, worldwide, are second only to mosquitoes in terms of their public health and veterinary importance. Some of these agents are only slightly pathogenic to livestock but may cause disease in people; others cause diseases in livestock that are of tremendous economic importance. In addition, ticks can harm their hosts directly by inducing toxicosis (eg, sweating sickness [see Sweating Sickness], tick paralysis [see Tick Paralysis] caused by salivary fluids containing toxins), skin wounds susceptible to secondary bacterial infections and screwworm infestations, and anemia and death. International movement of animals infected with the tick-transmitted blood parasites Theileria, Babesia, and Anaplasma spp and Ehrlichia(Cowdria) ruminantium is widely restricted.
Tumors of the Skin and Soft Tissues
Cutaneous tumors are the most frequently diagnosed neoplastic disorders in domestic animals, in part because they can be identified easily and in part because the constant exposure of the skin to the external environment predisposes this organ to neoplastic transformation. Chemical carcinogens, ionizing radiation, and viruses all have been implicated, but hormonal and genetic factors may also play a role in development of cutaneous neoplasms.
Ulcerative Dermatosis of Sheep
Ulcerative dermatosis is an infectious disease of sheep caused by a virus similar to the ecthyma virus. It manifests in two somewhat distinct forms, one characterized by formation of ulcers around the mouth and nose or on the legs (lip and leg ulceration), and the other as a venereally transmitted ulceration of the prepuce and penis or vulva.
Urticaria is characterized by multiple plaque-like eruptions that are formed by localized edema in the dermis and that often develop and disappear suddenly. It occurs in all domestic animals but most often in horses. Allergic urticaria may be exogenous or endogenous. Exogenous hives may be produced by toxic irritating products of the stinging nettle, the stings or bites of insects, topical medications, or exposure to chemicals (eg, carbolic acid, turpentine, carbon disulfide, or crude oil). Nonimmunologic factors such as pressure, sunlight, heat, exercise, psychologic stress, and genetic abnormalities may precipitate or intensify urticaria. Pruritus is not always present, particularly in horses.