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.
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.
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 such as pollens, environmental mites, molds, food allergens, and Malassezia and Staphylococcus organisms. The atopic phenotype is characterized by chronic pruritus, typical lesion distribution (All.see page Clinical Findings) , a family history of the disease, and breed predisposition. These signs can be seen in animals with IgE-mediated skin disease, 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.
Urticaria, in its most common form, is an acute immune response resulting in multifocal dermal edema. Diagnosis is generally achieved via history, physical examination, and response to treatment. Urticaria may resolve spontaneously 12–48 hours after occurrence. It is generally treated with administration of an antihistamine and a glucocorticoid. Rarely, urticaria may be chronic or may be a precursor to anaphylaxis. Atypical cases may require skin biopsy.
Dermatophilosis is a bacterial skin infection affecting multiple species of animals world-wide. It is most common in young or immunosuppressed animals or in animals chronically exposed to wet conditions. Signs include matted hair, crusts, and wart-like lesions that can have a wide distribution. Diagnosis is by cytology from lesions or bacterial culture. The disease is treated by systemic antibiotics, topical therapy, and changes in husbandry to keep animals dry.
Exudative epidermitis is a generalized staphylococcal infection that affects young pigs. Clinical signs include depression and reddening of skin, followed by rapid spread of exudative, nonpruritic pustules. Diagnosis is based on clinical signs and confirmed by bacterial culture of lesions. Treatment with antibiotics is most successful when combined with topical antiseptic sprays. Severely affected pigs may require fluids and electrolytes.
Interdigital furuncles are deep pyoderma lesions that form between the toes of dogs. They can be either single or multifocal. The nodules are painful areas of pyogranulomatous inflammation. Foreign body reactions to embedded hair shafts will prolong the infection. Lesions are treated with topical antimicrobial therapy and, if severe, with concurrent systemic antibiotics. It is important to identify the underlying trigger (eg, atopic dermatitis) if lesions are multifocal.
Superficial pyoderma is a bacterial infection confined to the upper layers of the skin and hair follicle. The infection is usually secondary to local trauma, keratinization disorders, parasitic infestation, hormonal factors, or allergies. In dogs, superficial pyoderma is the most common form of pyoderma, and it is also the most common reason for antimicrobial use in small animal practice. Performing skin cytology is key for the identification of bacteria and inflammatory cells, with cocci and neutrophils being the most common findings. Management of pyoderma has become, unfortunately, progressively more difficult due to the presence of methicillin- and multidrug-resistant bacteria. Identifying and controlling the underlying cause is critical to avoid recurrence.
Contagious ecthyma is an infectious dermatitis of sheep and goats that affects primarily the lips of young animals. The disease is usually more severe in goats than in sheep. People are occasionally affected through direct contact.
Pox diseases are 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.
Dermatophytosis (ringworm) is typically a superficial skin infection. It affects a wide range of animals, and several of the causative fungi also cause zoonotic infections. In otherwise healthy animals, it requires no treatment; however, treatment is usually recommended to shorten the course of the disease and avoid contagion. Disease is more common in young or stressed individuals, such as those in extremely crowded environments. Clinical signs can include any combination of hair loss, scaling, crusting, erythema, papules, hyperpigmentation, and variable pruritus. Diagnosis can be confirmed by direct examination of hairs or scales from lesions or by skin biopsy. Dermoscopy or a Wood's lamp can be used to identify hairs for culture and/or direct examination. Fungal culture can determine whether spores are present on the hair coat and must be used in conjunction with clinical examination findings. PCR testing confirms the presence or absence of fungal DNA on the hair coat. It cannot distinguish between viable and nonviable spores. In animals that need treatment, topical antifungal therapy disinfects the hair coat and eliminates infection from hair follicles.
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.
Infestation by rodent or rabbit bot fly larvae is most commonly seen in late summer or early fall and may affect dogs, cats, or ferrets. A fistulous swelling that may exude purulent exudate is typical. Definitive diagnosis is made by finding and identifying a larva. Treatment is manual extraction of the parasitic bot. The lesion is cleaned and allowed to heal by granulation.
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 Bartonellaspp 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.
Cutaneous habronemiasis is a skin disease of Equidae caused in part by the larvae of the spirurid stomach worms in the genera Draschia and Habronema (All.see page 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.
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 All.see page 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.
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 approximately 900 currently recognized species are exclusively blood-sucking 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 tickborne agents are only mildly 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, 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 ruminantium hard ticks is widely restricted.
Tumors of the skin and soft tissues 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.
True acanthosis nigricans is a genetic dermatosis found primarily in Dachshunds. Postinflammatory hyperpigmentation may appear clinically similar, but is a secondary reaction following an underlying disorder. Diagnosis is made based on the clinical presentation. Treatment includes topical anti-inflammatory drugs or antimicrobials if secondary bacterial or yeast infection is present.
Eosinophilic inflammatory skin diseases affect cats, dogs, and horses. The most common underlying cause is a hypersensitivity reaction to an allergen (insect, environmental, adverse food reaction). In some cases, the underlying trigger is unknown.
A hygroma is a false bursa that develops over bony prominences and pressure points, especially in large breeds of dogs. In young dogs the pathogenesis is believed to be due to trauma. In older dogs, hygromas tend to be result of impaired ambulation and excessive time spent in recumbency on hard surfaces. These are common in dogs with elbow arthritis. The trauma 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 or tarsus. If longstanding, severe inflammation may develop, and ulceration, infection, abscesses, granulomas, and fistulas may occur. The bursa contains a clear, yellow to red fluid.
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 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-week-old pigs characterized by lesions of the superficial layers of the epidermis. It results from a zinc deficiency 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 gastrointestinal disease. Parakeratosis is uncommon in commercial swine unless errors have been made in diet formulation; however, it may occur 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 is an increased susceptibility of skin to damage caused by ultraviolet light. Agents that cause the hypersensitivity include a variety of plant compounds, aberrant pigments, and compounds that accumulate after aberrant hepatic metabolism. Affected animals are photophobic and develop skin bullae, ulcers, and necrosis. Diagnosis is based on signalment and clinical signs, along with measurement of porphyrins in blood, urine, and feces. Treatment is primarily supportive and palliative care.
Pityriasis rosea (pustular psoriaform dermatitis) is a sporadic disease of unknown etiology of pigs, usually 8–14 weeks of age, but occasionally as young as 2 weeks and very rarely in pigs as old as 10 months. One or more pigs in a litter may be affected.
Saddle sores are pressure sores seen in horses over areas of wear from tack (especially if it is ill-fitting). 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. Prolonged focal pressure can lead to decreased capillary circulation, tissue damage, and even necrosis. Sores are frequently complicated by secondary bacterial infections. Emaciated horses are at increased risk.
Primary seborrhea is a very rare keratinization disorder characterized by greasy hair and scaling skin. Primary seborrhea is not pruritic. It is diagnosed by ruling out all causes of the much more common, secondary seborrhea. Primary seborrhea is treated with frequent bathing and administration of vitamin A or a retinoid.