Definitive diagnosis of the causes of various skin diseases requires a detailed history, physical examination, and appropriate diagnostic tests. Many skin diseases look alike, and a definitive diagnosis is made over time by including or excluding possible causes, evaluating responses to therapy, and/or process of elimination.
A careful dermatologic history is critical to interpret the physical examination findings and choose appropriate diagnostic tests. A complete general history should be obtained, including information about prior illnesses, vaccinations, husbandry (housing, feeding practices, etc), changes in attitude and food consumption, elimination practices, exposure to other animals, and travel within the past 6–12 mo. This should be followed by a detailed dermatologic history. Use of a preprinted history form can be very useful for chronic or complicated cases. A good history is important, because many skin diseases that look similar are differentiated based on interpreting clinical signs and historical patterns.
The following information should be obtained: 1) the primary complaint; 2) length of time the problem has been present; 3) age at which the skin disease started (distinct age predilections are seen in many diseases, eg, demodicosis and dermatophytosis in pediatric animals and signs of atopic dermatitis in animals 1–3 yr old); 4) breed (breed predilections include a predisposition of Cocker Spaniels to primary disorders of keratinization, and of terriers to atopic dermatitis); 5) presence and severity of pruritus (including licking, rubbing, scratching, or chewing behaviors—owners often do not realize licking may be a sign of pruritus); 6) how the disease started and its progression (diseases that begin with pruritus may lead to self-trauma and subsequent development of secondary skin lesions [alopecia, seborrhea] or infections [bacterial or yeast pyoderma]); 7) type and progression of lesions noted by the owner; 8) evidence of seasonality (suggesting fleas, allergic skin disease, or weather-related diseases); 9) area on the body the problem was first noticed (ie, regional patterns seen in atopic dermatitis [typically the face and feet], cheyletiellosis [primarily dorsal], scabies [primarily ventral], and endocrine hair loss [usually involves the trunk and spares the head and legs]); 10) any previous treatments and the responses to such (ie, antibiotic-responsive skin diseases suggest a bacterial cause; pruritus that responds to small doses of glucocorticoids, antihistamines, or essential fatty acids suggests allergic dermatitis); 11) frequency of bathing and when the last bath was given (recent bathing may obscure or change important clinical lesions, excessive bathing and wetting of the skin can predispose to skin disease); 12) presence of fleas, ticks, or mites; 13) other contact animals (ie, evidence of contagion, which suggests fleas, scabies, cheyletiellosis, or dermatophytosis); 14) the environment of the animal (housing changes can influence the development of certain skin diseases, eg, contact dermatitis, contagious diseases); and 15) signs or reports of systemic illness (endocrine [eg, hypothyroidism and hyperadrenocorticism] disorders and metabolic diseases [eg, diabetes mellitus, renal disease, liver disease] should be noted, because the skin can be the first place signs of systemic illness are noted).
A complete physical examination should always be performed. Many skin diseases are manifestations of systemic diseases, eg, hypothyroidism, hyperadrenocorticism, hepatocutaneous syndrome, systemic lupus erythematosus. (Also see Miscellaneous Systemic Dermatoses.) A good dermatologic examination requires very close inspection of the entire hair coat and skin under strong lighting; flashlights may be necessary to examine the skin of large animals. It is important to examine the ventrum of the animal, where many primary lesions and cutaneous parasites are found.
Clinical lesions are described in a variety of ways. Gross lesions can be described as focal, multifocal, or diffuse in distribution, followed by a description of the affected region (eg, mucocutaneous, truncal). On closer inspection, lesions may be further described as primary or secondary. Primary lesions include macules or patches (nonelevated areas of discoloration); papules or plaques (elevated lesions, the latter coalescing); pustules, vesicles, or bullae (fluid-filled lesions); wheals (flat-topped, steep-walled, solid elevations of the skin arising from histamine release); or nodules or tumors (large solid elevations of the skin). Secondary lesions include epidermal collarettes (late stage of a pustule), scars, excoriation (areas of self-trauma), erosions or ulcers (loss of the epidermis), fissures, lichenification (increased thickening and hyperpigmentation of the skin), and calluses. Some lesions may be either primary or secondary, depending on the cause of the disease. These include alopecia, scale, crusts, follicular casts (plugging of hair follicles with visible keratin), comedones (blackheads), and pigmentary changes.
Laboratory Procedures for Skin Diseases
Skin scrapings are part of the basic database for all skin diseases. There are two types of skin scrapings, superficial and deep. Superficial scrapings do not cause capillary bleeding and provide information from the surface of the epidermis. Deep skin scrapings collect material from within the hair follicle; capillary bleeding indicates that the sampling was deep enough. Skin scrapings are used primarily to determine the presence or absence of mites. Skin scrapings are best performed using a skin-scraping spatula, which is a thin metal weighing spatula commonly found in pharmacy or chemical supply catalogs. These spatulas are reusable and will not injure patients.
Combing of the Hair Coat
This technique, commonly referred to as “flea combing,” is useful to collect large amounts of skin debris and trap cutaneous parasites. Combings are particularly useful to find fleas, ticks, lice, and some mites. A clean scrub brush or curry comb can be used to collect material into a flat container (eg, pie plate) in large animals.
Examination of Hairs
Microscopic examination of hair shafts can be used to look for evidence of self-trauma, dermatophyte infections (requires clearing agents and special staining), dysplastic hairs, and, sometimes, genetic diseases of the hair coat.
Cutaneous and auricular cytology is helpful to identify bacterial, fungal, and, possibly, neoplastic skin diseases. At least 4–6 impression smears should be made; several slides should be saved for examination at a reference laboratory if necessary. When performing impression smears of the skin, the glass slide should be placed directly over the site to be sampled. An index finger or thumb should be placed directly over the slide and very firm pressure exerted. Alternatively, clear acetate tape can be used to sample the skin. Adequate sampling will produce a “thumb print” from the surface. At least one slide should be heat fixed with a match or lighter before staining. In most cases, a Romanowsky-type stain is adequate. In pruritic patients, material should be scraped from beneath nail beds and smeared onto glass slides for heat fixing, staining, and cytologic examination. Specimens should be examined under 4×, 10×, and oil immersion magnification.
Dermatophyte infections are best identified with a fungal culture on either dermatophyte test medium or on plain Sabouraud agar. Plates that are easily inoculated are preferred; glass, screw-topped jars are difficult to inoculate and obtain samples from and are best avoided. Cats are best sampled using a new toothbrush aggressively combed over the affected lesions. Dogs can be sampled with either a toothbrush or via a hair plucking technique. In large animals, hairs should be gently wiped with alcohol before collecting to minimize contaminant growth. Intermediate and deep fungal organisms are best cultured at a reference laboratory using a skin biopsy specimen (6–8 mm in size).
Intact pustules can be cultured by rupturing the pustule with a sterile needle and swabbing the lesion with a sterile culture swab. Lesions should not be scrubbed before sampling. Deep pyodermas are best cultured from a skin biopsy (6–8 mm). The reference laboratory should be informed as to what pathogens are suspected, because this may affect how the exudate is cultured. Systemic and topical agents should be withheld for at least 72 hr before sampling.
Skin biopsies are indicated in any case that appears severe, unusual, or does not respond to appropriate therapy. Lesions should not be scrubbed before biopsy, because surface pathology is important in the diagnosis of many skin diseases. Several samples from a variety of lesions should be submitted for examination. Primary lesions should be sampled whenever possible; otherwise, the report is often not very helpful in making a diagnosis or narrowing a list of differential diagnoses. Biopsy specimens require examination by a pathologist familiar with skin diseases of animals. Direct immunofluorescence is not necessary to diagnose autoimmune skin diseases; routine histopathology is the test of choice.
Routine Blood and Urine Tests
In most dermatologic cases, these tests do not help to make a definitive diagnosis. If systemic signs of an illness are present, then a CBC, serum chemistry panel, and urinalysis may be helpful to identify the cause. In dogs with recurrent infections, these tests may identify an underlying subclinical disease.
Intradermal Skin Testing
This test is not necessarily required to make a diagnosis of atopic dermatitis. A positive intradermal skin test reaction indicates past exposure to a particular allergen. Inhalant allergies are best diagnosed based on a compatible history, physical examination findings, and judicious use of intradermal skin testing or in vitro testing for allergies. Intradermal skin testing is recommended for animals in which immunotherapy is indicated because of the severity or duration of allergic signs. Potential drug interactions that can interfere with testing should be considered before intradermal skin testing is performed.
In Vitro Diagnostic Tests
In vitro diagnostic tests (ELISA or RAST tests) are an alternative to intradermal skin testing. Although in vitro tests are considered less reliable because of the large number of false-positive reactions, most complications in interpretation are the result of poor patient selection. Like intradermal skin tests, in vitro tests reflect exposure and must be interpreted in light of the patient's clinical signs and history.
Last full review/revision June 2013 by Karen A. Moriello, DVM, DACVD