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 months. 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:
The primary complaint
Length of time the problem has been present
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 years old)
Breed (breed predilections include a predisposition of Cocker Spaniels to primary disorders of keratinization, and of terriers to atopic dermatitis)
Presence and severity of pruritus (including licking, rubbing, scratching, or chewing behaviors—owners often do not realize licking may be a sign of pruritus)
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])
Type and progression of lesions noted by the owner
Evidence of seasonality (suggesting fleas, allergic skin disease, or weather-related diseases)
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])
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)
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)
Presence of fleas, ticks, or mites
Other contact animals (ie, evidence of contagion, which suggests fleas, scabies, cheyletiellosis, or dermatophytosis)
The environment of the animal (housing changes can influence the development of certain skin diseases, eg, contact dermatitis, contagious diseases)
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)
Whether the animal is receiving routine parasiticidal drugs, and if so what class and how frequently
A complete physical examination should always be performed to help diagnose a skin disease. Many skin diseases are manifestations of systemic diseases Miscellaneous Systemic Dermatoses , eg, hypothyroidism, hyperadrenocorticism, hepatocutaneous syndrome, systemic lupus erythematosus. 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 skin lesions include:
macules or patches (nonelevated areas of discoloration)
papules or plaques (elevated lesions, the latter coalescing)
vesicles or bulla (fluid-filled lesions; bulla are bigger)
wheals (flat-topped, steep-walled, solid elevations of the skin arising from histamine release)
nodules or tumors (large, solid elevations of the skin).
Secondary skin lesions include:
epidermal collarettes (late stage of a pustule)
excoriation (areas of self-trauma)
erosions or ulcers (loss of the epidermis)
lichenification (increased thickening and hyperpigmentation of the skin)
Some skin lesions may be either primary or secondary, depending on the cause of the disease. These include:
scale and/or crusts
follicular casts (plugging of hair follicles with visible keratin)
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 do not cause injury.
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.
Hair trichograms are part of the basic database for all skin diseases. These are increasingly being used in place of skin scrapings because the technique has been found to provide similar results. Hair trichograms are performed by grasping a group of hairs in the target hair with forceps close to the base (holding the forceps at 90° to the hairs) and gently plucking hairs in the direction of growth. Hairs are then mounted in mineral oil. It is important to use a coverslip. Microscopic examination of hair shafts can be used to look for evidence of mite infestations, dermatophyte infections, dysplastic hairs, and sometimes, genetic diseases of the hair coat. Note that clearing agents are not needed.
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. Heat fixing is no longer recommended because it has been shown not to increase visibility of yeast. Instead, the number of dips or time the slide is put into the "blue" stain should be increased. In most cases, a Romanowsky-type stain is adequate. In pruritic animals, material should be scraped from beneath nail beds and smeared onto glass slides, stained, and examined cytologically. Specimens should be examined under 4×, 10×, and oil immersion magnification. For acetate tape samples, specimens should be stained by holding an edge with forceps or a clothes pin, avoiding the fixative step, staining as usual, allowing to dry, and then mounting the specimen over a drop of immersion oil and examining. The tape should not be affixed to a glass slide and then stained, which results in poorly stained samples.
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. Animals are best sampled using a new toothbrush aggressively combed over the affected lesions. Plates can be incubated at room temperature and finalized at day 14 of culture.
Intact pustules can be cultured by rupturing the pustule with a sterile needle and swabbing the lesion with a sterile culture swab. If an intact pustule cannot be found, the tip of the swab should be moistened with the transport medium, aggressively swabbing the lesional area and being sure to rotate the swab 360° so the entire swab surface is used. 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 hours 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. A 6-mm or 8-mm biopsy punch should be used because samples may shrink as much as 50% in formalin.
Routine Blood and Urine Tests
In most dermatologic cases, routine blood and urine tests do not help to make a definitive diagnosis. If systemic signs of 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
An intradermal skin 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 animal’s clinical signs and history.
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