Diagnosis of Skin Diseases in Small Animals

Skin scrapings are part of the basic database for all skin diseases. There are two types: 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. They are best performed using a skin-scraping spatula, a thin metal weighing spatula commonly found in pharmacy or chemical supply catalogs. Skin-scraping spatulas are reusable and do not cause injury.

Hair combing, commonly referred to as “flea combing,” is useful for collecting large amounts of skin debris and trapping cutaneous parasites. Combing is particularly useful for finding fleas, ticks, lice, and some mites. For large animals, a clean scrub brush or curry comb can be used to collect material into a flat container (eg, pie plate).

Hair trichograms are part of the basic database for all skin diseases. Trichography is increasingly used instead of skin scraping because the technique has been found to provide similar results. A group of hairs is grasped with forceps close to the base (with the forceps held at 90° to the hairs), and hairs are gently plucked in the direction of growth. The plucked hairs are then mounted in mineral oil. It is important to use a coverslip.

Microscopic examination of hair shafts can reveal evidence of mite infestations, dermatophyte infections, dysplastic hairs, and sometimes genetic hair diseases. Clearing agents are not needed.

Cutaneous and auricular cytological examination 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 impression smears of the skin are performed, 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 for cytological skin samples because it has been shown not to increase the visibility of yeast. Instead, the number of dips or the amount of time the slide is in 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 4X, 10X, and oil immersion magnifications.

For acetate tape samples, specimens should be stained by holding the edge of the tape with forceps or a clothespin (see acetate tape staining image), skipping the fixative step, staining as usual, allowing to dry, and then mounting the stained specimen on the tape over a drop of immersion oil on a slide for examination. The tape should not be affixed to a glass slide and then stained, which results in poorly stained samples.

Acetate tape staining

Image

Courtesy of Dr. Karen Moriello.

Dermatophyte infections are best identified with a fungal culture either on dermatophyte test medium or on plain Sabouraud agar. Plates that are easily inoculated are preferred; glass, screw-top jars are best avoided because they are difficult to inoculate and to obtain samples from.

Animals are best sampled using a new toothbrush to aggressively comb 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 and then used to aggressively swab the lesion area, being sure to rotate the swab 360° to use the entire swab surface. Lesions should not be cleaned before sampling.

Deep pyodermas are best cultured from a skin biopsy (6–8 mm). The reference laboratory should be told which pathogens are suspected, because this information can affect how the exudate is cultured. Systemic and topical agents should be withheld for at least 72 hours before sampling.

Skin biopsies are indicated when clinical signs appear severe, are unusual, or do not respond to appropriate treatment. Lesions should not be cleaned 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 for making a diagnosis or narrowing a list of differential diagnoses.

Biopsy specimens from animals require examination by a veterinary pathologist.

Direct immunofluorescence is not necessary to diagnose autoimmune skin diseases; routine histological examination is the test of choice. A 6-mm or 8-mm biopsy punch should be used because samples can shrink by up to 50% in formalin.

In most dermatological 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 could be helpful to identify the cause. In dogs with recurrent infections, these tests might identify an underlying subclinical disease.

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.

Diagnosis of inhalant allergies is best 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 the severity or duration of allergic signs suggests that immunotherapy may be warranted.

Potential drug interactions that can interfere with testing (eg, topical or systemic glucocorticoids, oral antihistamines) should be considered before intradermal skin testing is performed.

In vitro diagnostic tests (ELISA or radioallergosorbent test [RAST]) are alternatives 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.