In-Hospital Diagnostic Testing for Dermatology Patients

Abstract

The successful practice of dermatology relies on performing standardized testing as part of routine consultations. Certain techniques can be used to increase the sensitivity and specificity of such testing, but simply performing them regularly is often sufficient for developing expertise. The tests to be reviewed include skin scrapings, diascopy, trichography, Wood’s lamp evaluation, dermatophyte cultures, cytology, and biopsies for histopathologic assessment.

Skin Scrapings

Skin scrapings are, or should be, the most common diagnostic test performed in veterinary dermatology. A dull scalpel blade or similar instrument is moistened with mineral oil and used to scrape away some of the epidermis, in which may reside a number of different parasites. After the scraping is obtained, the blade is then wiped onto a clean microscope slide and a microscope used to scan the slide for parasites. Parasites that might be recovered on skin scrapings include mites (Demodex, Sarcoptes, Cheyletiella, Otodectes, chiggers) and worms (Pelodera, hookworms, heartworms). The depth of the scraping and the locations to be scraped are determined by the specific parasite suspected. For example, Cheyletiella mites are surface dwellers, whereas scabies mites burrow into the stratum corneum. These require superficial scrapings using broad collections of surface debris. Most Demodex mites are follicular in orientation, so scrapings must be deep enough to recover these parasites, and this means scraping deep enough until there is oozing from the dermis (bleeding need not occur, and in fact, blood in the specimen makes it more difficult to find parasites.)

Diascopy

Diascopy is a technique that is used to determine whether redness in the skin is due to erythema associated with increased blood supply to the area, or whether the reaction is due to blood cells or pigment present extravascularly. It is accomplished simply by taking a microscope slide and pressing it firmly against the red-colored skin. With erythema, the redness disappears as the tissue blanches under the pressure of the slide. With blood cells or pigment present in the tissues, the color remains in the tissue.

Trichography

Direct microscopic examination of plucked hairs can be a very rewarding diagnostic experience. It is particularly helpful in cases of endocrinopathies, follicular dysplasias, telogen defluxion and traumatic hair loss. The hairs are evaluated for integrity of the shaft, stage (anagen, catagen, or telogen), and pigmentation. If most of the hairs have been sheared off, this is likely the result of licking, especially excessive grooming in cats. Damage to the shaft can be seen with several uncommon conditions, but dermatophytosis is the most common cause. It is imperative to understand the stages of the hair cycle when evaluating trichograms. Anagen is the growth phase, and the hair bulb is typically hooked and broad. Catagen is a transitional stage, and the hair bulb is often straight and fist-like. Telogen is a resting stage, and the hair bulb is spearlike, with little there to hold it in place. Hairs transition between stages as part of normal hair growth, as well as a reflection of different disease processes. For example, with endocrinopathies (such as hypothyroidism) many hairs get arrested in the telogen stage, so when samples are collected, telogen hairs predominate. With hair loss involving many follicular diseases (such as demodicosis, bacterial folliculitis, and dermatophytosis), hair loss is noted even while many of the hairs are in anagen. Finally, many of the follicular dysplasias cause hairs to be stuck in the transitional (catagen) stage, so there become more prevalent when hairs are assessed with trichography.

Wood's Lamp (Ultraviolet Light, Black Light) Evaluation

A Wood's lamp uses ultraviolet light filtered through nickel oxide to cause some fungi to glow green in a darkened room. A tryptophan metabolite is the fluorescing material, not the fungi or spores themselves. This metabolite is only seen when the fungus is growing on hair shafts; it is noticeably absent on scale, claws or material growing on a culture plate. Only Microsproum canis fluoresces (also M. distortum, M. adouinii and T. schoenleinii in humans), and then only about 50% of the time. The diagnostic value of the Wood's lamp is limited to a screening test for Microsporum canis. Negative fluorescence does not rule out M. canis because fewer than 50% of these infections routinely fluoresce, and is not useful for the diagnosis of dermatophytosis caused by other organisms, including M. gypseum and Trichophyton mentagrophytes.

Direct Microscopic Examination for Dermatophytes

The microscopic examination of hairs to identify characteristic spores and hyphae is a relatively specialized test in animals and is not commonly performed in general veterinary practices. However, when viewed by an experienced individual, a diagnosis may be rendered about 60 to 70% of the time. The inexperienced may find it difficult to distinguish spores from pigment and hyphae from keratin. In animals, direct microscopic examination can be made with saline or mineral oil alone, potassium hydroxide (KOH), KOH-dimethyl sulfoxide (DMSO), chlorphenolac solution (chloral hydrate, phenol, lactic acid) or a solution of KOH, DMSO, and chlorazol fungal stain. This last stain is often helpful in practice because hyphae stain green against a gray background and thus are clearly visible.

Dermatophyte Culture

Two types of media are used routinely for dermatophyte culture: dermatophyte test medium (DTM) and Sabouraud's dextrose agar. Dermatophyte Test Medium (DTM) is just a form of Sabouraud’s medium to which has been added an antibiotic (such as chloramphenicol), an antifungal capable of inhibiting fungal contaminants (such as cycloheximide), and a pH indicator (phenol red). Since dermatophytes tend to consume the protein in the medium first (whereas many non-dermatophytes consume the carbohydrate first), dermatophytes produce metabolites that often cause a pH change in the medium that in turn leads to a medium color change from amber to red. This is a useful indicator, but on its own is NOT diagnostic. A sample for fungal culture is obtained by selecting individual, representative hairs or scale. Broken hair shafts, hairs that fluoresce with a Wood’s lamp, or scale from nails are satisfactory specimens. A culture positive for dermatophyte growth must show colony growth and color change from yellow to red simultaneously, which can occur from 1 to 14 days but usually occurs within the first 7 days. Everything that turns the medium red is not a dermatophyte. Some non-dermatophytes also preferentially utilize the protein in the medium first, and virtually all fungi will cause color change by switching to protein ingestion once they have utilized the carbohydrate in the medium. All DTM culture media that have fungal growth, whether contaminant or pathogen, change from yellow to red over time.

Yeast Prep

Cytology is a quick and easy way to demonstrate yeasts. Material swabbed, scraped, or collected with clear adhesive tape from the ear canals, skin surface, or interdigital area can be applied to a clean microscope slide and heat-fixed for a few seconds. Samples can also be collected by impression smear, acetate tape collection or skin scrapings. Yeasts are usually seen adequately with the high dry objective, but oil immersion may be necessary to clearly depict them.

Cytology

Cytology is the study of free cells from tissues. Samples may be obtained in various ways, including fine-needle aspiration, impression smear, exfoliative procedures, and swab techniques. Each technique is determined by the ultimate availability of the tissue being sampled. Fine-needle aspiration is used primarily to sample papulonodular and vesiculopustular lesions. Impression smears are used primarily for erosive-ulcerative lesions, or where there is active “ooze” that can be readily sampled. Exfoliative procedures are used to gently scrape cells from the surface of lesions and swabs are used for exudative lesions in areas where impressions cannot easily be achieved (e.g., ears, lip folds, etc.)

Biopsies

Biopsy for histopathologic examination (microscopic tissue evaluation) is especially important in dermatology, since the tissue to be sampled (skin) is so readily accessible. Biopsies are valuable diagnostic tools but should not be expected to tell the entire story. They reveal the changes only in a small region of skin surface at a particular point in time. Biopsies can be taken by excising the entire lesion, by incising into the lesion and removing a representative section or, most commonly, with biopsy punches. When biopsying tissue, it is important to preferentially sample primary lesions or early secondary lesions. The most profound changes, such as ulceration, fibrosis and complete alopecia are less likely to be diagnostic. For this reason, it is best to submit several sections for evaluation, from a variety of different lesions and stages of development. Along with this it is important to submit to the pathologist a reasonable clinical history, along with clinical differential diagnoses.

Recommended Reading:

• Ackerman, L: Atlas of Small Animal Dermatology, Inter-Medica, 2008.
• Hnilica, KA; Patterson, AP: Small Animal Dermatology. A color atlas and therapeutic guide, 4th Edition, Elsevier, 2016
• Miller Jr., WH; Griffin, C; Campbell, S: Muller & Kirk’s Small Animal Dermatology, 7th Edition. Saunders, 2012.
• Helton-Rhodes, K; Werner, AH: Blackwell’s Five-Minute Veterinary Consult Clinical Companion: Small Animal Dermatology, 3 rd Edition. Wiley-Blackwell, 2018.
• Logas, D: Diagnostics and therapy in veterinary dermatology. Wiley-Blackwell, 2022.
• Nesbitt, G; Ackerman, L: Canine & Feline Dermatology, Veterinary Learning Systems, 1998, 498pp.
• Neuber, A; Nuttall, T: Diagnostic techniques in veterinary dermatology. WileyBlackwell, 2017.