Tufts University School of Veterinary Medicine, Dermatology & Allergy Services, Walpole, MA, USA; Bizvet, Inc., Westborough, MA, USA
© 2005 Lowell Ackerman DVM DACVD MBA MPA [No part of this material may be reproduced or distributed without express written consent of author].
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).
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.
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 M. 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, a chlorazol fungal stain. This last stain is often helpful in practice because hyphae stain green against a gray background and thus are clearly visible.
Two types of media are used routinely for dermatophyte culture: dermatophyte test medium (DTM) and Sabouraud's dextrose agar. 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. All DTM culture media that have fungal growth, whether contaminant or pathogenic, change from yellow to red over time.
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 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.
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.
1. Nesbitt, G; Ackerman, L: Canine & Feline Dermatology, Veterinary Learning Systems, 1998, 498pp.