Page 2191 - Cote clinical veterinary advisor dogs and cats 4th

1092 Dermatologic Diagnostic Procedures

Possible Complications and ○ Impression smears made from the surface Dermatophyte culture (p. 247):
Common Errors to Avoid of intact lesions or from cut surfaces of • Generally performed in clinics on commercial
VetBooks.ir it is important to clip hair before performing ○ Impression smears made after lancing able in glass jars or flat plates. The culture
dermatophyte test medium (DTM), avail-
surgically excised nodules or tumors
• Skin scrapings: to avoid false-negative results,
medium consists of a Sabouraud-dextrose
skin scrapings. However, it is preferable to
pustules or papules or after lifting or
use scissors when surface mites such as
or collerette
Cheyletiella are suspected because they can gently scraping away a superficial crust agar, antibacterial and antifungal agents to
inhibit growth of contaminants, and phenol
be lost if electric clippers are used. NOTE: A ○ Fine-needle aspiration (FNA) of cells or red (pH indicator).
positive scraping allows the clinician to find material from lesions • Pluck hairs from the edge of newly develop-
and identify a parasitic infestation, but its ○ Smears made by rolling the cotton-tipped ing lesions. Broken or frayed hairs and those
sensitivity in ruling out a diagnosis depends applicator across a glass slide (particularly that fluoresce with Wood’s lamp are the
on the parasitic disease and the aggressiveness useful for ear specimens). Humidify the best specimens. The plucked hairs should
of sampling. tip of applicator when lesions are dry. be firmly pressed onto the surface of the
• Skin biopsies: do not scrub—or wipe or rub ○ Scrapings of surface epithelial cells and medium. Alternatively, vigorous brushing of
with alcohol or antiseptic—the surface of a debris with a dry, dull scalpel blade. This the haircoat with a sterile toothbrush or a
biopsy site before performing punch biopsies. material is then smeared onto a glass slide. small piece of sterile carpet can effectively
Pathologic changes on the skin surface, which • After the specimen has dried, the slide is collect hairs and scales (more useful to
often are critical in making a diagnosis, may stained with a modified Wright’s stain (e.g., identify carriers not showing obvious
be altered or removed. For the same reason, Diff-Quik) and examined microscopically. lesions). Collected material is removed from
do not shave the area of interest; gently clip When a drop of mineral oil is put on the the toothbrush with a sterile hemostat and
the hair with scissors if necessary. This does stained dried specimen and then covered placed into the culture medium. Alternately,
not apply for excisional biopsies. with a large coverslip (e.g., 22 × 40 mm), if plates are used, the DTM is inoculated
visualization of cells and most microorgan- by gently embedding or repeatedly dabbing
Procedure isms with 40× objective is enhanced, and the toothbrush into the medium.
Skin scraping: examination under oil immersion (100× • Ideally, the culture medium should be
• Apply a few drops of mineral oil to the area objective) in not needed in most cases. incubated at 22°C-30°C (72°F-86°F) for
of skin selected for scraping, or coat the dull Wood’s lamp examination: 2 weeks and should be checked daily for
scalpel blade that is used for performing the • This ultraviolet light is a very useful tool for fungal growth. Desiccation hinders growth.
scraping with mineral oil. Broad superficial the diagnosis of dermatophytosis (p. 247). The culture medium should be kept for up to
scrapings that collect scales and crusts should The animal should be examined in a dark ≈3 weeks in animals with antifungal pretreat-
be performed when looking for mites living room. In untreated animals, the majority ment or suboptimal culture environment
on the surface (Cheyletiella, Demodex gatoi) of M. canis lesions fluoresced. This inherent fungal growth. Look for a whitish, fluffy to
or in the superficial layers of the epidermis property of M. canis causes infected hairs (not powdery colony with a red color change in
(Sarcoptes, Notoedres). Deeper skin scrapings scales or crusts) to fluoresce an apple green the medium around the same time that the
must be performed for deep-dwelling mites color. It is a fast and inexpensive screening colony is first visible. Contaminants (envi-
(Demodex). In the latter case, the skin must tool for dermatophytosis. False-positives ronmental molds) eventually turn the media
be squeezed to help extrude the mites from are unfortunately frequent. Glowing hair red; however, colony growth is usually well
the hair follicles first, and the scrapings should be plucked and the proximal end established before any color change appears
should be deep enough to create capillary examined further with the Wood’s lamp in the medium. Most saprophyte colonies
oozing. or used for culture or direct examination are pigmented (black, gray, green, or multi-
• The skin scraping material that is collected on for fungal elements under microscope. A colored). Nonetheless, identification of the
the scalpel blade is smeared on a glass slide; negative examination does not rule out fungi is essential if a suspected dermatophyte
mineral oil is added and a coverslip mounted. dermatophytosis. is grown on culture. Macroconidia must be
Examination is done with 4× (Cheyletiella,
Sarcoptes, Notoedres) or 10× (Demodex) objec-
tive. Demodex mites are part of the skin’s
normal flora, but it is rare to find them on B C
skin scrapings in healthy dogs. If one or two A
mites are found, more deep skin scrapings
should be taken to confirm the diagnosis of
demodicosis. Conversely, numerous negative
skin scrapings from appropriate areas should
reliably exclude demodicosis and notoedric
mange. In areas that are difficult to scrape
(e.g., eyelids, paws), hair can be plucked
and the proximal ends examined under a
microscope for the presence of Demodex
mites.
• Negative skin scrapings (even if several are G
performed) do not rule out sarcoptic mange F
or cheyletiellosis.
Skin cytologic examination:
• Allows microscopic examination of fluid or E H
material collected from nodules, tumors, D
cysts, plaques, draining tracts, ulcers, DERMATOLOGIC DIAGNOSTIC PROCEDURES Material used for performing dermatologic diagnostic
pustules, vesicles, papules, and surface of tests. A, Cover glass. B, Glass microscope slides. C, #10 or #21 scalpel blades. D, Culture swab (collection and
the skin or ears. Several techniques may be transport system). E, Biopsy punches. F, Mineral oil. G, Acetate tape (clear adhesive tape). H, Cotton-tipped
used for obtaining samples: applicators.

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