Page 1456 - Clinical Small Animal Internal Medicine

1394 Section 12 Skin and Ear Diseases

Sabouraud dextrose agar. The most common dermato- Contaminants are also likely when there is a color change
VetBooks.ir phytic agents include Microsporum canis, Microsporum but the colony is pigmented. Cultures should be incu-
bated for three weeks before declaring them negative to
gypseum, and Trichophytan mentagrophytes.
When procuring samples for DTM, hairs should be
Cultures grossly suspicious for dermatophyte growth
plucked from within the lesion close to the periphery. A ensure that there is no late growth of a pathogen.
medical‐grade plug‐in long wave Wood’s lamp with should be evaluated microscopically. The sticky side of
magnification can be helpful for identifying fluorescing clear acetate tape is gently brushed against the colony
hairs that can be selected for culture in approximately growing on Sabouraud dextrose agar and placed over
50% or more of cases caused by M. canis and rarely several drops of lactophenol cotton blue stain on a glass
with M. gypseum. The Burton Ultraviolet (Wood’s) slide. The sample should sit for 5–10 minutes before
Exam Light (Philips Burton, Franklin Park, IL, USA) is examination. This allows for proper absorption of the
an excellent model as it provides magnification, and stain by the spores and thus better visualization. The
replacement bulbs can be purchased. Medical‐grade sample is then ready to be evaluated with the ×10 and
Wood’s lamps do not require a period to warm up, but ×40 objectives. The macroconidia of M. canis are spin-
the room should be darkened and the evaluator allowed dle‐shaped and have thick, spiny walls that have six or
to let their eyes adjust for 2–3 minutes. The lamp should more divisions with a knob at the terminal end. M. canis
be held close to the skin (i.e., 4–10 cm). Infected hair does not produce as many macroconidia as M. gypseum,
shafts will fluoresce a bright apple green. These are which are thin‐walled and ellipsoid with rounded ends
often hiding under crusts, and thus it is important to lift having less than six divisions. Microscopic evaluation of
crusts and examine the hairs beneath. Fluorescing hairs T. mentagrophytes will typically reveal large numbers of
are then plucked for culture and can also be evaluated spherical microconidia, rare cigar‐shaped macroconidia,
by direct examination to confirm an infection (ectothrix and spiral hyphae. The website https://drfungus.org/
arthroconidia may be observed). Besides directly pluck- contains helpful information regarding the macro‐ and
ing hairs for culture purposes, the MacKenzie tooth- microscopic evaluation of dermatophytes.
brush technique can be used. Individually packaged
toothbrushes can be purchased readily online. This Diagnostic Treatment Trials
method is especially helpful for sampling poorly cir-
cumscribed lesions and nonlesional patients when look- Treatment trials are performed often in veterinary derma-
ing for asymptomatic carriers. A newly unwrapped tology and are used to confirm a suspected diagnosis
toothbrush is used to brush the lesions (or the entire where the response to therapy is either the only or the best
patient in asymptomatic animals), and the scales and way to diagnose the possible underlying disease. Examples
loose hairs caught in the bristles are gently embedded of this include the improvement or elimination of clinical
onto the agar. A sterile needle or hemostat can also be signs with insecticidal therapy in presumptive cases of flea
used to remove trapped hair and scale from the tooth- bite hypersensitivity, sarcoptic mange, and D. gatoi infes-
brush for placement onto the culture medium. tation in cats. In other cases, confirmation of the underly-
After inoculation, the culture plate should be placed in ing disease is made when a relapse is observed after
a sealable plastic baggie to prevent contamination and discontinuing the trial with subsequent resolution on
dehydration, and put in an incubator. The incubation restarting the trial, as is the case when food elimination
temperature should be maintained at 75–80 °F and the trials are performed for diagnosing adverse food reac-
humidity should be approximately 30%. A digital aquar- tions. Another example of this concept is the use of gluco-
ium thermometer can be used to monitor the tempera- corticoids to support a diagnosis of allergic dermatitis
ture, and placing a pan of water in the incubator should when other infectious and parasitic diseases have been
be enough to sustain adequate humidity. The plates ruled out. Further discussion of specific treatment trials
should be kept away from sunlight or fluorescent light. can be found in the relevant chapters of this section.
Cultures should be examined daily and color change
(which occurs due to a change in the pH of the media) Biopsy and Histopathology
and colony growth findings must be recorded. Pale, buff‐
colored to white colonies that simultaneously develop a There are several indications for performing skin biopsy
red ring of color around them as they grow are indicative early on in the clinical work‐up. These include the pres-
of dermatophytes. The colonies are typically cottony or ence of unusual lesions or lesions that evolve dramati-
velvety and growth will often occur in 4–7 days. A con- cally and/or severely, as in the case of a possible cutaneous
taminant is likely when there is colony growth and adverse drug reaction. Erosive to ulcerative draining der-
no associated color change of the media or the color matoses, especially those that are nodular, also should be
change appears more than 10 days after colony growth. biopsied early on. At any point when there is concern of