Page 1455 - Clinical Small Animal Internal Medicine

157 Approach to the Patient with Dermatologic Disease 1393

an appropriate number of deep skin scrapes along with (punch biopsy technique is described in detail below).
VetBooks.ir the sampling of appropriate lesions, a negative skin Local anesthesia is accomplished by infusing 1–2 mL of
2% lidocaine subcutaneously using a 25 gauge needle.
scrape can rule out D. canis and D. cati. However, the
diagnosis of pododemodicosis may require biopsy and
ficially as it can inhibit microbial growth within the
certain breeds require biopsy to confirm a diagnosis, Care must be taken not to inject the lidocaine too super-
particularly shar‐peis. The identification and treatment specimen. When in doubt, a ring block may be more
of parasitic diseases will be discussed in Chapter 165. appropriate, especially if subcutaneous tissue is also
going to be submitted. Sterile instruments should be
Culture used as well as gloves. The epidermis is then aseptically
Bacterial Culture removed with a No. 10 surgical scalpel and the remain-
In this age of methicillin and multidrug resistance, bacte- ing deep dermal tissue is used for culture. Removal of the
rial culture and sensitivity has become a more routinely epidermis prevents overgrowth of the tissue culture by
implemented test in patients with superficial bacterial surface bacteria that are not relevant to the deep infec-
folliculitis. While performing an aerobic bacterial culture tion. The dermal tissue is placed in a sterile red‐top tube
is never contraindicated, there are some instances which with just enough nonbacteriostatic sterile saline to keep
make it a necessary test. These include the following: the sample moist, and refrigerated and shipped over-
poor response and the emergence of new lesions in night to the laboratory. The tissue sample can be submit-
●
spite of appropriate antibiotic therapy ted for aerobic, anaerobic, and mycobacteria culture
patients with chronic recurrent pyoderma, especially purposes. The laboratory should be notified of the dif-
●
those with a history of multiple courses of antimicro- ferential diagnoses, as culture media adjustments will be
bial therapy made to facilitate the growth, isolation, and identifica-
when rods or unusual organisms are present on cytology tion of the suspected organisms.
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in cases of deep pyoderma
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Fungal Culture
In cases of superficial bacterial folliculitis, pustules are Fungal Culture for Deep Mycoses When deep fungal
the ideal lesion to sample; however, epidermal collarettes infection is suspected, samples are obtained using the
and crusts can also be sampled. The skin is not cleaned same principles as described above for bacterial cultures
prior to sampling. The pustule is gently lanced with of deep infection. Laboratories should be contacted in
a sterile 25 gauge needle and a mini‐tip culturette is advance of the differential diagnoses so that modifica-
touched to the purulent material, taking care to avoid tion of culture techniques can be implemented in
contact with the surrounding nonlesional skin. If crusts advance, not only for diagnostic purposes but also
are present, the culture should be obtained from under- because of the human health risks associated with vari-
neath the crust. When epidermal collarettes are present, ous fungal diseases. Laboratories should also be con-
the sample should be obtained from the leading edge sulted regarding special requirements for tissue
where the epithelium is lifted to avoid surface contami- submission. Depending on the suspected organism,
nants. The culturette is then placed back into the trans- additional tests beyond culture may be required, such as
port media device and refrigerated until sent overnight polymerase chain reaction (PCR) testing.
to the laboratory. Mini‐tip culturettes are commercially
available through most microbiology laboratories. Dermatophyte Culture In cases of suspected dermato-
Bacterial cultures of deep infection can vary somewhat phytosis, cultures (considered the gold standard for diag-
depending on the type of lesion being sampled. Fine nee- nosis) can be performed in hospital using dermatophyte
dle aspiration can be performed to obtain specimens test medium (DTM) or, alternatively, samples may be sub-
from bullae and dermal or subcutaneous lesions with mitted to a diagnostic laboratory. The clinician should
pockets of fluid. Tissue cultures using sterile technique check with their laboratory regarding the preferred means
are best when a nodule, tumor or cellulitis is present. It is of sample submission. Dermatophyte test medium is most
also ideal to obtain a tissue culture in the presence of useful because the red color indicator is a helpful diagnos-
deep infection characterized by exudative draining tic aid and it also contains inhibitors of bacterial and sap-
tracts, rather than merely swabbing any drainage, which rophytic contaminants (e.g., Sab‐Duets, Hardy
will often be contaminated with superficial organisms Diagnostics, Santa Maria, CA, USA). Sab‐Duets are also
while deeper organisms may be missed. popular in that they are double plated: one side contains
The surface should be aseptically prepared by gently DTM and the other side contains plain Sabouraud dex-
swabbing the area with 70% alcohol and can be followed trose agar. Dermatophyte test medium may decrease
by a rinse with nonpreserved saline. Specimens are development of conidia, and therefore it is best to perform
obtained via punch biopsy from nonulcerated areas microscopic identification using the sample plated onto