Why should I perform dermatologic diagnostics?
There are many reasons to perform dermatologic diagnostics: to guide therapeutic choices, to make decisions regarding further
diagnostic testing, to judge response to therapy as well as to actually make a diagnosis! Incorporating these tests into
a busy practice schedule mandates that these tests be quick and easy to perform, useful information must be gained in an efficient
manner and the results must be easily interpreted within the medical record. The basic and most commonly used tests include
skin scrapings, surface cytology (ears, skin) and trichograms.
When appropriately performed, negative skin scrapings will rule out canine demodicosis. Skin scrapings for demodicosis are
focused on lesional areas (select a minimum of 3 lesions consistent with folliculitis) and are deeper than those for surface
dwelling parasites. In contrast, broad superficial scrapings are performed for sarcoptic mange; negative skin scrapings in
this instance DO NOT rule out scabies as a differential diagnosis for intense pruritus. In order to maximize results when
performing superficial skin scrapings to locate scabies mites, focus on heavily crusted areas such as elbows or ear margins.
Cytology is your friend! In order to make this sampling technique useful, apply a few rules.
• Only sample areas where, depending upon the results, you will change your therapeutic plan.
• This technique is most useful for surveying areas for Malassezia spp. organisms. A negative surface cytology sample does
not rule out a bacterial pyoderma since most organisms are below the surface layers of the skin.
• Use an adhesive technique to maximize organism recovery, such as clear acetate tape or Durotak® adhesive slides manufactured
by Delasco. **These preparations are not placed into the fixative solution during the staining process.
• If the area is moist or excessively greasy, firmly press a plain glass slide to the area and heat fix prior to staining
with Diff Quik® stain.
• Roll out the swabs from both ears on a single slide to minimize staining and microscope time: place the sample from the
left ear on the left side of the slide and from the right ear on the right side of the slide. Remember to heat fix these
prior to examination under 1000x magnification with immersion oil.
Papule/ Pustule cytology
This technique is very useful when differentiating infectious causes for papules/ pustules from allergic papules and
pustules caused by autoimmune disease (pemphigus foliaceus).
• Degenerate neutrophils with intracellular cocci = requires oral and/or topical antibiotic therapy.
Nondegenerate neutrophils without bacteria, but with acantholytic keratinocytes = requires skin biopsy for histopathology
and culture (aerobic and fungal).
• Eosinophils without evidence of organisms = most likely a lesion associated with an allergic response, such as insect exposure.
• Some neoplasias can appear papular prior to forming nodules—consider skin biopsy for histopathology if the cells do not
appear consistent with an inflammatory response.
• Deeper lesions associated with a deep pyoderma, such as those that drain or bleed easily may reveal pyogranulomatous inflammation.
Any cause of follicle rupture resulting in furunculosis can create this lesion. Both deep pyoderma and fungal kerions can
be the cause for abundant neutrophils and macrophages found on cytology. If intracellular cocci and eosinophils are also
present, this implies a foreign body reaction, such as the reaction to free hair and keratin found with interdigital lesions
of pododermatitis, which will require weeks to months of antibiotic therapy. If bacteria are not identified, consider a fungal
culture to rule out dermatophytosis as an infectious cause.
Every lump and bump deserves a fine needle aspirate. The majority of these lesions will also require biopsy for histopathology
in order to make a final diagnosis, however it is helpful to be able to anticipate a diagnosis of neoplasia or something infectious
or immune-mediated (sterile) in advance. A mast cell tumor is typically easily identifiable and proper surgical plans can
be made prior to removal, whereas a sterile or infectious cause will require aerobic and fungal cultures in addition to histopathology
to make the diagnosis. **Tip—a sterile disease cannot be declared sterile if cultures have not been performed.
This technique is as simple as pulling a few hairs from lesional sites, laying them out straight in a drop of mineral oil
and covering with a cover slip. The hairs are then examined with the 10x objective, just like skin scrapings. Hairs with
ectothrix spores, indicative of a dermatophyte infection (usually Microsporum canis) can be identified this way, indicating the need for fungal culture. In addition Demodex spp. mites can also be found clinging to the hairs in affected animals. This is very useful when the only lesions of folliculitis
are present around the mouth or eyes where skin scrapings can be technically challenging.