Essentials of dermatological diagnostic test (Proceedings)


Essentials of dermatological diagnostic test (Proceedings)

Nov 01, 2010

There are a limited number of tests a veterinary practitioner will be required to perform when presented with a patient with skin disease. For some of these tests, subtle and simple techniques can influence the accuracy of the results.

Cytology: Cytology is an invaluable test for a veterinarian to master. Any primary, and many secondary lesions such as papules, pustules, crust and exudate can provide helpful information. Staining with Diff-Quik® is usually adequate for most in-house cytologies. A number of methodologies are used to obtain samples, including aspiration, tape preps, swabbing, impression smears, and scraping of surface material. Some of the more important lesions to sample and what to consider include:

Pustules: Neutrophils and possibly eosinophils will be present. Are bacteria present? If so, what is the shape (rod vs. coccoid) and are they intracellular, extracellular or both? Are acantholytic cells present which would be suggestive of pemphigus foliaceus?

Nodules: Fine needle aspiration or multiple insertions of a 20-22ga needle into a nodule can yield cells and help distinguish neoplasia from inflammatory disease. If a homogenous population of cells such as lymphocytes, mast cells etc are seen then biopsies to evaluate for neoplasia are indicated. A mixed inflammatory response could direct the veterinarian to also consider infectious or sterile inflammatory conditions. Any bacterial or deep fungal infection could cause nodule formation. Sterile granuloma, sterile histiocytosis, and sterile panniculitis are examples of conditions which could cause nodule formation without microorganisms.

Exudate: Cytology of exudate can be enlightening regarding potential infectious causes. Exudate is often found in ears, interdigital areas, and the ventral neck region.

Crust: Gentle removal of the crust and sampling the material under the crust can be useful in obtaining information regarding the cause of the crust.

When microorganisms are present, it is helpful to quantify the numbers so that response to therapy can be monitored. We use a 1 through 4 plus system as defined
      1+ Organisms are rare to scattered at oil power
      2+ Many organisms in every field
      3+ Organisms difficult to count
      4+ Complete coverage of the field with organisms

Hair-pluck (trichogram)

In our practice we will pluck hairs for three reasons. Ectothrix hyphae of dermatophytes may be seen although this requires practice and should not be used alone to diagnose dermatophytosis. Plucking hair and placing in mineral oil can be useful in recovering demodex mites, especially in hard to scrape regions such as interdigital areas. Finally patients with color dilution alopecia will show melanin clumping within the hair shaft resulting in deformation of the medulla and cortex of the hair.

Skin scrapings

Skin scrapings are another commonly performed diagnostic test, and is performed to recover mites. Simple techniques to improve chances of recovery of mites start with shaving the area to be scraped. Otherwise the hair will deflect the blade off the skin, resulting in false negative scrapings. Placement of mineral oil on the skin increases the recovery of mites, otherwise the scraped material is to dry to effectively "pick up" mites. When scraping for demodex, gentle "pinching" of the skin before scraping may force mites in the follicle closer to the surface. Scrape to a depth until capillary bleeding occurs, but avoid vigorous or excessive pressure. When scraping for Sarcoptes there will often be excessive surface crust and debris present on the skin, and subsequently on the slide. Examine this debris carefully. Mites may be more motile after the slide has sat on the microscope for a minute and warmed the contents of the slide. Scraping skin with papules can also be a good area more likely to yield mites. Demodex injai inhabits sebaceous glands in canine skin, so scraping the dorsal trunk where it is most greasy will be necessary to recover this particular strain of Demodex. The numbers of recovered D. injai mites will be lower compared to D. canis populations.

Because of increasing prevalence of methicillin resistant Staphylococcus it is advisable to keep spatulas or dull scalpel blades in a cold disinfectant solution such as benzalkonium chloride with an added anti-rust ingredient