Simple diagnostic tools in veterinary dermatology (Proceedings)

Apr 01, 2010

Skin and ear problems are very common reasons for dogs and cats being presented to a veterinarian. These animals can suffer from many different skin diseases with a wide range of underlying causes. Because the skin has a limited range to react to the different insults a straight-forward diagnosis is commonly not possible, especially in patients with chronic skin diseases. To identify the underlying diseases it is crucial that a thorough history is obtained followed by a series of diagnostic tests. Although many of the tests used in veterinary dermatology appear simple, it still requires that microscopes are properly used, slides are stained sufficiently, and samples are handled correctly. However, you should always keep in mind that tests are rarely 100% reliable. This is especially true in patients with obvious lesions but negative test results. For this reason it is important to take the time to perform these tests properly and to become experienced in evaluating your test samples so that you and your veterinarian will be a successful team.

Methods of finding ectoparasites

     A. Flea combing/coat brushing
          - Indication: pruritic animals, seborrheic skin, previous history of fleas
          - Look for: Fleas, lice, Cheyletiella, and their eggs
          - Combs should be washed after each use and kept in a container with a disinfectant to prevent contamination and transmission of infectious diseases (e.g. dermatophytosis)

     B. Paper towel test (only fleas)
          - Look for: flea dirt in material obtained from brushing
          - It is not unusual that fleas cannot be seen, but flea dirt very often indicates a situation of flea infestation if an animal has an infestation flea dirt (feces) can be found in most cases. The feces is composed of digested blood which explains, that it dissolves in water and leaves behind red-brownish stains discoloration on white paper tissues.

     C. Skin scraping
          - Indication: Pruritus, alopecia, seborrhea, papulo-pustular eruptions, erosions
          - Method: Use a #10 blade dip the blade in mineral oil or apply a drop onto the skin transfer the sample onto a microscope slide put a cover slide on top microscopic examination at x4-x10 with the condenser turned down
          - Look for: Demodex, Cheyletiella (broad head with 2 large hooks), Sarcoptes (short legs with long stalks), Notoedres, Otodectes (ear mites; long legs and very short stalks), Chigger mites (3 pair of legs)
          - Demodex: because these mites live in hair follicles deep skin scrapes are necessary. Sometimes these mites are located deep in the hair follicle (especially in the skin of the feet) which makes it hard to find them. Since Demodex mites are a part of the skin flora, it usually requires the presences of multiple mites to consider the findings relevant. Squeezing the skin prior to scraping increased the chance of finding the mites.
          - Other parasites: these parasites live on the skin surface but tend to be present in lower numbers than Demodex (the presence of one single parasite is diagnostic!). For this reason multiple superficial and larger areas should be scraped

     D. Unstained adhesive tape strips
          - Indication: Pruritus or scaling
          - Look for: Lice, Cheyletiella ("Walking dandruff"), and Chigger mites

     E. Ear smear for otic parasites
          - Indication: Otitis, pruritic ears with brown-black crumbly discharge
          - Look for: Otodectes, Demodex (rare)

     F. Cytology
          - Indication: Pruritus, seborrhea, pustules, ulcers, draining tracts, non-healing wounds, otitis or lumps/swelling
          - Techniques: Cotton swab (draining tract, ears), acetate tape impression (especially for dry, difficult to access lesions), direct impression smear, fine needle aspiration (mass, tumor, abscess, large vesicles/pustules), skin scraping. Most common in-hous stain is Diff Quick. High power (x100 with immersion oil) is used to evaluate microorganisms. Evaluate 5-10 HPF (high power fields) and score them either by averaged number of organisms or by giving a score of + to ++++.
          - Look for: Bacteria (cocci and rods), Malassezia yeast, other fungal organisms (e.g. Sporothrix, Blastomyces), acantholytic cells (e.g. Pemphigus foliaceus), inflammatory cells (e.g. neutrophils, macrophages, eosinophils, mast cells), and neoplastic cells