What can I possibly teach you about that most basic test in veterinary dermatology, the skin scraping? Taking a skin scraping seems like a simple enough technique, yet at least once per month I am able to find a mite on a pet that previously had a “negative” scraping by their primary care veterinarian. In this first installment of the SkinVet Blog, I’m going to share 5 tips for more productive skin scrapings.
Tip #1. Adjust your technique for the type of mite you suspect. Demodex canis, Demodex cati, and long-bodied Demodex injai all live in hair follicles and sebaceous ducts. They require deep skin scrapings, forcing the mites to the surface by pinching and going deep enough to get capillary bleeding.
Demodex gatoi of cats and the mite tentatively named Demodex cornei of dogs are short, stubby mites that live superficially and may be more scarce. You should use a superficial skin scraping technique, covering a large area of skin, but not going as deeply. Similarly, Sarcoptes scabiei, Notoedres cati and Cheyletiella spp. live superficially and you need only scrape superficially, but over a larger area.
Tip #2. When suspicious of Sarcoptes, scrape the ear margins, even if they are minimally involved or unaffected. The margin of the pinnae is the place Sarcoptes go to party. I’ve had more positive scrapings from ear margins than all other body sites.
Tip #3. Learn to recognize Sarcoptes and Demodex eggs. Sarcoptes are hard enough to find on scraping, but finding even one oval egg is diagnostic, and enough to send me jumping for joy. Demodex eggs are described as fusiform, sort of like a deflated football. Finding a lot of Demodex eggs on follow up scrapings of a patient that you hoped and thought was improving is a downer. It means the mites are still feeling well enough to be amorous.
Tip #4. Adjust the condenser of your microscope down to increase contrast. This is especially important for mites with rather faint exoskeletons, like D. gatoi. They can be nearly transparent if the light is too bright and contrast is low. Recall, this cause of feline pruritus and alopecia is often rather scarce and hard enough to find. If suspected but not found, you’re going to have to do lime sulfur dips weekly to rule it out. Don’t stink up your clinic or your client’s home because you forgot to lower the condenser!
Tip #5. OK, this last one is more of a pet peeve of mine and has nothing to do with the results. If you’re still doing it, it’s time to ditch the old-school technique of re-using scalpel blades under the guise that it is better if they are dull. If your dermatologist came at you with the option of using a dull, used scalpel blade or a new one (that she could easily dull before using), which would you choose? If you are in need of something really dull, I recommend a stainless steel spatula. These work great around eyelids and other delicate areas, especially when scraping squirmy dogs. Every pug should come equipped with one.
Do you have a skin scraping tip to share?
Next week’s blog will be on diagnosing atopic dermatitis.
Jon Plant, DVM, DACVD