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Dermatology: Top 5 questions technicians ask about diagnostic testing
Tips on performing skin cytology, dermatophyte cultures, and more.
While veterinarians diagnose a pet's dermatologic issue, veterinary technicians play an integral role in performing and evaluating diagnostic tests. Here are answers to questions technicians commonly ask about collecting samples and running tests related to dermatology.
1. When is skin cytology necessary and how is it done?
Skin cytology—the microscopic evaluation of material collected from the skin—provides valuable information that helps determine:
Technicians can collect samples for skin cytology via direct impression smear using clear adhesive tape to lift the sample, a cotton swab to obtain exudate, or metal spatulas (or something similar) to obtain material from nail beds. Different slides need to be used for different locations and lesions, and the slides should be labeled accordingly.
Figure 1: Microscopic evaluation of skin cytology can reveal Malassezia pachydermatis(Diff -Quik; 100X magnifi cation).
For most samples, other than tape, heat-fix them and then stain them with a modified Wright's (Diff-Quik—Dade Behring) or comparable stain. Dry the slides either naturally or with a hair dryer, a lighter, or bibulous paper (a hair dryer is preferred). Then examine the samples microscopically. Begin at a low magnification to find an area where cellular material is present, and then go to oil immersion for identification.
2. What is the difference between superficial and deep skin scrapings?
The goal of most skin scrapings is to identify parasites, the most common being the mites Notoedres cati, Demodex species (Figure 2), Sarcoptes scabiei (Figure 3), and Cheyletiella species. The scrapings are done differently if the parasites are more superficial in the stratum corneum (surface of the skin) versus deeper in the hair follicles.
Figure 2: A photomicrograph of a Demodex canis adult mite and egg(10X magni ficati on).
Superficial scrapings are obtained using a broad-sweeping scrape, and surface material is often collected from a larger area. Because S. scabiei (causing sarcoptic mange) is usually found on superficial skin scrapings, the prime spots to scrape are the ear margins, elbows, and hocks. Notoedres and Cheyletiella species also can be found in superficial scrapings.
Figure 3: A photomicrograph of Sarcoptes scabiei, the causative mites of canine sarcoptic mange (10X magni fi cati on). The main parasite being evaluated for with a deep skin scraping is Demodex species. A No. 10 or 15 blade (often dulled) is used to scrape the skin (preferably after a mineral oil swab) until capillary oozing occurs. Technicians would typically squeeze the skin before and during the scrape to aid in capillary oozing. The veterinarian will usually decide the ideal place to scrape, as Demodex species can be located anywhere.
Regardless of which scraping is done, examine the entire slide with the condenser turned low to make the mites more refractory. Use a low magnification, such as 10X.
3. What is the proper process for dermatophyte cultures?
Dermatophytes, the fungi responsible for the condition know as dermatophytosis or ringworm, are a common cause of hair loss, redness, itching, and circular crusty lesions in dogs and cats. Three kinds of dermatophytes commonly infect dogs and cats: Microsporum canis, Microsporum gypseum, and Trichophyton mentagrophytes. These fungal organisms can be contagious to people.
A fungal culture is the gold standard for diagnosing ringworm. Collect hair, scale, and crust, and plate them on a dermatophyte culture. Collect these samples with clean, dry hemostats, or use the MacKenzie toothbrush technique—commonly used when screening an inapparent carrier or if the lesions are subtle—to collect hair from all over the body. To perform this technique, run a sterile toothbrush through the hair thoroughly, pull the hair off the toothbrush head, and gently place it onto a plate or slant of Sabouraud's dextrose agar and incubate. Fungal cultures should be stored in a dark place at a consistent room temperature and protected from ultraviolet light and desiccation.
Dermatophytes use protein and produce an alkaline by-product that produces a red color change. But after all the carbohydrates are used, saprophytic contaminants can use the protein and turn the media red over time as well. For this reason, dermatophyte test medium cultures should be inspected daily, and any suspected fungal growth should be lifted with clear plastic tape and stained with lactophenol cotton blue to look for characteristic macroconidia, the most reliable method for fungal identification. You should always perform speciation of the dermatophytes to determine the source of infection to help prevent reinfection. A red color change without identification of macroconidia is not a positive result.
4. How useful is a Wood's lamp?
A Wood's lamp emits ultraviolet light that is filtered through cobalt or nickel. It is used to identify the fungal dermatophyte M. canis (i.e. ringworm). Wood's lamps are useful tools for initially screening patients, for selecting the hairs to perform a fungal culture, and in monitoring a patient that is receiving treatment for dermatophytosis. (Note: It is best to let the lamp warm up for five minutes before use, and animals should be examined in a dark room.)
In 30 percent to 50 percent of M. canis cases, the infected hairs will fluoresce a characteristic candy apple green color under the Wood's lamp (Figure 4). It is important to realize that the other two common dermatophytes in small animals, M. gypseum and T. mentagrophytes, do not fluoresce. Also note that in many cases (50 percent to 70 percent), M. canis does not fluoresce; that is, a negative Wood's lamp test result does not rule out dermatophytosis.
Figure 4: A Woodâs lamp can be used to identify Microsporum canis infection (ringworm). Infected hairs will fl uoresce, but a negative result does not rule out ringworm infection.
Furthermore, false positive results can occur and typically are caused by topical medications or dead skin cells. (Note: The hair should be glowing, not the skin.) Pets that have a positive result on a Wood's lamp test always should have the dermatophytosis diagnosis confirmed with a fungal culture.
5. What do clients need to know about allergy testing for atopic dermatitis?
Accurate allergy testing is currently available for animals with atopic dermatitis (environmental allergies) but not food allergic dermatitis. It is important to realize that atopy is a diagnosis of exclusion, meaning that all other causes of the allergy must be ruled out first. Then allergy testing to identify the specific allergen can be attempted. In other words, allergy testing is performed after the diagnosis is made rather than to make the diagnosis.
After the veterinarian diagnoses atopic dermatitis, allergy testing is performed to determine what the animal is allergic to in the environment, ideally so the animal can start receiving allergen-specific immunotherapy (allergy vaccine). Allergy testing can be performed via intradermal skin testing or serum serologic testing.
Intradermal skin testing is considered the gold standard, but the blood serology test can be a valuable tool when skin testing can't be performed. Because of the expense and labor involved in maintaining the allergens, only veterinary dermatologists or practitioners with a large caseload of owners willing to pursue immunotherapy typically perform it. Before testing, animals should have stopped receiving injectable corticosteroids for six to eight weeks; oral corticosteroids for one month; and antihistamines, fatty acids, and topical corticosteroids for two weeks. Serum serologic testing can be performed while animals are receiving these medications.
Dr. Ghubash, DACVD, is a veterinarian at the Animal Dermatology Clinic in Marina del Rey, Calif.