Dermatology Diagnostics: Skin Scrapes, Hair Plucks, and More
Chris Reeder, DVM, DACVD
Blue Pearl Veterinary Partners, Franklin, Tennessee
Parasites, such as chiggers and scabies-causing mites, can cause tremendous itching. Fungal organisms, such as dermatophytes, can affect the hair, resulting in fracturing and large areas of crusting, nodules, or excoriations. Not all dermal problems come from external threats; various breed-related issues, such as color-dilution alopecia, pattern baldness, and many autoimmune diseases, can develop over time. Normal inhabitants of the skin (eg, Demodex mites) can also create problems for many dogs. This article looks at how to investigate some dermal conditions in cats and dogs.
Skin scrapes are typically performed with a #10 scalpel blade and are either deep or superficial (Figure 1). The key to success is to sample multiple sites and evaluate the microscope slide thoroughly in an orderly fashion (eg, scan in a “down, across, then down, across” pattern). Although skin scrapes are helpful in diagnosing external parasites, false-negative results are possible. Chinese shar-peis may require a skin biopsy to determine the presence of Demodex mites.1 Scabies mites on dogs are found only 20% to 50% of the time, depending on the number of sites scraped.2
Deep Skin Scrapes
Deep scrapes are largely used to diagnose dogs with Demodex canis mites and cats with Demodex cati mites; however, more surface-oriented mites, such as scabies mites or Cheyletiella species, may also be found.
- Place a few drops of mineral oil on a glass microscope slide.
- Hold a #10 scalpel blade firmly in one hand. With the other hand, gently collect a fold of skin on the trunk, or evert a pinna or interdigital space.
- Scoop a small amount of mineral oil onto the scalpel blade.
- Squeeze the skin in a fold of tissue between 2 fingers. In a single direction, scrape the skin fold, pinna, or paw with firm and constant pressure over a single site until a small amount of capillary oozing occurs.
- Apply the collected material and exudate to the glass slide containing mineral oil. Repeat 3 to 5 times to cover the slide.
- Evaluate under 4×magnification, then move to greater objectives for mite evaluation.
Superficial Skin Scrapes
Superficial scrapes are used to detect Demodex gatoi mites in cats, Notoedres cati mites, and occasionally Cheyletiella mites.
- Place a drop of mineral oil on a microscope slide.
- Apply a drop of mineral oil to a #10 scalpel blade.
- In a single, uniform direction, scrape with gentle and light pressure across a broad lesional surface.
- Place the debris on the microscope slide with mineral oil and examine on low power (4×and 10×) for mites.
TRICHOGRAPHY (HAIR PLUCKS)
This is one of my favorite diagnostic techniques because it is an easy and effective test that can be used for many disease processes, from infectious and parasitic conditions (eg, dermatophytosis, demodicosis) to various forms of alopecia (eg, color-dilution alopecia, alopecia areata). A good reference guide for hair shaft evaluation is Muller & Kirk’s Small Animal Dermatology, 7th edition; this topic is discussed in the diagnostic methods section.
A good hemostat is essential for hair plucks. Many inexpensive hemostats do not grasp the hair well enough to pluck out 10 to 20 hairs at a time, and the hairs can slip through the jaws (Figure 2).
- Apply a few drops of mineral oil to a glass microscope slide.
- To pluck the hairs, grasp about 10 to 20 hairs
at a time at just about the surface of the skin and pull with a quick and deliberate motion in the direction perpendicular to the skin surface (this will help minimize patient discomfort, which is rare).
- Place the hairs on the microscope slide with mineral oil.
Evaluate the entire shaft of the hair for such conditions as dermatophytosis or color-dilution alopecia (Figure 3).
- Examine the hair bulb region for alopecia areata.
- Assess the area just above the bulb region for Demodex mites (Figure 4).
Dermatophytes can be observed on hairs with spores or hyphae. Some authors recommend digesting the keratin to observe these structures more readily, but this is not always needed. A formula called chlorphenolac has been used to replace KOH preparation solution; this formulation consists of 50 g of chloral hydrate added to 25 mL liquid phenol and 25 mL of liquid lactic acid (note: it may take several days for crystals to go into solution).1
Wood’s lamp evaluation for dermatophytosis is an easy and useful tool. It is a good screening test, although false-positive and false-negative results often occur. Only 30% to 80% of Microsporum canis isolates fluoresce, and Microsporum gypseum and Trichophyton mentagrophytes do not fluoresce under a Wood’s lamp.1 Occasionally, lint, dander, and organic debris can fluoresce and give a false-positive result. Dermatophytes produce an apple-green color upon fluorescence (Figure 5). Other, less common, species of dermatophytes that fluoresce are Microsporum audouinii, Microsporum distortum, and Trichophyton schoenleinii.
Clear acetate tape impressions are helpful to identify mites, such as Cheyletiella species, poultry mites, and cat fur mites.
- Press the tape, sticky side down, directly onto lesional sites.
- Place the tape onto a microscope slide for direct evaluation.1
Flea combs are easy to use and allow direct evaluation for mites, such as Cheyletiella species; fleas; and flea excrement.
- Place a few drops of mineral oil on a glass microscope slide.
- With the comb, use long, broad strokes from cranial to caudal to collect hair and debris. The material is collected on the tines of the comb.
- Remove the material from the comb and visually inspect it for fleas and flea debris.
- Place the hair onto the microscope slide containing mineral oil and evaluate it microscopically for mites.
- Place suspected flea excrement on a moistened piece of paper towel or gauze; dissolution into a reddish-brown color indicates flea excrement.
A recent study found that fecal flotation (Sheather’s solution, filtered and centrifuged) from cat stool samples detected various parasitic mites, including D gatoi, N cati, Lynxacarus radovskyi, Cheyletiella species, chigger mites, and Otodectes cynotis.3 This study used flea combing, acetate tape preps, superficial skin scrapings, and ear swabs to evaluate detection methods in community cats in the Ohio River Valley region of the United States. With the exception of only chigger mites and O cynotis being found on ear swabs, all mites listed above were found with each detection method. Although the numbers of mites found with each detection method varied, fecal flotation detected more free-living mites than parasitic mites. This must be considered a potential false-positive result unless one is well educated in distinguishing parasitic from free-living mites in stool samples. Many of these cats did not have dermatologic disease upon examination, and fecal flotation may serve as a good surveillance method for mite detection when evaluated at yearly examinations.
Photographs of positive findings using any of the diagnostic methods can provide the pet owner a good image of whatever pathogen is found. Photographs can also be uploaded to a pet’s file in many of the currently used patient software systems. A digital camera, iPhone microscope adapter, and specialized cameras that attach to microscopes are all good options.
- Miller WH, Griffin CE, Campbell KL. Diagnostic methods. Canine demodicosis. In: Muller & Kirk’s Small Animal Dermatology. 7th ed. Philadelphia: W.B. Saunders, 2013:78-92.
- Miller WH, Griffin CE, Campbell KL. Parasitic skin disease. Canine demodicosis. In: Muller & Kirk’s Small Animal Dermatology. 7th ed. Philadelphia: W.B. Saunders, 2013:315-319.
- Milley C, Dryden M, Rosenkrantz W, et al. Comparison of parasitic mites retrieval methods in a population of community cats. J Feline Med Surg 2016:1-8.