Skin Scrapings

The primary parasites identified by skin scrapings are feline Demodex mites, Cheyletiella mites, and Notoedres mites. Otodectes mites can sometimes be found on skin scrapings. Positive skin scrapings rule in a parasite, but negative skin scrapings do not necessarily rule out these parasites. Skin scrapings in cats should only be performed with a skin-scraping spatula (Figure 22-2). Skin-scraping spatulas are safer and less expensive and provide better sample material than dulled scalpel blades. Optimal samples are obtained by putting several drops of mineral oil on the skin and using the length of the spatula (not the tip) to dislodge material from the surface and follicular material by gently scraping with short sweeping movements. Using the length of the spatula allows the veterinarian to gently scrape large or small areas of the skin. The mites of interest are more superficial in the cat’s skin than in the dog’s, so unless there is thickening of the skin, it is rarely necessary to scrape until capillary bleeding is apparent. It is more fruitful to scrape more frequently and over larger areas when dealing with cats with skin disease.

FIGURE 22-2 Skin-scraping spatula used for collecting cytologic specimens. This tool makes collection of specimens much easier and safer than doing so with a scalpel blade.

(Photo courtesy University of Wisconsin School of Veterinary Medicine.)

After obtaining the sample, the material is transferred to a clean glass microscope slide and cover slipped. The latter is important because the pressure from the cover slip “pushes” mites to the margins of the slide. Examine the slides using 10× magnification and starting at the margins of the cover slip. It is often helpful to move the condenser down to increase contrast and scan first for movement. Demodex spp. mites can vary in shape, and the short, fat Demodex gatoi mite is easily overlooked, particularly at 4× magnification. The following are artifacts: red-brown globules (blood cells), brown-black granules (melanin granules), colored threads, broken hair shafts, and plant pollen or mold spores (usually darkly pigmented). Microsporum canis macroconidia are never seen on skin scrapings.

Flea Combing

Flea combing is indicated in any cat with hair loss, scaling, or pruritus. The fine-tooth combs can be used to find fleas, flea excreta, ticks, lice, and Cheyletiella spp. mites. The hair coat is combed using a fine metal- or plastic-tooth comb. The material can be examined with a handheld magnifying lens or under a microscope. Soil particles can be confused with flea excreta, and water can be used to distinguish between the two substances; the flea excreta dissolve, leaving a reddish brown smear. Shampoo residue (usually uncommon in cats) may mimic excessive scaling. It appears as fine, powdery debris on the distal tips of the hairs.

Acetate Tape Preparations

Acetate tape preparations can be used to capture fleas and flea excreta and ticks and to collect scales to look for Cheyletiella spp. mites. Cheyletiella spp. mites are more often found on skin scrapings or flea combings; the usefulness of this technique is greatly overemphasized. The sticky side of a clear piece of acetate tape is pressed against the object of interest and then placed on a glass microscope slide over a drop of mineral oil. Another drop of mineral oil is placed on top of the tape, and then a cover slip is added. This enhances visualization of mites. The slide is examined under increasing magnification. Common artifacts include crinkling of the tape, frosted tape, clothing treads, pigmented fungal spores, and plant matter.

Ear Swab Cytology

It is important to collect specimens for ear swab cytology before collecting specimens for mineral oil examination. Ear swab cytology examines debris, wax, and exudate from the ear canal. It is indicated in all cases of otitis or head and neck pruritus. A dry cotton-tipped swab is used to collect material and rolled onto a clean glass microscope slide. The slide is heat fixed and stained using a rapid Giemsa stain. Slides should be examined at low power (4× or 10× magnification) to find areas of cellularity for closer scrutiny. Bacteria and yeast are best seen under oil immersion. If rods are evident, an ear culture is indicated.

Mineral Oil Ear Swab Cytology

Mineral oil ear swab cytology is most commonly used when Otodectes spp. mites are suspected; however, it is an underused diagnostic test in cats. D. gatoi and Demodex cati can cause pruritic otitis in young and adult cats. Ear swabs are indicated whenever ear mites are suspected, debris is in the ear, or the cat has head and neck or ear pruritus. A dry cotton-tipped swab is used to collect ear debris. To look for mites, the veterinarian should roll the tip of the swab in a drop of mineral oil on a glass microscope slide and then place a cover slip over the oil drop.

Skin Cytology and Nail Bed Cytology

Skin cytology and nail bed cytology is an underused diagnostic test in feline dermatology. It is most commonly used to identify bacterial and yeast overgrowth. It can also be used to help identify acantholytic cells that are seen in pemphigus foliaceus (PF). There are three common methods for taking samples from the skin: glass slide impressions, clear acetate tape, and spatula collection:

FIGURE 22-3 Acetate tape cytology specimen. Clear acetate tape facilitates the collection of cytologic specimens from the skin of cats.

(Photo courtesy University of Wisconsin School of Veterinary Medicine.)

Two common normal findings are easily mistaken for bacteria: melanin granules (darkly pigmented and slightly refractile) and surface lipids (very small and slightly basophilic, granular in appearance). Cocci, diplococci, rods, and yeast stain deeply basophilic and are not refractile.

Wood’s Lamp Examination

A Wood’s lamp is an ultraviolet light with a wavelength of 273.7 nm filtered through a cobalt or nickel filter. The light interacts with a metabolite on the surface of infected hairs, resulting in an apple-green fluorescence (Figure 22-4). M. canis is the only species of importance in veterinary medicine that fluoresces with a Wood’s lamp. After allowing the lamp to warm up for several minutes, the veterinarian should turn off the room lights and hold the light over suspected lesions for several minutes. The hair examination should continue for at least 3 or 4 minutes to ensure that the examiner’s eyes have adapted to the light. Positive hairs can be plucked for fungal culture or hair trichogram. Not all strains of M. canis fluoresce. Any topical medication can cause a false-positive or false-negative test. The most common false-positive result occurs with the blue-white fluorescence of dust and scales.

FIGURE 22-4 Wood’s lamp positive hairs. Note the apple-green fluorescence of the hairs.

(Photo courtesy University of Wisconsin School of Veterinary Medicine.)

Hair Trichograms

Hair trichograms are used to examine hair shafts and hair bulbs for evidence of fungal infections, stage of growth (anagen or telogen), and Demodex mites. Target hairs should be plucked in the direction of growth, placed on a drop of mineral oil, cover slipped, and examined. Mites will be found around the hair bulb. When examining hairs for fungal spores, the veterinarian does not need to use a clearing agent such as potassium hydroxide. Infected hairs will have a cuff of spores around the shaft and appear wider and more filamentous than normal hairs (Figure 22-5). Fungal spores are more refractile. If suspicious glowing hairs cannot be found on a microscope slide, the Wood’s lamp can be used to locate them on the glass slide. The veterinarian should turn out the lights again and hold the lamp over the glass slide to locate them.

FIGURE 22-5 Hair trichogram of M. canis–infected hair. Note the large cuff of spores seen around the base of the hair. This is a mineral oil mount of an infected hair.

(Photo courtesy University of Wisconsin School of Veterinary Medicine.)

Dermatophyte Cultures

A fungal culture is the best way to diagnose a dermatophyte infection. It is indicated whenever a Wood’s lamp examination yields a positive result or the veterinarian suspects a dermatophyte infection.

Samples are most commonly collected by way of a toothbrush combing of suspect lesions or plucking of suspected hairs. A newly wrapped toothbrush is combed over the coat for several minutes to obtain a specimen. This is a particularly useful technique for obtaining samples from cats. Alternatively, broken hairs from the margin of a lesion can be gently plucked in the direction of growth. Plucked hair samples are firmly pressed to the surface of the fungal culture plate.

Toothbrush samples are inoculated by gently stabbing the surface of the plate 10 to 20 times with the bristles, taking care not to lift the media off the plate. Commercial media plates should be placed in a self-closing zip-lock bag and labeled. The plastic bag will help minimize cross-contamination of samples, help keep the plate moist, and prevent media mite infestations. Plates are incubated at 21° C to 23.8° C (70° F to 75° F).⁹⁹ The most commonly used fungal culture media are Dermatophyte Test Media (DTM), which contain a color indicator and plain Sabouraud dextrose agar. It is important to remember that the red color change in the DTM media is only suggestive of, not diagnostic of, a pathogen. Definitive diagnosis requires microscopic examination. If DTM is used, suspect colonies are those that are pale with a red color change surrounding them as they are growing (Figure 22-6). Most pathogens will grow by day 14 of culture. All suspect cultures should be identified microscopically using lactophenol cotton blue stain or new methylene blue stain. A piece of clear acetate tape is pressed to the surface of the growth and placed over a drop of stain. A second drop of stain is placed on top of the tape, and a cover slip is added. Additional information can be found at http://www.doctorfungus.org regarding the identification of common pathogens, particularly M. canis.

FIGURE 22-6 Positive culture of M. canis. This is a dual culture plate with DTM on one side (note red color change around the growing colony) and Sabouraud dextrose agar on the other side. The difference in gross characteristics is because the phenol color indicator in the DTM changes gross colony morphology.

(Photo courtesy University of Wisconsin School of Veterinary Medicine.)

Skin Biopsy

Skin biopsy specimens can be obtained using a scalpel blade or a skin biopsy punch (Figure 22-7). Primary lesions (e.g., pustules, vesicles) should be sampled. If these are not found, several representative samples should be obtained. When multiple skin biopsy specimens are being obtained, lesions should be circled with a black marker so that the veterinarian can identify the sites to biopsy after the local anesthetic has been injected. The skin is not washed or scrubbed because surface pathology is very important in the diagnostic evaluation. A local anesthetic is injected subcutaneously directly beneath the sample. Skin biopsy punches are placed directly over the lesion and twisted in one direction, using gentle pressure; twisting clockwise and counterclockwise produces shear, which ruins the specimen. Because cat skin is very thin, the veterinarian must be careful to cut through to the dermis without penetrating the underlying musculature. Using fine-tipped forceps, the veterinarian harvests the sample by grabbing the subcutaneous fat pedicle; the sample is harvested in the same manner as a flower is plucked (see Figure 22-7, B). If junctional samples are desired, an elliptical biopsy is collected. Blood is gently blotted from the sample, and the subcutaneous side is placed on a piece of tongue depressor and fixed in 10% neutral buffered formalin. If there is a particular lesion or lesions that the clinician wants cut into the block, he or she can place a small black dot on the site and tell the pathologist to section over this site since discriminating features such as erythema can be lost by tissue in fixative. The veterinarian should always include digital pictures and a complete history. The samples should be sent to a laboratory where there are veterinary pathologists with a declared interest in dermatopathology.

FIGURE 22-7 A, Collection of skin biopsy specimen. This diagram shows the correct placement of a skin biopsy punch for harvesting a sample of tissue. Given the small size of the punch, marginal lesions are not recommended because there is no way to ensure that the pathologist will know how to section the sample. Take a sample from an abnormal area and normal area. If it is important to see margins, use an elliptical incision. B, It is important to grasp the biopsy specimen by the subcutaneous pedicle and cut it free from the skin without crushing the specimen.

(A and B courtesy University of Wisconsin School of Veterinary Medicine.)

Otodectes

The most common mite infestation of cats is the ear mite, Otodectes. Ear mites live on or in the ears. Eggs are cemented to the hairs and skin on the margin of the ears. Mites feed on the epithelium, and the ear canal fills with black-brown crumbly material. Kittens are recognized as a high-risk group, but any cat can be affected; adult animals should be examined with the same care as kittens. The classic clinical signs are a coffee-ground discharge and pruritus. Hypersensitivity reactions in cats can develop secondary to mite infestations. Pruritus will be severe, yet mites are often not found. Untreated or inappropriately treated ear mite infestations can lead to secondary infections, purulent otitis externa (OE) or otitis media (OM), and aural hematomas and may be involved in the development of chronic OM in adult cats. Severe mite infestations can occur over the entire body, leading to a pruritic, papular skin disease that can mimic many other parasitic and nonparasitic diseases (Figure 22-9).

FIGURE 22-9 Adult cat with intense otic pruritus due to hypersensitivity reaction to Otodectes.

(Photo courtesy University of Wisconsin School of Veterinary Medicine.)

Diagnosis of ear mite infestations is made by finding mites either by direct examination with an otoscope or, more commonly, on mineral oil ear swab cytology. One mite or egg is diagnostic for an infestation. Positive pinna–pedal reflexes are common in cats with gross or subclinical infestations; when the ear is manipulated or swabbed, the cat scratches with the ipsilateral hind limb. Mites can often be found on skin scrapings of ear margins in animals in which mites are suspected but cannot be found on ear swab cytology.

There are many well-accepted and effective treatment protocols for ear mite infestations. The key treatment points are as follows:

The application of otic ear mite preparations at night has been recommended on the basis of a report from a human infested with ear mites. This person reported that the mites were more active at night than during the day. In this fascinating report, a veterinarian self-inoculated his ears three times with ear mites.⁵⁰

Fur Mites and Lice

The most common fur mites of cats include Dermanyssus gallinae, Lynxacarus radovskyi, chiggers (Eutrombicula spp., Walchia americana), and Cheyletiella spp. (see below).^40,43,55,66 D. gallinae, or the poultry mite, is most common in wild birds and pet birds. Pet birds can be affected if they are in contact with wild birds. Contact need not be direct; mites can be mechanically transmitted to pet birds through contact with contaminated material or close exposure to nests. Clinical signs vary from none to pruritic papular eruptions. Contact with poultry or exposure to wild birds is an important part of the history. Pet cats are most commonly exposed when homes have wild birds nesting near screened porches. The diagnosis can be difficult, but one helpful finding in the history is if the owner reports finding dead baby birds near these nests.

L. radovskyi infestations have been reported in Hawaii, Texas, and Florida.²² Clinical signs varied from mild to severe pruritus and papular eruption. Mite infestations were generalized and produced large amounts of scale. In the few cases seen by the authors, the mites were not difficult to find.

Chiggers are an underdiagnosed cause of pruritus in cats. Chiggers live in organic material, and it is the larvae that are parasitic and feed on animals. Bites can occur anywhere but are most common in areas in contact with grass or soil. The most common clinical sign is a papular eruption. In the cases seen by the authors, both owners and cats were affected. The infestations are seasonal and tend to occur in the late summer and fall.

Cheyletiella mites are the most well-known and common fur mite infestation of cats. These mites can also affect dogs, rabbits, and other small mammals. The mites are highly contagious and of zoonotic importance. There are several species but all have the same general appearance. Infested animals can be asymptomatic and not identified until people or other cats become affected. The classic clinical presentation is dorsal scaling with mild to moderate pruritus that can be severe.

Notoedres cati, also known as “feline scabies,” is an intensely pruritic skin disease of cats. It is uncommon and is most often found in catteries and multiple-cat households. Affected cats present with intensely pruritic crusting and scaling on the face, ears, head, neck, paws, and perineum. If left untreated, the skin becomes lichenified, hyperpigmented, alopecic, and excoriated. These mites are easily found on skin scraping, and the mite is similar in appearance to Sarcoptes spp.

Lice are species specific and are contracted by direct contact with another infected host. Cats are afflicted with only one species of lice, Felicola subrostratus. Infested animals may be asymptomatic or more commonly present with restlessness, pruritus, scaling, hair loss, and irritability. Lice cement their eggs to the hairs, and these eggs are often referred to as nits.

Definitive diagnosis of lice and fur mite infestations can be difficult because there is no single best diagnostic test to find these parasites. Skin scrapings, flea combings, hair trichograms, acetate tape preparations, and fecal examinations are recommended. In many cases, however, definitive diagnosis or ruling out of mite infestations can be done only by identifying a response to treatment trial. Lice infestations are most often made by finding the lice or nits (or both) while making a visual inspection of the cat’s hair coat.

Treatment options for lice, fur mites, Cheyletiella, and Notoedres infestations are similar. The key to successful treatment is use of a product that is applied to the entire hair coat. If possible, the cat should be bathed to remove debris, excess scales, egg cases, and nits from the hair coat. Nits can be loosened from the hair coat with a 1 : 4 dilution of white household vinegar in water. The solution is applied to the coat for 2 to 3 minutes, and then the hair coat is rinsed. If the cat has long hair and the infestation is severe or if soaking the hair coat is difficult, it may be necessary to clip the coat. These parasites generally have a life cycle of 3 weeks, so a treatment plan of 4 weeks is recommended. Whole-body treatments include lime sulfur rinses, fipronil spray, and pyrethrin sprays.²³ Water-based pyrethrin sprays labeled as safe to use in kittens are recommended to minimize the risks of toxicity from pyrethrins. Permethrins are very toxic in cats and should never be used. Whole-body treatments should be done at least once weekly. Concurrent systemic treatment options include ivermectin (0.2 mg/kg to 0.4 mg/kg orally, once daily for 4 weeks) and milbemycin (0.5 to 2 mg/kg orally, once daily for 4 weeks). Selamectin used every 2 weeks for three treatments is also effective; however, it is important to mechanically remove debris, scales, and nits from the hair coat if selamectin is used.

Demodicosis

Demodicosis is increasingly being recognized as a cause of skin disease and pruritus in cats. The disease is caused by D. cati, a long slender mite, or D. gatoi (Figure 22-10), a short, rounded mite. Skin disease may be limited to the ears, causing pruritic otitis. Localized or generalized skin lesions may also be seen. As with dermatophytosis, the clinical presentation is highly variable. Localized lesions are usually characterized by patchy hair loss, scaling, and erythema around the eyes or on the head or neck. Erythema, scales, crusts, easily epilated hairs, symmetric alopecia, or just intense pruritus mimicking feline “hyperesthesia-like” quivering may be all that is found. D. cati is most commonly found in cats with pruritic ears or in cats with skin lesions and concurrent disease such as diabetes mellitus, feline hyperadrenocorticism, feline immunodeficiency virus (FIV), feline leukemia virus (FeLV), toxoplasmosis, systemic lupus or other immune-mediated diseases, and neoplasia.⁷⁰

FIGURE 22-10 Demodex gatoi mite. This mite lives in the superficial layers of the skin and can be difficult to find. This mite was found on a fecal examination of an intensely pruritic cat (400× magnification).

(Photo courtesy University of Wisconsin School of Veterinary Medicine.)

D. cati is not considered to be a contagious mite. D. gatoi is increasingly being recognized as a pruritic skin disease of cats and is a known contagion; it can be difficult to find on skin scrapings.⁸⁷ The mite lives in the superficial layers of the stratum corneum, and routine grooming of cats often removes the mite. It is increasingly being found in cats with symmetric alopecia, which suggests that this mite is an overlooked and underdiagnosed cause of this reaction pattern in cats (Figure 22-11).³² Diagnosis is made by demonstration of the mite on skin scrapings, fecal flotation, ear swab cytology, or hair trichogram. In cats with symmetric alopecia, diagnosis is often made by response to treatment.³² D. cati often resolves without treatment if the underlying disease can be identified or managed. It has been successfully treated with lime sulfur rinses every 4 to 7 days for to 8 weeks.²³ Feline demodicosis responds well to a number of treatment options, including twice-weekly lime sulfur rinses (e.g., Lime Sulfur Dip, Vet Solutions, Fort Worth, Texas) alone or paired with daily oral ivermectin (200 µg/kg) for 4 to 8 weeks or oral daily milbemycin (0.5 mg/kg) for 4 to 8 weeks. If lime sulfur is not an option, the author (KM) has used topical fipronil spray once weekly. Cats with a suspected or known D. gatoi infestation must be isolated during treatment. All cats in contact with the afflicted cat should be treated.

FIGURE 22-11 D. gatoi–infested cat. This cat was presented for the problem of over grooming and feline symmetric alopecia. D. gatoi mites were found, and the pruritus resolved with treatment.

(Photo courtesy University of Wisconsin School of Veterinary Medicine.)

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