Hair trichograms are used to examine hair shafts and hair bulbs for evidence of Demodex spp. mites, Cheyletiella spp. mites, dermatophyte infections, barbering or breakage due to disease, and stage of growth (anagen or telogen). Samples should be mounted in mineral oil; clearing agents are neither needed nor recommended to find dermatophyte spores. Target hairs should be plucked in the direction of growth, placed on a drop of mineral oil on a glass microscope slide with a cover slip, and examined at 10× and 40×. Demodex spp. mites will be found around the hair bulb. Dermatophyte-infected hairs will have a cuff of spores around the shaft and appear wider and more filamentous than normal hairs (Fig. 25.2). Fungal spores are more refractile than surrounding debris. If suspicious glowing hairs cannot be found, a plug-in Wood’s lamp can be used to locate them on the glass slide. The veterinarian should turn out the lights and hold the lamp over the glass slide to locate them.

The primary parasites identified by skin scrapings are Demodex spp., Cheyletiella spp., Notoedres spp., and Sarcoptes spp. mites. Otodectes spp. mites can sometimes be found on skin scrapings. Skin scrapings can also be used to diagnose dermatophytosis. In one study, Microsporum canis infections were confirmed in 35 of 40 cats (87.5%) when hair trichograms and skin scrapings in mineral oil were used.9 A positive skin scraping rules in a parasite, but a negative skin scraping does not necessarily rule out parasites. Skin scrapings in cats should only be performed with a skin-scraping spatula ( e-Fig. 25.2). Skin-scraping spatulas are safer, 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 flat edge of the spatula (not the tip) to dislodge material from the surface and follicular material by gently scraping with short sweeping movements like spreading butter on bread. Using the length of the spatula allows one 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, 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.

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. The slides are examined using 10× magnification starting at the margins of the cover slip. Higher magnification (40×) can be used to confirm mite species, if necessary. 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, but it is possible to see fragments of infected hairs.

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, Lynxacarus radovskyi, 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. Water can be used to distinguish between the two substances; the flea excreta will dissolve, leaving a reddish-brown smear (Fig. 25.3). 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 can be used to capture fleas, flea excreta, and ticks and to collect scales to look for Cheyletiella spp. mites; however, Cheyletiella spp. mites are more often found on skin scrapings or flea combings. The usefulness of the acetate tape technique depends upon making sure that the surface of the skin is sampled and not just random sites on the distal hairs. This is a very effective diagnostic test for finding Cheyletiella spp. mites, Felicola felis, and L. radovskyi fur mites compared to hair trichogram.10 The sticky side of a clear piece of acetate tape is pressed against the surface of interest and then mounted over a drop of mineral oil on a glass microscope slide. Another drop of mineral oil is placed on top of the tape, and 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 threads, pigmented fungal spores, and plant matter.

Mineral oil ear swab cytology is most commonly used when Otodectes spp. mites are suspected; however, it is an underused diagnostic test in cats ( Video 25.1). Demodex gatoi (Fig. 25.4) and Demodex cati can cause pruritic otitis in young and adult cats. A dry cotton-tipped swab is used to collect ear debris. To look for mites, the tip of the swab is rolled in a drop of mineral oil on a glass microscope slide and a cover slip is placed over the oil drop.

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

A Wood’s lamp is an ultraviolet light (320 to 400 nm wavelength) that is used to detect microbial organisms that produce phosphors as a result of their growth on skin and/or hairs. A Wood’s lamp is a point of care diagnostic tool that is used to identify hairs for direct examination and/or fungal culture of M. canis–infected hairs. The characteristic green fluorescence of M. canis–infected hairs is due to a water-soluble pigment located within the cortex or medulla of the hair. An evidence-based review found that many of the commonly held beliefs about the usefulness of Wood’s lamps are incorrect and that this is an excellent point of care diagnostic tool for identifying hairs for direct examination or culture. When data was objectively reviewed, it showed that positive fluorescence in cats with experimental infections and cats with spontaneous disease was 100% and 91%, respectively.11 Contrary to what has been published in anecdotal reports, the Wood’s lamp is a useful tool for identification of hairs for cytological examination. Positive M. canis fluorescence in cats that previously received treatment (predominantly longhaired cats) was lower than in untreated cats, varying from 39% to 53%.¹¹ This would make sense since fluorescence disappears as the disease resolves. Topical therapy did not destroy fluorescence as is commonly believed. It was found to be a useful tool to assess response to treatment as positive response was associated with a loss of fluorescence at the base of the hair and in the hair shaft. Wood’s lamp examinations have been found to be an important point of care examination tool in shelters to screen cats for dermatophytosis. For example, in one report of management of M. canis dermatophytosis in a shelter, 1226 cats were surrendered to the shelter in a 7 month period.¹² Of these cats, 273 (22.3%) were culture-positive but only 60 of 273 had lesions, and were positive on Wood’s lamp and direct examination. The other 213 culture-positive cats had no lesions and were negative on Wood’s lamp examination; they were determined to be fomite carriers. The use of the Wood’s lamp at intake allowed for rapid identification of infected cats (50 of 60 being kittens), and prevention of reintroduction of the contagion to the facility. Table 25.1 summarizes helpful hints for Wood’s lamp use. Plug-in lamps with built in magnification are recommended ( e-Fig. 25.3). Fluorescence is found on the hair shafts and it can be helpful to keep a positive control for reference. This is easily made by pressing clear sticky tape to an area of fluorescence, mounting it on a glass slide and sealing the edges with clear nail polish. The fluorescence will last for years (Fig. 25.5). Contrary to what has been previously written, the lamp does not need to warm up, but users do need to allow time for their eyes to adapt to the darkness. Having a positive control can help determine if enough time has been allocated for light adaptation.

Dermoscopy is a point of care tool used to identify hairs for direct examination and/or fungal culture. A dermoscope is a noninvasive, handheld tool that allows for illuminated magnification of the skin and hair. There are several studies in the veterinary literature describing normal cat skin and hair and dermoscopic examination of cats with dermatophytosis.13–15 Infected hairs are opaque, wider than normal thin hairs, and can be slightly curved or broken (Fig. 25.6). These infected hairs have been called “comma hairs” because of their appearance. Microscopic examination of hairs shows hyphae and spores. Dermoscopes vary in cost and their primary advantage is magnified illumination of broken hairs for the purpose of sampling for direct examination and fungal culture. This tool can be used with or without a Wood’s lamp.

Fungal culture is still the most common method to confirm the diagnosis and to identify the causative pathogen in suspect dermatophytosis cases. It is also the only medical–legal test used to confirm fungal species identification. Samples are most commonly collected by way of a toothbrush combing of suspect lesions or plucking of suspected hairs. A newly unwrapped 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, keep the plate moist, and prevent media mite infestations. Plates are incubated at 21°C to 23.8°C (70°F to 75°F). The most commonly used fungal culture media is Dermatophyte Test Media (DTM), which contains 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 for, a pathogen. Definitive diagnosis requires microscopic examination. If DTM is used, suspect colonies are those that are pale white with a red color change surrounding them as they are growing (Fig. 25.7). Fungal culture plates can be finalized by day 14 instead of day 21 based upon findings in two studies.^16,17 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 on a glass microscope slide. 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.mycosesstudygroup.org regarding the identification of common pathogens, particularly M. canis. It is worth noting that a study revealed good correlation between point-of-care cultures and reference laboratories when both gross colony formation along with microscopic features were used to identify colonies.¹⁸ However, there was a high error rate when color change alone was used.

Skin biopsy specimens can be obtained using a scalpel blade or a skin biopsy punch ( e-Fig. 25.4). 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. Unless the face is being sampled, use a 6- to 8-mm punch. Because cat skin is very thin, it is important to carefully cut through to the dermis without penetrating the underlying musculature. Using fine-tipped forceps, the sample is harvested by grabbing the subcutaneous fat pedicle; the sample is harvested in the same manner as a flower is plucked ( e-Fig. 25.4B). If junctional samples (from the border where normal meets abnormal) are needed, an elliptical biopsy incision that spans the normal, abnormal, and normal areas is used to collect the specimen. Instruct the pathologist to section the specimen lengthwise. 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. Digital images and a complete history should always be provided to the pathologist. The samples should be sent to a laboratory where there are veterinary pathologists with a declared interest in dermatopathology.

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 inoculated his own ears three times with ear mites.21

Lynxacarus radovskyi are fur mites that are found on the hair shafts of cats living in tropical environments (Fig. 25.8, Video 25.2). Clinical signs vary from mild to severe pruritus and papular eruption. Mite infestations are generalized and produce large amounts of scale. Mites can be found via flea combings, hair trichograms, or acetate tape preparations. These mites are susceptible to flea control products including oral fluralaner and moxidectin/imidacloprid.²²

Chiggers are an underdiagnosed cause of pruritus in cats.23 Chiggers live in organic material, and it is the larvae that are parasitic and feed on animals. Bites can occur anywhere on the cat’s body 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 spp. 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 multicat 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. Sarcoptes spp. infestations have been documented in cats. Cats presented with facial, pedal, and auricular crusting.^24,25 Pruritus was variable in these cats.

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

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 response to a treatment trial. Lice infestations, called pediculosis, are most often diagnosed 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 spp., and Notoedres spp. 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 hair coat for 2 to 3 minutes, then 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 hair. 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.²⁶ Water-based pyrethrin sprays labeled as safe to use in kittens are recommended to minimize the risks of toxicity. Permethrins (synthetic pyrethroids) are very toxic in cats and should never be used. Whole-body treatments should be done at least once weekly. Spot-on flea control products used every 2 weeks for three treatments are effective, but it is important to mechanically remove debris, scales, and nits from the hair coat if selamectin is used.

Demodicosis is increasingly 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 (Fig. 25.4), a short, rounded mite. A third unnamed mite has been identified but its role in disease is unknown.^27–29 Skin disease may be limited to the ears, causing pruritic otitis. Localized or generalized skin lesions may also be seen. 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. Demodex cati is most commonly found in cats with pruritic ears or in cats with skin lesions and concurrent disease such as diabetes mellitus, hyperadrenocorticism, feline immunodeficiency virus (FIV), feline leukemia virus (FeLV), toxoplasmosis, systemic lupus or other immune-mediated diseases, and neoplasia.³⁰ Localized demodicosis due to D. cati developed on the muzzle of two cats treated with inhalant glucocorticoids, emphasizing the need for a complete medical history when assessing skin diseases of cats.³¹

Demodex cati is not considered to be a contagious mite. Demodex gatoi is increasingly recognized as a pruritic skin disease of cats and is a known contagion; it can be difficult to find on skin scrapings.³² The mite lives in the superficial layers of the stratum corneum, and routine grooming of cats often removes the mite. It is increasingly found in cats with symmetric alopecia, which suggests that this mite is an overlooked and underdiagnosed cause of this reaction pattern in cats (Fig. 25.9).³³ 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. Demodex cati often resolves without treatment if the underlying disease can be identified or managed. There are no large case series evaluating the efficacy of treatment; however, demodicosis responds well to a number of therapies. Twice-weekly lime sulfur rinses alone or paired with daily oral ivermectin (200 µg/kg) for 4 to 8 weeks or daily oral milbemycin (0.5 mg/kg) for 4 to 8 weeks have been effective. Most recently, D. gatoi has been successfully treated with imidacloprid/moxidectin spot-on therapy.³⁴ Fluralaner spot-on therapy has been reported to be effective against canine demodicosis and the feline formulation has also been reported to be effective against feline demodicosis.^35–37 The authors currently use this fluralaner spot-on therapy as their treatment of choice for aggressive parasite control and for a response-to-treatment trial for feline demodicosis in place of lime sulfur.

Cats that are not allergic to fleas may show no signs of disease even if their hair coat is heavily infested with fleas. This represents a state of tolerance and is not common in pet cats. Most cats with a flea infestation are pruritic. Hair loss, scaling, papular eruptions with or without crusting, and areas of self-trauma (i.e., eosinophilic plaques) are common. The severity of pruritus in a flea-allergic cat is disproportionate to the number of fleas found. No fleas may be found because cats will bite, nibble, hunt, and ingest fleas. Hair loss over the lumbosacral area, hind legs, and neck is common (Fig. 25.10). Miliary dermatitis (small red crusts of serum and blood) is common, especially on the face and abdomen, as are ulcerated lips and symmetric alopecia. “Rodent ulcer” lesions are frequently the result of flea bites around the lips. Fleas are common causes of many of the eosinophilic reaction patterns observed in cats (see later).

Treatment of obvious infestations can start with oral nitenpyram (1 mg/kg, every 24 to 48 hours for 1 to 2 weeks until no more fleas are seen). At the same time, spot-on treatments can be started ( e-Table 25.1). Anecdotal reports about regional variations in efficacy are common, and the clinician should use products known to be effective in the practice area. Owners should be advised to use only products labeled as safe for use in kittens and cats as canine products can be highly toxic and warned that focal areas of hair loss at the site of spot-on products can occur. Treatment of kittens requires aggressive removal of fleas as they can cause life-threatening anemia. Nitenpyram has a wide safety margin and is licensed for use in kittens as young as 4 weeks of age.

Tick infestations can and do occur in cats and are important to control because they can transmit infectious diseases. Owners often do not notice infestations or believe they do not occur because ticks are mechanically removed by self-grooming in many cases. Tick bites can lead to small nodular reactions in the skin at the site of attachment. The lesions tend to occur several weeks after the tick bite. Another common complication of tick infestation is invasion of the ear canal. An affected cat may present with otitis or possibly severe vestibular disease.

The most common allergic skin diseases of cats include flea allergy dermatitis and environmental allergy.7 Evidence-based studies have found that food allergy as a primary cause of pruritus is uncommon.⁷ The pathogenesis of these allergic diseases involves combinations of type 1 and type 4 hypersensitivity reactions to allergens. The diagnosis is based on history, compatible clinical signs, ruling out of other, more common causes of allergic disease, and response to treatment trials. In the case of feline atopic dermatitis, it is important to remember that allergy testing (in vitro or intradermal) reflects exposure and is not a definitive diagnostic test. In other words, it does not answer the question “Is this cat allergic or not?”