Two of the most important steps when you’re assessing dermatology issues in dogs and cats are (1) identifying primary and secondary skin lesions and (2) obtaining an accurate history. Most lesions are easily distinguishable from each other, but crusts and scales can be more of a challenge—especially when they occur on the same patient at the same time.
Melissa Hall, DVM, DACVD, shares her advice on how to tell the difference between the two, and how to zero in on accurate diagnosis and treatment of the most common causes of non-allergic crusting in cats.
One of these things is not like the other
First things first: Let’s talk about the difference between crust and scale.
Scale is an accumulation of loose fragments of the skin’s cornified cell layer. Scales can look like fine powder or large flakes, they can be greasy or dry, loose or adherent, and they can be various colors, such as white, brown or beige. Normal desquamation of the epidermis is invisible, so when larger flakes are apparent, abnormalities in keratinization or desquamation can be the underlying cause. Hall says most scaling disorders in cats result from chronic inflammation or poor grooming and therefore are usually a secondary manifestation.
A crust is an accumulation of dried exudate, serum, pus, blood or other cells, or scale that combines with other cellular debris to adhere to the skin surface. Crusts often become thickened in haired regions as hair and crust can mat and become tightly adherent. Crust can be variable in color—hemorrhagic crusts may be reddish, purulent crusts may take on a greenish or yellow hue, honey-colored crusts may be more infectious in nature, and crusts predominantly of keratinocytes and scale may be dark brown, silvery or black.
Crusts are often associated with inflammation, excoriation and pruritus, Hall says. They’re usually secondary types of lesions and may contain diagnostic clues such as infectious organisms, microbial elements or, if you’re lucky, dermatophyte hyphae. The diagnosis is often lurking in the crust, Hall says.
You’ve identified a crusty cat. Now what?
The first step, of course, is the same as in any other workup.
Get a history. How long has this been happening? Has it happened before? Is it seasonal? Are other cats, dogs or people in the household affected? Is the cat itchy?
Finding evidence of pruritus in the history may be difficult. Often an owner’s assessment is influenced by how much time they actually spend with the cat and how observant they are. We all know cats that are closet lickers.
To find evidence of pruritus, Hall advises looking at the shape of the crusts. Are they linear (see Photo 1)? Hall says that’s evidence of an itchy cat! Next obtain a trichogram. Pluck hairs from regions of thinning hair or alopecia and microscopically evaluate the ends for trauma. Tapered ends are normal. Broken and blunted follicular tips indicate that the hair has been sheared by overgrooming.
Create a diagnostic plan. The diagnostic plan for cats with scales or crusting is consistent with a routine workup for most cases of feline dermatologic disease. It includes a physical exam, superficial and deep skin scraping, flea combing, direct cytologic impression smears, dermatophyte cultures, Wood’s light examination and histopathologic examination of the skin via skin biopsies. Additional tests based on baseline results can include immunohistochemistry, bloodwork and imaging studies.
For superficial skin scraping, Hall recommends applying the oil to the cat and then scraping. For a deep scraping, apply oil to the blade and then scrape until the skin bleeds—this is less messy than applying oil to the lesion. You can put both scrapings on the same slide. She doesn’t advocate squeezing the skin during a deep skin scraping: Demodex cati is very rare in cats.
Is your patient crusty but not itchy? Scrape anyway. Infectious mites may or may not cause pruritus.
Mites to suspect in crusty cats
Here are your chief suspects:
Notoedres cati. Scabies in cats is caused by Notoedres cati. Like other sarcoptic mites, these burrowers live in the epidermis and are obligate parasites; they live their entire life cycle on the host. Notoedres is highly contagious. Clinically, crusts develop initially on the face and medial proximal edge of the pinnae with subsequent secondary clinical signs that include erythema, scaling and pruritus (see Photo 2). The areas affected may spread to involve the rest of the body over time. Focus your superficial skin scrapings along the ear margins: Notoedres loves pinnae! These mites are generally much easier to find than their counterpart in the dog.
Otodectes cynotis. These mites live predominantly in the ear canal and occasionally on the face, neck or body. Otodectes mites are also obligate parasites but are not particularly host-specific. They may affect dogs and small mammals in the environment as well. They cause disease both by direct mechanical irritation and by hypersensitive reactions. Classic clinical signs include young age of onset (although any age may be affected) with a typical black-brown ceruminous discharge that is usually bilateral.
These cats can be severely itchy, with erosions and significant crusting on the face and ears from excoriations. If the mite travels to another area of the body, lesions can mimic flea allergy dermatitis on the caudal dorsum. Chronic relapsing cases of otodectic mange are unlikely, and cats that have recurrent ear disease require an investigation into underlying allergic skin disease. These mites are also generally easily detected either by direct examination of the ear canal with an otoscope or in microscopic examination of direct smears of otic debris on a slide with mineral oil.
Demodex cati. Mange caused by Demodex cati is a relatively uncommon disease associated with the proliferation of these mites in the hair follicle or sebaceous glands, Hall says. The life cycle of this mite is similar to that of Demodex canis in dogs. Localized or generalized conditions can occur; however, the localized form in the cat is uncommon. Localized presentations are often limited to the head, eyelids and face or present as a ceruminous otitis externa (see Photo 3). These may be self-limiting or require topical, but rarely systemic, therapy.
The generalized form of the disease is more common, although it is still relatively rare. Burmese and Siamese cats are overrepresented. Clinical signs include alopecia, erythema, scale and crusting, and there may or may not be pruritus. Generalized disease is most often associated with underlying metabolic immunosuppression from such conditions as diabetes mellitus, hypercortisolism (either spontaneous or iatrogenic), malignant neoplasia or FeLV/FIV-positive status.
Demodex gatoi. This short-bodied Demodex mite lives in the epidermal pits of the stratum corneum as opposed to the hair follicles. Its life cycle is less understood; however, it is suspected to live its entire life in the superficial layers of the skin. Clinically this condition can present as anything from a nonpruritic case of alopecia and scale with crusting to an intensely pruritic case that has significant crusting with excoriations and trauma. The abdomen, lateral thorax and medial aspect of the legs are generally affected. These mites seem to be more common in southern U.S. states and areas of higher humidity.
D. gatoi mites are small and translucent and can be easily missed on skin scraping. Plus pruritic cats can easily remove them through grooming. Fecal flotation may be helpful in finding these mites in cases when skin scrapings are nondiagnostic. In addition, Hall recommends scraping along the margins of the areas of hair loss to increase the mite yield on superficial broad skin scraping.
Cheyletiella blakei. This mite is specific to feline patients, although on occasion other species of Cheyletiella (including C. yasguri and C. parasitovorax) can also infest cats. Often called “walking dandruff,” this condition is likely underdiagnosed and can be particularly problematic in catteries. The mites live on the skin surface and feed on cutaneous debris. They are more often diagnosed in young cats, but adults can be affected and are often asymptomatic. C. blakei presents a zoonotic concern as it can trigger a pruritic papular rash in people.
These mites are slightly larger than others and can be visualized as moving white spots in severely affected cats. Infestation will create significant scaling. Skin scraping or tape preparations can locate the mite.
Treatment for mites. If you see any mite, that’s considered conclusive—treat for that mite. If all tests come back negative, Hall considers the results inconclusive and treats empirically for mites with selamectin every two weeks for three applications. However, selamectin is ineffective for any Demodex species. In these cases the only treatment available is a lime sulphur dip every two weeks for three applications, Hall says.
Bacterial infection possibilities
Although most textbooks and references label superficial bacterial pyoderma or folliculitis as “uncommon” or “rare” in cats, most dermatologists and even recent literature disagree with this assessment. Superficial bacterial pyoderma is commonly undiagnosed and in many cases untreated in cats.
Crusted papular eruptions or miliary dermatitis are the most common presentations for folliculitis in cats; however, large areas of erythematous and erosive dermatosis can also be associated with large numbers of bacteria.
Pyoderma can be diagnosed with skin surface cytology: tape preps and direct impression smears of the affected area are indicated, and regular modified Wright’s stain is adequate. Eosinophils are often seen. Hall does not think culturing is necessary in routine cases; however, if rods are present or the patient is not responding to appropriate antimicrobial therapy, perform a culture.
Collect samples from the leading edge of the crust or collarette; swab under the crust and sample multiple sites. If the cat is on an antibiotic, do not stop the antibiotic before obtaining the sample. It’s also a good idea to perform cytology at the same time you collect your culture sample. If you see intracellular bacteria or bacteria with numerous inflammatory cells on cytology, you can assume that you are not dealing with normal flora. If you are concerned that the bacterial species present is resistant to the antibiotic, then the culture should not come back negative as there is active bacterial growth present.
While pyoderma is generally still considered a secondary complication of underlying disease, management of the bacterial component can be a critical factor in achieving control and remission of the primary disease.
Treatment is with empirical systemic therapies: amoxicillin-clavulanate, cephalexin, clindamycin and cefovecin are all good options. Treat for three to four weeks, and continue treatment for two weeks after resolution of clinical signs. If using cefovecin, administer two injections two weeks apart. Topical therapy with mupiricin cream is also an option because it is safe if ingested—i.e., licked off.
In cats, dermatophytosis is most often caused by Microsporum canis (see Photo 4). It’s common and can present in a myriad of ways. It can be characterized by either scale or crusting. Route of infection is contact with an arthrospore on an infected animal or in a contaminated environment.
The most commonly affected animals are young, older or immunocompromised due to poor nutrition, ectoparasitic concerns or metabolic disease. Most healthy cats experience spontaneous remission, and treatment goals should be aimed at reducing environmental contamination and thus zoonotic potential.
Hall finds a trichogram to be a very helpful: dermatophytes like to hang out on the outside of the hair shaft, and irregularity or fuzziness on the hair shaft can indicate fungal spores. You may also get lucky and see haloed fungal spores or ghostlike hyphae (see Photo 5).
A Wood’s lamp can help you determine where to take culture samples; some dermatophyte strains glow a bright apple-green color along the hair shaft. Scale and debris typically glow green-blue. By the way—Hall says it’s a myth that a Wood’s lamp needs five to 10 minutes to warm up. Delayed fluorescence is all a matter of perception and your eyes adjusting to the dark.
Recent evidence suggests that infected arthrospores can be found numerous centimeters away from the obvious clinical lesions. Because of this, most cases that don’t resolve spontaneously should be considered generalized and treated as such, Hall says.
Fungal culture is Hall’s diagnostic of choice, with dermatophyte test medium being the most reliable, inexpensive in-house method available. Dermatophytes consume the protein in the agar, creating alkaline metabolites that change the color to red. Dermatophyte cultures will grow in seven to 14 days. Color change noted after 21 days is usually due to contaminants, Hall says. She recommends incubating the samples at 82 degrees F, placing them next to a computer tower for warmth. Hall stores her cultures inside a plastic baggie with a moistened gauze square so the heat and humidity will encourage the fungus to grow. Check the samples daily for two weeks, then every other day for a total of 30 days.
“I check for 30 days just to make sure there isn’t something slow-growing,” Hall says. “It’s unlikely but it makes me feel more sure of the results.”
Dermatophyte colonies are white and fluffy; contaminant growth is usually darker, Hall says. You can use a piece of Scotch tape to lift the colony off the agar and place it on a microscope slide to identify. Hall recommends the website mycosesstudygroup.org as a resource with lots of pictures to help you identify dermatophyte species.
If you need a more rapid diagnosis, such as in households with higher zoonotic risk (pregnant women, children, immunocompromised individuals), Hall advises taking a biopsy of the leading edge of a lesion and requesting PSA or silver stain with the lab.
Treatment options for systemic therapy include the use of itraconazole, fluconazole or terbinafine as the drugs of choice. Because of the zoonotic potential of ringworm, Hall always starts oral antifungal therapy before culture results are back.
Coming up negative on your diagnostics for anything infectious and your paracitidal trial failed? Then you may be dealing with allergy, autoimmune disease or neoplasia. Further workup is required, including biopsy and allergy diagnostics, Hall says.