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Feline dermatophytosis
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Dermatophytosis (often known as “ringworm”) is a common dermatological fungal infection in cats that should be diagnosed and treated as early as possible, as Amelia White explains.
Amelia White
DVM, MS, Dip. ACVD
Dr. White graduated from the University of Georgia in 2010 and served as a small animal intern in the College of Veterinary Medicine at Auburn University between 2010-2011. She completed a three-year dermatology residency at the University of Illinois in Champaign-Urbana before returning to Auburn in 2014 where she currently serves as Associate Clinical Professor of Dermatology. She received the Dean’s Award for Excellence in Teaching in 2020 and has authored or co-authored various dermatology papers.
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Key points
- Dermatophytosis is a common superficial fungal infection in cats that is both contagious and zoonotic.
- Dermatophytosis is self-limiting, but treatment is always recommended in order to prevent contamination of the environment and spread of disease to other cats or people.
- Diagnosis is achieved easily through a combination of PCR, Wood’s lamp, trichography, and fungal culture.
- The treatment goals are to kill the fungal organisms and reduce spread within the environment through a combination of systemic and topical therapies.
Introduction
Fungal dermatoses frequently occur in veterinary medicine, and dermatophytosis represents one of the most common causes of infectious superficial folliculitis in cats. Dermatophytes are keratin-loving organisms that invade skin and hair shafts, leading to clinical signs of folliculitis. The most common dermatophyte species affecting cats is Microsporum canis, and the cat serves as its environmental reservoir.
Dermatophytosis is both contagious and zoonotic, therefore rapid diagnosis and treatment will prevent propagation within the environment and spread of disease, which is especially important in multi-cat households, catteries, and shelter environments. The traditional means of diagnosis (including Wood’s lamp, trichography, and dermatophyte culture) remain widely accepted, and PCR is a useful tool to shorten time to treatment and clinical resolution. Although the disease is self-limiting, treatment is recommended to prevent spread of infection.
Prevalence and predispositions
While dermatophytosis is recognized as a common fungal dermatosis in cats, the true prevalence is unknown 1. Disease occurs worldwide, and research trends suggest that animals living in warm environments, group housing, free-roaming cats, young cats, immunodeficient animals, and cats with clinical lesions more commonly culture positive for dermatophyte 1 2 3. There are no known predispositions, although evidence suggests that Persian cats are over-represented, especially for the subcutaneous form of dermatophytosis (mycetoma or pseudomycetoma) 1 4.
Pathogenesis
Most fungi are opportunistic pathogens that invade the body if there is failure of the host’s innate immune defense system. This can be defined as the non-specific, naturally present component of the immune system which is not dependent on prior antigen sensitization – e.g., the physical skin barrier, temperature, pH, and antimicrobial peptides. The innate immune system includes cells (e.g., natural killer cells, macrophages, and neutrophils) which recognize conserved regions on pathogens (known as pathogen-associated molecular patterns, or PAMPs) and mount an immune attack.
Dermatophytes invade the superficial layers of skin, the hair shaft/follicle, and the claws, quickly infecting the host while bypassing the innate immune defenses by producing fungal proteases (e.g., fungalysins, lipases, ceramidase, adhesins) that promote penetration into the keratinous tissue 5 6 7. Arthrospores released from the skin, hair, and claws of infected cats into the environment form the infectious stage of the lifecycle. These infective arthrospores develop when fungal hyphae fragment, and directly or indirectly (via clippers, brushes, bedding, etc.) make contact with a new host and create infection within hours of exposure. Once in contact with the skin, arthrospores create germ tubes to penetrate the stratum corneum and hair 5. Fungal invasion is more likely to occur in animals with microtrauma to the skin (e.g., scratches from allergies, clipper blade trauma), ectoparasites, and increased humidity 7. Clinical signs of infection develop 2-4 weeks after exposure, but infected cats shed infectious spores before signs appear 8. Infected hairs and spores remain viable in the environment for 12-18 months, but rarely serve as a source of reinfection 1.
The host immune response to the presence of dermatophytic hyphae and arthrospores, mediated by neutrophils, macrophages and cytokine release, eventually leads to spontaneous resolution of infection over a course of weeks to months; however, infection may persist in cats with a compromised immune system. This could happen for numerous reasons, such as physical trauma to the skin barrier, surgery, poor husbandry, underlying disease (e.g., allergic dermatitis, endocrinopathy, neoplasia), and immune-compromising medications (e.g., steroids, chemotherapeutics).
Clinical appearance
Dermatophytes need keratin to survive, so lesions occur in areas of the skin with the most keratin: epidermis, hair follicles and claws. Folliculitis is the hallmark of infection, and clinical lesions include papules, pustules, alopecia, broken hairs, scales, crusts, follicular casts (keratin plugs), and skin hyperpigmentation (Figures 1-3). Infected claws will become misshapen and are prone to fracture and break. Most affected cats are not pruritic. Cats with subcutaneous forms of the disease may present with nodules in the deep dermis and subcutis that develop fistulae and ulcers with purulent exudate. As is typical of most infectious dermatoses, clinical lesions are asymmetrically distributed over the body; they may be single or multifocal in nature, and the severity of clinical presentation is usually dependent upon the cat’s immune response to infection.
![Figure 1a. Multifocal areas of alopecia, erythema, hyperpigmentation, scaling and crusting on an adult female spayed domestic shorthaired cat with M. canis infection. © Amelia White](/sites/default/files/images/media/image/Schermafbeelding%202021-05-29%20om%2022.49.16.png)
Figure 1a. Multifocal areas of alopecia, erythema, hyperpigmentation, scaling and crusting on an adult female spayed domestic shorthaired cat with M. canis infection. © Amelia White
![Figure 1b. Multifocal areas of alopecia, erythema, hyperpigmentation, scaling and crusting on an adult female spayed domestic shorthaired cat with M. canis infection. © Amelia White](/sites/default/files/images/media/image/Schermafbeelding%202021-05-29%20om%2022.49.29.png)
Figure 1b. Multifocal areas of alopecia, erythema, hyperpigmentation, scaling and crusting on an adult female spayed domestic shorthaired cat with M. canis infection. © Amelia White
![Figure 2. A well-demarcated, focal area of alopecia and minor scaling on the right lateral stifle on the same kitten. © Amelia White](/sites/default/files/images/media/image/Schermafbeelding%202021-05-29%20om%2022.49.46.png)
Figure 2. Multifocal areas of alopecia and crusting on the dorsal nasal bridge and rostral muzzle of a kitten with M. canis infection. © Amelia White
![Figure 3. A well-demarcated, focal area of alopecia and minor scaling on the right lateral stifle on the same kitten. © Amelia White](/sites/default/files/images/media/image/Schermafbeelding%202021-05-29%20om%2022.49.54.png)
Figure 3. A well-demarcated, focal area of alopecia and minor scaling on the right lateral stifle on the same kitten. © Amelia White
Differential diagnoses
The prime differential in cats is superficial folliculitis, which occurs secondary to Staphylococcus spp. and Demodex spp. infections, as well as allergic dermatitis and eosinophilic granuloma complex. Less common considerations would include psychogenic alopecia, anagen/telogen defluxion, pemphigus foliaceus, pseudopelade, thymoma-associated exfoliative dermatitis, and cutaneous lymphoma.
Nodular forms of the disease may appear similar to other opportunistic bacterial (e.g., Mycobacteria spp., Nocardia spp.) or fungal (e.g., phaeohyphomycosis, hyalohyphomycosis, zygomycosis) infections, neoplasia, or sterile nodular panniculitis.
Diagnosis
Many cats go undiagnosed for days to weeks after developing infection due to camouflaging of lesions within the hair coat, or misdiagnosis as another similar-appearing dermatosis such as pyoderma (bacterial dermatitis) or allergic dermatitis. Any delay in diagnosis leads to increased contamination of the environment and risk of spread within the population of cats, dogs and people. Rapid diagnosis is critical to eliminate infection as quickly as possible. Dermatophytosis is commonly over-diagnosed based on clinical lesions alone, so additional testing is required. Many diagnostic techniques are available but they vary in reliability; however, it is usually possible to accurately diagnosis infection through a combination of tests. [...]
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