Feline dermatophytosis: Clinical features and diagnostic testing

Feline dermatophytosis is caused by the superficial fungal infection of keratinized cutaneous structures in cats. Dermatophyte fungal organisms may be zoophilic, geophilic, or anthropophilic. Zoophilic dermatophyte species are adapted to living on animal hosts. The organisms bind to keratin in superficial skin and use it as a nutrient source, when appropriate local cutaneous conditions are available to them (1,2). Microsporum canis, M. equinum, Trichophyton equinum, T. verrucosum, and T. mentagrophytes are some of the important zoophilic dermatophytes in veterinary medicine. Geophilic dermatophyte species such as M. gypseum are primarily non-pathogenic and cause decomposition of hair, feathers, and horn present in soil post-shedding, although they may sporadically infect animals through contaminated soil (1).

Microsporum canis is the most common and important cause of dermatophytosis in cats, due to its pathogenicity and significant zoonotic potential (2,3). Rarely, dermatophytosis in cats may be caused by simultaneous infection with 2 different fungi (2). Studies on the normal fungal flora of healthy pet cats have shown that M. canis is not part of the normal skin microbiome of cats (4,5). Thus, dermatophyte organisms isolated from the feline hair coat are not true flora as is the case for bacterial or Malassezia colonization.

The infective form of dermatophytes is the arthrospore, which is formed by fragmentation of fungal hyphae into small infective spores. Infection can be transmitted by direct contact with infected hair and scales on animals, in the environment, or through fomites (1,2). Cutaneous trauma is an important factor in development of clinical infection. Traumatic injury from an infected animal (cats being the most important source), as well as combs, brushes, clippers, bedding, collars, transport cages, and ectoparasites can favor disease transmission by creating micro-trauma to the skin. Transmission from contaminated environments without micro-trauma is not efficient. Experimental dermatophyte infections required that the skin surface was lightly abraded prior to dermatophyte inoculation and kept moist as the infection developed (1,6). Interestingly, normal grooming in cats likely serves as a host defence mechanism, as clinical infection is difficult to establish in laboratory cats experimentally infected with dermatophytes, with use of Elizabethan collars and prevention of self-grooming needed to allow for clinical infection to develop (6).

Studies on prevalence of skin diseases diagnosed in cats have found that dermatophytosis is an uncommon diagnosis, even in cats presented with skin disease. In a Canadian study, dermatophytosis was diagnosed in only 4 of 111 cats (3.6%) presented for skin disease (1,7). In US and UK studies of a greater number of cats, dermatophytosis was found in an even lower percentage of cats with more common diagnoses of allergy, feline atopy, bacterial skin infections, Otodectes mites, cat bite abscesses, and flea infection being reported (1,5,8). In a study on the causes of pruritus in cats, only 2.1% were diagnosed with a fungal disease (9). The incidence of dermatophytosis is higher in hot humid climates and lower in cold dry climates.

Immunosuppressive diseases are historically thought to predispose cats and dogs to the development of dermatophytosis. However, the recently published Clinical Consensus Guidelines of the Word Association for Veterinary Dermatology (WAVD) on dermatophytosis reported that seropositive feline immunodeficiency virus (FIV) and/or feline leukemia virus (FeLV) status in cats alone does not increase the risk of dermatophytosis (1). While FIV and FeLV seropositive cats may exhibit a greater diversity of saprophytic fungal carriage and increased carriage of Malassezia in general, dermatophyte carriage is largely not different from that of seronegative cats (10,11). When immunosuppressed cat are culture positive for M. canis, they may be free of clinical signs of dermatophytosis (11). The development of concurrent dermatophytosis in cats receiving immunosuppressive drugs for the treatment of pemphigus foliaceus is not common (1). Immunosuppression may yet be a factor to consider as demonstrated by the report of 1 cat developing M. canis dermatophytosis while receiving cyclosporine for treatment of pseudopelade (12). Poor nutrition, pregnancy, lactation, and anti-inflammatory or immunosuppressive drug therapy including corticosteroids, should be considered in patients presenting with clinical signs suggestive of dermatophytosis (3,13). Presence of ectoparasites such as fleas and Cheyletiella mites can be important in establishment and spread of dermatophytosis in multiple-cat households (13).

Clinical signs of dermatophytosis in cats

Feline dermatophytosis most often appears as a superficial dermatosis due to the pathogenesis of the disease, affecting keratinized tissues. The most consistent clinical sign is single or multifocal circular patches of alopecia with variable scaling (Figure 1). Some patients may experience the classic ring lesion with central healing and fine follicular papules and crusts at the periphery. However, signs and symptoms are highly variable and depend on the degree of inflammation based on host-fungus interaction (2). Typically, lesions are asymmetrical. Pruritus is usually minimal or absent; however, it may be marked when present. When pruritus is present, self-trauma can mimic lesions associated with pyotraumatic dermatitis or ulcerative eosinophilic lesions in cats (1,14).

Figure 1.

Generalized alopecia and hair loss in a cat affected with Microsporum canis dermatophytosis (photograph provided courtesy of Dr. Martín Acevedo Arcique).

Lesions noted can be quite variable and include irregular or annular areas of alopecia, scale, crust, comedo formation, erythema, hyperpigmentation, seborrhea, and follicular papules. Lesions tend to occur most commonly on the face, ears, and muzzle, later progressing to paws and other cutaneous sites. Less commonly, miliary dermatitis, chin folliculitis, or onychomycosis may be noted. Dermatophytosis is a differential diagnosis in cats with pododermatitis, including changes in nail growth and appearance (1,2).

If clinical signs alone are relied upon for a diagnosis, dermatophytosis has the potential to be over-diagnosed in both cats and dogs. On the other hand, the diagnosis may be missed because of the variable nature of skin lesions (2). Several other dermatoses, especially staphylococcal folliculitis, can mimic the classic “ringworm” lesion. Clinical signs and patient presentation can be suggestive of pemphigus foliaceus in some patients due to scaling and crusting over the face and pinna, with or without paronychia. Generalized dermatophyte infection is more common in cats than in dogs.

Nodular lesions due to feline dermatophytic infection

Cats can occasionally develop nodular dermatophyte infections, including kerion, pseudomycetoma, and mycetoma reaction patterns. Clinically, these animals are presented with 1 or more sub-cutaneous nodules. Concomitant superficial dermatophytosis lesions may be present, or the cats may exhibit normal skin apart from the nodules.

According to the Clinical Consensus Guidelines of the WAVD, indirect evidence is available that Persian cats are predisposed to dermatophytosis (1). Cats presented with superficial and nodular forms of dermatophytosis most commonly include long-haired breeds, particularly the Persian cat. As with dogs, dermatophytosis is more common in younger cats, less than 1 y of age (2).

Diagnosis of feline dermatophytosis

As dermatophytosis is an infectious and contagious disease with zoonotic potential, rapid confirmation or rule out of the condition is desirable. Point-of-care tests are available in addition to diagnostic tests that can be pursued through reference laboratories. While there is no specific gold standard test available for dermatophytosis, a range of diagnostic modalities is available. Often more than 1 of the available diagnostic tests is needed in the work-up of a cat suspected of dermatophytosis. Diagnostic testing should be tailored based on the presenting patient’s stage of infection, types of lesions, presence or absence of previous treatment, quality and availability of testing options, as well as clinician’s experience and training. Some of the diagnostic tests are also useful in monitoring treatment and confirmation of cure in affected cats. These tests include:

1. The Wood’s lamp is an inexpensive, non-invasive point-of-care diagnostic tool, which can be used in clinic for immediate results. Characteristic green fluorescence is observed on M. canis infected hair shafts due to a chemical interaction that occurs as a result of the infection. As per the Clinical Consensus Guidelines of WAVD, Wood’s lamp examination is an important diagnostic test that is likely to be positive in most cases of M. canis dermatophytosis, if used appropriately (1). Fluorescing hairs are most likely to be found in untreated infections but may be difficult to find in treated cats. False positive and false negative results can occur due to inadequate equipment, lack of magnification on the lamp, patient compliance, poor technique, or lack of training.

2. Dermoscopy is a non-invasive point-of-care diagnostic tool that allows for illuminated magnification of the skin. Unique to the cats with dermatophytosis are the findings of comma hairs (opaque, slightly curved or broken hairs with homogenous thickness) with or without variable amounts of brown to yellow crusts. Dermoscopy findings in cats with dermatophytosis are distinctly different from findings in the skin of cats that are normal or with disease due to other causes (15–17).

3. Direct microscopic examination has been used to rapidly confirm the presence of a dermatophyte infection. It involves microscopic examination of hair and scales for hyphae or fungal spores, or both (Figure 2). Hairs and scales can be mounted in mineral oil, compounded chlorphenolac, or potassium hydroxide.

4. Fungal culture is often regarded as the gold standard of diagnosis. Dermatophyte Test Medium is commonly used and contains a nutrient growth medium with antibiotics to suppress bacterial and contaminant fungal overgrowth and a color indicator to aid in the early recognition of possible dermatophyte species. While it is a vital diagnostic and treatment monitoring tool, the test merely detects the presence or absence of fungal spores on the hair coat or hair sample. False positive and false negative test results can occur. The test is helpful in identifying the dermatophyte species involved. Three sampling techniques for small animals have been described in the literature: hair coat brushings, hair plucking, and sticky tape sampling. The “Mackenzie” brush technique is the most widely used. While there is no standard description of the technique, soft bristle tooth brushing for 20 brush strokes, or 2 to 3 min of brushing, or brushing until the bristles are full of hair are all sampling end points that have been described in the WAVD Consensus Guidelines (1). The technique is very sensitive with respect to detecting spores on the hair coat, even in carrier animals.

5. Dermatophyte DNA detection using polymerase chain reaction (PCR) is a newer diagnostic test. The test has a rapid turnaround time, although a positive PCR does not necessarily indicate active infection. Non-viable fungal organisms from a successfully treated infection as well as from non-infected carrier animals will still be detected. False negative test results can also occur.

6. Biopsy sampling for histological examination of tissue is rarely required as a routine diagnostic test for superficial dermatophytosis. The test can be useful in cases presenting with undiagnosed non-healing wounds or nodule(s) due to kerion, pseudomycetoma, or mycetoma. Special stains such as periodic acid Schiff (PAS) and Grocott methenamine silver (GMS) are used for confirming or ruling out dermatophytosis.

Figure 2.

Fungal hyphae and arthroconidia evident on a direct hair examination (photograph provided courtesy of Dr. Martín Acevedo Arcique).

To summarize, while feline dermatophytosis is an uncommon disease of cats, even in cats presented for skin disease, it is of significant importance due to pathogenicity, zoonotic potential, and impact on multi-cat households. The disease may be presented with a large range of potential dermatological signs, thus lending itself to over-diagnosis and under-diagnosis of the condition without appropriate diagnostic testing. The diagnosis of dermatophytosis is typically obtained by using a number of complementary diagnostic tests, including point-of-care testing such as Wood’s lamp, direct hair examination, and dermoscopy, if available, in order to document active hair infection; dermatophyte culture and/or PCR testing to help diagnose the fungal species involved; and tissue biopsy including special fungal stains in cases of nodular or atypical dermatophyte infections. Monitoring of response to therapy includes clinical response, use of Wood’s lamp, and fungal culture. Negative fungal culture from a cat with no lesions and a negative Wood’s lamp (except for glowing tips) is compatible with cure. Negative dermatophyte PCR in a treated cat is also compatible with cure. The disease is treatable and curable once a diagnosis has been established. Dermatophytosis treatment and dermatophytosis management in catteries and shelters is beyond the scope of this discussion; the reader is referred to excellent available resources on these topics (1–3,18).