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Dermatophytosis in cats: Recent evidence-based recommendations, Part One

Aug 21, 2018

Moriello KA, Coyner K, et al.  Diagnosis and treatment of dermatophytosis in dogs and cats: Clinical consensus guidelines of the World Association for Veterinary Dermatology.  Vet Dermatol. 2017;28:266-e68.

Part I.  Etiology, risk factors, and diagnostic testing

Dermatophytosis, colloquially called "ringworm," is a superficial fungal disease of the skin resulting from infection with various fungal species, and can be found in many species of animals, including cats, dogs, and humans.  Common etiologic agents of dermatophytosis in cats and dogs are fungi of theringworm genera Microsporum and Trichophyton. In feline dermatophytosis, the most commonly isolated pathogen is Microsporum canis, but other species associated with dermatophyte infection include other members of the genus Microsporum, as well as Trichophyton spp.  The goal of this clinical consensus statement from the World Association for Veterinary Dermatology is to present the best evidence-based recommendations for diagnosis and treatment of this disease, distilled from evaluation of published studies dated from 1900 to 2016 on canine and feline dermatophytosis.

It is important for clinicians to know that dermatophytes are being reclassified taxonomically based on molecular testing.  Most members of the genus Microsporum, including M. canis, are being reclassified as part of the Arthroderma otae complex.  For the near future, however, laboratories will still report these pathogens under their common, familiar species names.

Because dermatophytosis is a known zoonotic, infectious disease, dermatophyte infection in pet animals is a significant concern, although the skin lesions caused by dermatophytosis in man and animals are treatable and curable.  However, even though dermatophytosis is a common skin infection in humans, the true rate of transmission from animals to people is unknown, largely because dermatophytosis is a non-fatal and non-reportable disease. The etiologic agent in the majority of human dermatophyte infections is Trichophyton rubrum, which not transmitted by animals. Although dermatophytosis is generally self-limiting within weeks to months in immunocompetent hosts, diagnosis and treatment is important in order to shorten the course of the disease and prevent transmission to other animals and to people.  

Microsporum canis, a zoonotic dermatophyte and the most common cause of feline dermatophytosis, is not a part of the normal skin flora of cats. One study reviewed in this paper indicates that dermatophytosis is actually uncommon, even in cats with skin disease, and is less prevalent than allergic dermatoses, bacterial dermatitis, ear mites (Otodectes), and fleas.  The authors also found that seropositivity for feline leukemia virus or feline immunodeficiency virus is not in itself a risk factor for dermatophyte infection.  Microsporum canis infections usually result from contact with an infected animal, most commonly a cat.

Dermatophyte fungi invade keratinized structures, principally hair and nails.  Therfore, dermatophytosis can present with a wide variety of dermatologic disease signs.  Pruritus is variable, but is usually minimal to absent.  The variety of clinical signs and extent of disease are a reflection of the individual cat's immune and inflammatory responses. Cats living in high-stress situations such as hoarding environments are at greater risk of developing dermatophytosis than those from less stressful living situations.  A strong cell-mediated immune response on the part of the host is required for clinical cure of dermatophytosis and protection against re-infection.

Due to the zoonotic and infectious character of dermatophyte infections, a significant amount of fear has developed amongst health professionals in both the human and veterinary fields as well as among the general public.  The clinical consensus guideline panelists considered it important to reassure concerned individuals that:

                  --Dermatophytosis is self-resolving in healthy cats

                  --The primary mode of dermatophyte transmission is direct contact with                           infected hairs or lesions

                  --Transmission by fomites requires micro-trauma to the skin as a                                     predisposing factor; therefore, excellent flea control is essential, as fleas are                   fomites and cause micro-trama to the skin

                  --Acquistion of dermatophytosis from a contaminated environment by                             people or animals is rare

                  --Infective spores are shed by cats long before clinical signs are apparent;                     direct examination of hairs and skin with a Wood's lamp is a useful                                 diagnostic modality; when M. canis is present, fluorescence is common and                   most likely an inherent property of this pathogen

A variety of tests are available for the diagnosis of dermatophytosis. The panel was unable  to identify any single diagnostic test as the "gold standard" test for diagnosis of dermatophytosis.  They stated that the most important consideration regarding diagnostic testing was to determine what test(s) confirm the presence of active infection so that treatment and management decisions can be made, and what test(s) confirm the absence of active infection, identifying animals that pose no transmission risk and/or are clinically cured.

Direct examinations (including Wood's lamp evaluation) and biopsy are the only two modalities that can definitively identify actual invasion of hairs and/or skin by dermatophytes. Biopsy and histopathology of tissue is necessary when dermatophytosis may present in an uncommon form such as nodules (mycetoma/pseudomycetoma), or has a similar appearance to another uncommon dermatologic disease such as pemphigus or neoplasia.  In these cases, pathogen identification requires tissue culture or PCR (polymerase chain reaction) testing.  Dermatophytosis may present in a nodular form in long-haired cats, especially Persians.

With direct examination techniques, the combined use of a Wood's lamp as well as direct examination of superficial skin scrapings and hairs plucked from lesions was found to be positive in 87.5% of cases of M.canis dermatophytosis.  The type of Wood's lamp most likely to yield an accurate diagnosis is a plug-in electric lamp with magnification.  A "black light" is not the same as a Wood's lamp.  While both emit portions of the ultraviolet light spectrum, in contrast to a Wood's lamp, a black light emits a substantial amount of visible light, which makes it hard to see the fluorescence associated with dermatophyte infection.

PCR is a useful modality, but is not universally available.  Fungal species can be accurately identified using PCR; however, a positive PCR does not necessarily imply that there is active infection. Dead dermatophytes left behind from a succesfully treated infection and non-infected carriers will be PCR positive. A negative dermatophyte PCR in a treated cat is compatible with cure.

Fungal culture is also not a "gold standard" test.  Fungal culture may be negative in cases of infections confirmed on direct examination.  Other false negative findings on fungal culture can occur with overgrowth of contaminant fungal organisms.  Also, false positive fungal cultures may be associated with fomite carriage and environmental contamination.  A negative fungal culture with no lesions and a negative Wood's lamp evaluation is also compatible with cure.  Glowing hair tips on Wood's lamp examination of a treated cat does not imply the animal is not cured.  Because the fluorescence is associated with a water-soluble pigment (pteridine) produced by M. canis and not with arthrospores or infection itself, residually fluorescent hair tips without fluorescence in the proximal portion of the hair shaft is a common finding in cured cats.
continued in part two [PJS]

See also:

Meason-Smith C, Diesel A, et al.  Characterization of the cutaneous microbiota in healthy and allergic cats using next generation sequencing.  Vet Dermatol. 2016;28:71-e17.

 

dermatophytosis ringworm fungal fomite

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