by Lorraine Shelton

The World Association for Veterinary Dermatology has recently released their guidelines for the treatment of feline ringworm, most commonly caused by the infection of the outermost layer of skin with the fungal organism arthroderma otae (previously known as microsporum canis).

Although ringworm is often a self-limiting disease, where the immune system of the cat can clear signs of infection over the period of a few months even without treatment, the fact that it can be so easily spread to other cats, and even humans, makes it a disease that we need to take seriously as cat fanciers. Appropriate treatment can shorten the course of the disease and prevent transmission.

In the general cat population, ringworm is less prevalent than allergic dermatitis, bacterial infections, ear mites, and fleas, so these potential causes of skin issues need to be considered as well. The severity and presentation of ringworm infection are a reflection of the individual cat’s immune and inflammatory responses. Cats living in high-stress situations, including high-density environments, are at greater risk of developing dermatophytosis than those from less stressful living situations. Fewer cats in more space tend to be healthier.

A cat must have a strong enough immune system to be able to produce a strong cell-mediated immune response to infection, in order to completely respond to treatment.

The primary mode of dermatophyte transmission is through direct contact with infected hairs or lesions. Keep in mind, however, that infective spores are shed by cats long before clinical signs are apparent. Despite commonly held fears of cat owners, infection from a contaminated environment is rare. Trauma to the skin can predispose a cat to infection, therefore fleas and mites must be controlled as part of the treatment plan.

Diagnosis

• Wood’s lamp examination
• Culture
• PCR (polymerase chain reaction) testing

Wood’s lamp examination under magnification of superficial skin scrapings and hairs plucked from lesions was found to be positive in 87.5% of cases of M.canis dermatophytosis. A “black light” is not the same as a Wood’s lamp: while both emit portions of the ultraviolet light spectrum, a black light emits a substantial amount of visible light, which makes it hard to see the fluorescence.

A negative dermatophyte PCR in a treated cat can be used to establish that the cat is cured and no longer infectious. Fungal cultures are associated with a significant level of false negative and false positive results, however a negative fungal culture in the absence of lesions, combined with a negative Wood’s lamp evaluation can be used to demonstrate that treatment has been successful.

Glowing hair tips on Wood’s lamp examination of a treated cat does not necessarily mean the cat is not cured. Because the fluorescence is associated with a pigment (pteridine) produced by the organism, hairs that fluorescence only at the tips is a common finding in cured cats.

Because skin lesions and infected hairs transmit the infection, isolation of infected cats, topical treatment of the skin and coat, and removing shed hair completely from the premises are the initial, essential steps in treatment. Whole body, twice a week lime sulfur or enilconazole dips have been demonstrated to be effective topical treatments. Accelerated hydrogen peroxide, climbazole, and terbinafine topical treatments have also been used, but efficacy of these agents has not been demonstrated through studies.

Treatment

Treating only lesions is not recommended, as infectious spores can be found distant from these lesions. Unfortunately, topical treatment often fails without concurrent systemic treatment with itraconazole, fluconazole, or terbinafine. One study demonstrated that with combined topical and systemic treatment, cats can be non-infectious to humans and other cats within a week. However, a longer treatment course is needed to prevent re-infection.

Whole body clipping may actually spread the infection to other areas of the body, and may cause trauma to the skin, spread infectious hairs to the environment, and increase stress to the cat. Infected cats should be confined to an easily cleaned area, but not isolated from human contact. People interacting with infected cats should wear gloves as well as shoes and clothing that are washable.

Twice weekly cleaning of the environment is recommended, with special care taken to remove shed hair, followed by application of a disinfectant. Ordinary washing with laundry detergent of washable items is sufficient for disinfection; bleach and hot water have been found to be unnecessary. Since bleach is a respiratory irritant, accelerated hydrogen peroxide or enilconazole (where available) can be used in lieu of bleach on washable surfaces requiring disinfection.

Ringworm does not “live” or “multiply” when not on a live host, unlike mildew or mold, it needs keratin (skin and hair) as a source of nutrients. Spores are easily removed by mechanical cleaning and washing.

Prognosis

Although feline ringworm can infect people, it is easily treated in humans, even immunocompromised ones. A ringworm infected cat needs to be treated appropriately and completely, but this infection should not be used as a reason for euthanasia in a shelter or relinquishment from a loving home.

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