by Arnold Plotnick MS, DVM, ACVIM, ABVP

Introduction

We all know the joys of sharing our lives and our homes with our feline companions. Some things, however, are best not shared. Ringworm, a fungal infection of the hair and skin, is one of a handful of feline infectious diseases that can be transmitted from cats to humans. Cat owners are often afflicted along with their pets during an outbreak, and this has naturally generated concern among physicians. As a feline practitioner, I am very familiar with the challenges of ringworm. In fact, just last week I formulated an extensive treatment protocol for Sophia, Jules, and Barkley Schmidt – three very fluffy longhaired Persian cats from the same household, all afflicted with ringworm. Their owner, Rachel Schmidt, had the misfortune of contracting it as well, on her arm. In this article, we’ll discuss the pathogenesis of ringworm, how to diagnose it, and how to treat it.

It’s not a “worm”, it’s a fungus

Despite the name, ringworm is not a “worm”. It is a skin fungus. Depending on their natural habitat, skin fungi can be categorized as geophilic (living in soil), anthrophilic (living on people), or zoophilic (living on animals). Microsporum canis, the species of ringworm that most commonly affects cats, is a zoophilic fungus that has become so well-adapted to cats that it may live on their hair and skin without causing any clinical signs of disease. Microsporum canis, however, is not part of the flora that normally resides on the skin, and the discovery of this fungus on the skin should always be considered an abnormal finding. Two other species of fungus, Microsporum gypseum, and Trichophyton spp, are less common, but may also cause ringworm in cats.

The mere presence of a ringworm spore on the hair coat isn’t enough to cause infection and disease. Cats (and humans) must contact a minimum number of spores before an infection becomes established. The minimum number of spores varies with the individual and the circumstances of exposure. Many factors, including young age, concurrent disease, drugs that suppress the immune system, compromised immune status, poor nutrition, stress, and overcrowding will predispose cats to acquiring ringworm. Cats in animal shelters and catteries are much more likely to harbor ringworm than pet cats, and isolation of this fungus from even one cat or kitten in a cattery warrants treatment of the entire cattery.

Grooming is one way that cats help to remove ringworm spores from their coat. Persians, Himalayans, and other long-haired cats tend to be less efficient groomers, making them more inclined to acquiring ringworm infection. Genetic influences, however, may play a role in these breeds as well. Regardless of whether a cat is infected with ringworm or is just mechanically carrying the fungus on its hair coat, veterinary care is required because both cats can be sources of infection to other animals and to people.

Pathogenesis

Ringworm is the most common infectious skin disease in cats. Cats become infected with ringworm when they’re exposed to infective spores through contact with an infected animal, a contaminated object, or a contaminated environment. Spores are small and may also be carried on air currents and on dust particles. Once the spores reach the coat, if they survive the cat’s natural defense mechanisms (for example, grooming and sunbathing), they adhere to and invade cells called keratinocytes on the hair shaft and skin (and occasionally, the nails) and they germinate, initiating the infection.

The fungus uses keratin protein as its nutritional source. It produces enzymes that digest the dead, keratinized tissue, allowing it to penetrate the hair and continue growing. Hair shafts become weak, brittle, and easily broken. Hair fragments and skin scales are shed into the environment, along with thousands of spores. Spores can remain in the environment for months or years, serving as a reservoir of infected material for humans and other cats brought into the environment.

Cats of any age, sex, or breed are susceptible to infection. Kittens and geriatric cats, however, are more frequently affected, as are longhaired cats. Longhaired cats are believed to be more susceptible because the long hairs protect the spores from being removed by grooming. Long hair can get matted, and matted hairs are also more susceptible to ringworm infection. When cats groom themselves, they’re engaging in an important activity that helps limit ringworm infection. This may explain why older cats may be more likely to develop the condition; some cats have difficulty grooming as they age because of loss of flexibility or concurrent illness.

Kittens in general are the most susceptible population, with the head, face, ears, and forelimbs primarily involved. It is speculated that the face and ears are commonly affected spots in kittens because these areas, though groomed well by the mother, tend to be not very well groomed by kittens. Interestingly, lesions often appear in kittens at or around the time of weaning, which supports this hypothesis.

Concurrent disease can have an effect on the susceptibility to infection. For example, cats infected with feline immunodeficiency virus (FIV) are three times more likely to acquire ringworm than uninfected cats. Genetics may also play a role in a cat’s susceptibility to ringworm. Studies have shown that chronic ringworm problems are most common in catteries in which members were genetically related, and breeders may be unintentionally selecting for susceptible cats when they breed for certain coat characteristics.

Clinical Signs

The classic clinical appearance of ringworm includes one or more areas of patchy hair loss with mild or moderate crusting, but ringworm in cats can have a wide variety of presentations. Infected cats can present with any combination of the following:

• pruritus (itchiness) – in general, ringworm is not an especially itchy disease, although some cats have mild itchiness, while others are severely itchy and will scratch to the point of self-mutilation
• hair loss – ringworm causes hairloss, and this may be subtle or dramatic, and may show a symmetrical pattern or be totally asymmetrical
• crusting and scaling – ringworm lesions tend to be exfoliative. Usually, the scaling is mild, but in some cats it’s quite severe.
• blackheads – ringworm infection occasionally causes blackheads on the chin in young cats
• hyperpigmentation – ringworm infection can cause a darkening of the skin in some cats
• nail infection only – a few cats with ringworm develop crusty or greasy nail infections as their only clinical sign of ringworm
• redness – areas of hair loss are often reddened when they first develop
• overgrooming – cats with symmetrical hair loss from apparent overgrooming will sometimes be found to have ringworm infection.
• itchy ears only – unilateral or bilateral itchiness of the ears is an uncommon presentation for ringworm, but it has been seen occasionally.

Diagnosis

Diagnosis is achieved through fungal culture. Hairs from affected areas are plucked with a sterile hemostat and placed on a special fungal culture medium. Most veterinary practices use “dermatophyte test medium” as their culture medium, because it contains a color indicator that turns the medium red when ringworm starts to grow on it. Shining a fluorescent light, called a Wood’s lamp, on the hair coat may help better identify infected hairs, for better sampling for the fungal culture. Half of the strains of Microsporum canis will glow “apple-green” or “yellow-green” under the lamp. The source of the fluorescence is a metabolite that growing fungi secrete onto the hairs.

The Wood’s lamp is a screening tool; it cannot be used to definitively diagnose ringworm. If the cat with fluorescent hairs is later confirmed to be infected via fungal culture, subsequent Wood’s lamp examinations can be used to monitor the response to treatment. Depending on the stage of infection, glowing hairs will show various fluorescence patterns. During an active or early infection, the entire hair shaft will glow. As the infection resolves, only the more distal portions (i.e. the tips) of the hairs will glow, presumably because the hairs are healing and the base of the hair is no longer infected. The number of glowing hairs should also decrease during treatment.


Treatment

People can be become infected with ringworm, especially if the person is immunocompromised in some way, i.e. HIV infection, organ transplant recipients, people receiving chemotherapy, the very old, or the very young. It has been reported that 50% of people exposed to infected cats develop ringworm lesions, and that in approximately 70% of all households with an infected cat, at least one person became infected. With this in mind, veterinarians should set two goals when devising a treatment plan for ringworm: a cured cat, and a decontaminated environment. Both are required to prevent reinfection in cats, and exposure of people to this disease.

Cats that test positive for ringworm need some kind of treatment. Treatment plans may vary somewhat for each individual cat, but they all involve some combination of clipping the hair coat, topical therapy, systemic (oral) antifungal medication, and suggestions for environmental contamination. The two most common questions I get asked, as a feline practitioner, is whether it is absolutely necessary to treat cats topically (i.e. bathing), and whether cats can be treated successfully with topical therapy alone (i.e. just baths, and no oral medication). Frankly, we’ve found that the greatest success is achieved with aggressive therapy – clipping the coat, bathing with medicated shampoos, and using oral medications.

The treatment plan must take into consideration all other fur-bearing animals in the household. A fungal culture, therefore, should also be performed on all other animals living with or in contact with the infected cat. While waiting for the fungal culture results, these other pets should be bathed twice a week.

Step 1 of therapy for the infected cat is to determine whether the cat needs to have its coat clipped. When ringworm invades the hairs, they make them fragile and easily broken. When the hair shafts break, infected hair fragments and spores are shed into the environment and onto the cat’s hair coat. This shedding increases the risk of spreading the ringworm, and also increases the probability that the cat becoming re-exposed to spores in the environment. This can cause the cat to continuously test positive on fungal culture. Clipping the coat removes infected hairs and minimizes continued shedding of hair fragments and spores. It also allows for more thorough penetration of topical antifungal shampoos. Because children, elderly people, or anyone with immunosuppression is at higher risk of catching ringworm from an infected cat, any cat that lives in a household with someone who fits the above description must have its coat clipped. Otherwise, as a general rule, if a shorthaired cat has five or more discreet spots of ringworm, the entire coat should be clipped. If a shorthaired cat has less than five discreet spots, the hair around the individual spots can be clipped. If the cat is longhaired, the entire coat should be clipped regardless of how many discreet ringworm spots are visible on the coat.

All cats that test positive for ringworm should receive some kind of topical therapy. Topical therapy minimizes the spread of infective spores into the environment, and helps remove infective crusts, scales, and spores from the coat. Without topical therapy, treatment would take longer and would be more costly.

There are many effective topical products. Miconazole, either as a sole therapeutic agent, or in combination with chlorhexidine, has been shown to be effective. Manhattan Cat Specialists recommends Malaseb shampoo, which is a combination of miconazole and chlorhexidine. Studies have shown this shampoo to be very effective against ringworm. Cats should be bathed twice weekly. For the shampoo to be effective, it is important that there be a contact time of 10 minutes with the cat’s fur.

Local topical therapy with ointments and creams is generally not recommended for treatment. From a practical perspective, they are messy, easily groomed off, and tempt clients to try spot therapy instead of aggressive treatment with shampoos and oral medications. In our experience, however, Conofite lotion, applied to the affected areas on the days that the cat is not being bathed, appears to be beneficial.

The cornerstone of treatment for ringworm is systemic therapy with an oral medication. Ideally, these drugs should not be used in kittens less than 8 weeks of age. Drugs that have been shown to be effective include griseofulvin, terbinafine, ketaconazole, and itraconazole. At Manhattan Cat Specialists, we’ve found itraconazole to be very effective when given at a dose of 10 mg per kg once daily for 6 weeks. Side effects, if they occur, tend to be limited to vomiting and decreased appetite. Itraconazole needs to be prepared by a compounding pharmacy into a form that allows proper dosing. We have the pharmacy prepare a flavored liquid that makes dosing much less objectionable to the cat.

Several years ago, a study was published that suggested that lufenuron, a drug used for controlling fleas, might have antifungal activity. Subsequent studies have been disappointing. Most veterinary dermatologists do not feel that lufenuron has any appreciable efficacy against ringworm and no longer recommend using it for prevention or treatment of ringworm.

A ringworm vaccine has been available for several years, but it is rarely used. The vaccine does not prevent cats from being infected with ringworm. The vaccine, however, does temporarily reduce the clinical signs of ringworm. Used alone, they’re ineffective for treatment. They may be useful, however, if combined with topical and systemic therapy.

Response to Therapy

Cats receiving treatment for ringworm usually show marked improvement in clinical signs within 2 to 4 weeks of therapy.

Four weeks after beginning treatment, cats should be reexamined. A Wood’s lamp exam should be performed to screen for infected hairs (for cats whose infected hairs fluoresced during their initial visit). Any infected hairs discovered during the exam should be removed. Infected hairs discovered on the muzzle, face, and ears suggest that the owner is having difficulty or has been hesitant to apply the shampoo to these areas.

A fungal culture should be performed at every recheck. Once a negative culture is obtained, weekly fungal cultures should be performed. Two consecutive negative fungal cultures indicates successful treatment in single-pet situations. In multi-cat situations, it’s probably best to continue therapy until three negative cultures are obtained.

Environmental Decontamination

Ringworm spores can persist in the environment for a long time, perhaps 18 to 24 months. The spores are microscopic and can be spread easily by air currents and contaminated dust, and through heating ducts and vents.

To minimize environmental contamination in households where only one single pet cat is infected, the cat or kitten should be kept in a small, easily cleaned room (such as a bathroom) that does not have carpeting. The cat should be quarantined in this room until it has received oral antifungal medication for two weeks, and a minimum of four medicated baths. At this point, the cat can be given greater access in the home, ideally in uncarpeted and easily cleaned rooms.

Thorough and repeated vacuuming, and wiping of surfaces on a daily basis, should prevent the home from becoming contaminated. Cat beds and blankets should be washed daily in hot water and bleach. Bathrooms and smooth surfaces can be disinfected with a bleach solution (1 part bleach, 9 parts water). Routine cleaning and disinfecting should continue until the cat is considered cured of ringworm.

One veterinary dermatologist with extensive expertise in ringworm recommends the following decontamination protocol:

Initially…
• Discard all cat rugs, blankets, collars, brushes, and fabric toys
• Discard any cat objects that cannot be repeatedly scrubbed, vacuumed, and disinfected easily
• Purchase a new, inexpensive vacuum cleaner with hose attachments that can be thoroughly cleaned.
• Remove and clean all drapes and decorations. In multicat households, remove and clean all heating duct and vent plates, and install disposable house dust filters behind the duct plates before replacing them. This will keep spores out of the heating ducts
• If possible, put a fan in the window so that it draws air out of the room to the outside
• Vacuum all surfaces of the room
• Dust all surfaces and ledges with a disposable electrostatic cloth (like “Swiffer”)
• Scrub all surfaces with a detergent that is safe to use around cats. Rinse all surfaces well. Apply a 1:10 dilution of bleach to all non-porous surfaces. Leaving the bleach solution on the surface for 10 minutes is ideal.
• Using a portable dehumidifier is beneficial, because humid environments allow spores to remain viable.

Then, daily…
• Vacuum all surfaces and use the disposable electrostatic dust-trapping cloths to remove dirt and spores

And, weekly…
• Apply disinfectant to all surfaces. Disinfectants can be used daily, but they are harsh and irritating to people and cats. Weekly application is acceptable.

Preventing reinfection

Indoor cats are unlikely to become reinfected once the ringworm is treated successfully, whether it’s a single cat household or a multicat household. The most likely sources of exposure are the addition of a new cat to the household, or exposure to other infected animals at boarding or grooming facilities. Any new cats to be added to a household should be screened for ringworm, especially if coming from a pet store or animal shelter, and it should probably be bathed with a medicated shampoo while waiting for the culture results.

Treating ringworm infections can be frustrating for cat owners and veterinarians. Aggressive topical and oral therapy combined with diligent environmental decontamination is necessary. At Manhattan Cat Specialists, our treatment protocol has been quite effective at controlling this pesky fungus.
 

Updated 3/29/06