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.
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
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.
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
• 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 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.
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
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.
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:
• 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
• 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.
• Vacuum all surfaces and use the disposable electrostatic dust-trapping
cloths to remove dirt and spores
• Apply disinfectant to all surfaces. Disinfectants can be used daily,
but they are harsh and irritating to people and cats. Weekly application
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.