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Name
Ringworm, Canine and Feline
Short Description
Dermatophytosis, Microsporum canis, Trichophyton mentagrophytes,
Microsporum gypseum
Affected Animals: Dogs, cats, humans, horses, cows, and other
mammals. Ringworm can be transmitted between humans and animals.
Overview:
The fungal skin disease dermatophytosis has come
to be called ringworm because of the appearance of the skin lesion
that characteristically occurs with this disorder: a circular area
of hair loss with a red, raised outer rim. These lesions result from
an inflammatory reaction to the fungus. Most often, dogs and cats
are infected by the Microsporum canis fungus, but other types of
fungi cause ringworm infections as well.
Cats, especially longhaired breeds, have a more generalized form of
infection than dogs. These animals can be chronic carriers of a
fungus even though they may not show any signs of infection
themselves.
Ringworm fungi can be transmitted to humans; therefore, owners of
infected animals should consider quarantining the pet indoors until
the infection is cured. Precautions should be taken while treating
animals in order to prevent human infection and environmental
contamination.
Clinical Signs: Clinical signs include circular or patchy
alopecia; scales; follicular papules and pustules; erythema;
hyperpigmentation; and pruritus. The skin lesions most commonly are
localized to the tail, paws, face, and pinnae. However, clinical
syndromes can vary. Therefore, dermatophytosis should be considered
in the differential diagnosis list of other skin eruption patterns.
Symptoms: Hair loss that is patchy or circular may be
noted. Increased scales, reddened skin, bumps or pimples, darkened
skin tone, and itching may be present. The face, ears, feet, and
tail are the most commonly affected areas.
Description:
Ringworm is an infection by a fungus that most
often affects the hair, nails, and superficial layers of the skin.
The most commonly noted fungal types seen in cats and dogs are
Microsporum canis, Trichophyton mentagrophytes, and Microsporum
gypseum.
Animals can come into contact with infective fungal spores in the
indoor or outdoor environment. Contaminated soil is a common source
of infection, as are other animals infested with ringworm. Not all
animals that are exposed to fungal spores develop a fungal
infection, and if an infection does occur, the dog or cat may not
show clinical signs of the disease but instead serve as asymptomatic
carriers.
The classic clinical sign of ringworm is the circular patch of hair
loss with a red ring of inflammation. However, not all animals
infected by ringworm will have this type of lesion. In fact, because
the symptoms of this disease can vary greatly, ringworm should be
considered as a possible cause of skin disease in any eruptive skin
disorder.
Although most healthy dogs and cats can rid themselves of a fungal
infection on their own, some cases can be very frustrating to cure.
The asymptomatic carrier state can complicate matters. Since the
presence of disease is hidden in these cases, owners will not know
to take precautionary measures to protect against the spread of
infection. Animals that do not respond to treatment, especially
those living in multiple-cat households, should be referred to a
veterinary dermatologist or specialist.
Diagnosis:
Following a thorough history and physical exam,
testing will be performed to rule out other skin diseases that have
similar signs, such as a bacterial skin infection and skin mite
infestation. A special light, called a Wood's lamp, can be used as a
crude screening test for ringworm. Unfortunately, only 50 percent of
a specific type of ringworm called Microsporum canis will fluoresce
within the animal's fur with the characteristic apple green color.
Therefore, a negative result from a Wood's lamp does not rule out
the possibility of ringworm.
A more reliable way to diagnose ringworm is to conduct a fungal
culture on hairs taken from around the skin lesions by plucking them
with a clean instrument or brushing them with a new toothbrush. In
order to identify the source of the infection, the fungal growth is
evaluated under a microscope to determine the type of fungus
present. This assessment of the material subsequent to its growth in
a medium will rule out false positives that would otherwise be
caused by environmental contaminants.
The veterinarian may evaluate plucked hairs under a microscope to
look for evidence of fungal units associated with the hair shaft.
However, this test is more time-consuming and only carries a 40 to
70 percent success rate in detecting a ringworm infection.
In animals with severe skin abnormalities, skin biopsies may be
obtained. Although a skin biopsy can indicate a true fungal
infection of the skin as opposed to a temporary presence, this
procedure offers a less reliable diagnosis than a fungal culture.
Often, this test is performed when the skin lesions are impossible
to culture for ringworm.
Prognosis: Most healthy animals are capable of clearing a
fungal infection on their own, but this process takes months.
Because of the zoonotic potential of the disease, medical treatment
should be used in order to expedite the elimination of ringworm and
to decrease the contamination of the environment with infective
fungal spores.
Risk factors include poor nutrition, poor hygiene, and housing
situations in which a large number of animals are closely grouped
together. In addition, there is an increased risk for animals that
are immunocompromised due to disease or immunosuppressive
medications.
Treatment:
Because ringworm is infectious, animals with the
disease should be quarantined within the owner's home until the
disorder can be cured. All infected animals or asymptomatic carriers
within the household should receive topical therapy, which may
include clipping down the hair and applying an antifungal ointment
to the skin or shampooing and dipping the entire dog or cat in
medicated products. The examining veterinarian will recommend the
best approach depending on the location of the lesions. Topical
treatment is continued until a negative fungal culture is obtained.
Animals that do not appear to respond to topical treatment within
two to four weeks may be given supplemental oral drug treatment in
order to eradicate the infection more quickly. The most commonly
used oral antifungal medication is griseofulvin, but some fungal
infections may be resistant to it. Also, some animals, especially
cats, cannot tolerate griseofulvin and may develop a serious side
effect of fatal bone marrow suppression. Thus, serial complete blood
count tests are performed on cats taking this drug to watch for
evidence of bone marrow problems. Also, cats with the feline
immunodeficiency virus should not be given this drug. Ketoconazole
and itraconazole, two drugs that are not licensed currently in the
United States for the treatment of ringworm, nevertheless are used
effectively as an alternative to griseofulvin for animals that
cannot tolerate this medication. Typically, griseofulvin is safe for
dogs.
A vaccination against Microsporum canis has been developed for cats,
but the safety and efficacy of this vaccination still needs to be
researched. The use of the vaccine may be recommended in frustrating
cases of ringworm infection.
Infections can be very difficult to eradicate in multiple-cat
households or breeding facilities and often require the consultation
of a veterinary dermatologist. Humans should wear gloves while
treating the infected animal and follow the recommended protocol for
avoiding infection, including a thorough disinfection of the indoor
environment. If human infection does occur, prompt medical attention
is advised.
Prevention: Avoid geographical areas suspected of containing
fungal spores. The animal's environment, brushes, bedding, and other
potentially contaminated objects should be disinfected periodically
with a 1:10 dilution of bleach to water. In multi-animal households,
all animals should be tested with a fungal culture even if they are
not exhibiting clinical symptoms of infection. Some animals,
especially longhaired cats, can be asymptomatic carriers of ringworm
for long periods of time. The examining veterinarian may recommend
additional measures of prevention.
