| An occasional cause of skin
conditions in pets is caused by Ringworm. The scientific name for
this disease is Dermatophytosis. It is caused by a fungus not a worm,
and the lesion is not always in the shape of a ring. Since fungi are
everywhere in our environment, it is difficult to determine which
pets will develop the problem. The fungus that causes Ringworm can be
cultured from the hair coats of normal dogs and cats. These pets
might be carriers of the disease to other pets along with people. We
tend to see the problem more in young animals.
People will sometimes pick up a case of Ringworm
from their pet, but just because a pet has Ringworm does not necessarily mean
that the people that interact with that pet will develop the problem. A dog
or cat can transmit Ringworm to a person without showing any symptoms at all.
Cause
There are 3 specific fungi of
significance in this disease.
- Microsporum canis
The source of this species of
Ringworm is almost always a cat.
- Microsporum gypseum
This species of Ringworm is
usually from dogs and cats that dig into contaminated
soil.
- Trichophyton
mentagrophytes
This species infects dogs and
cats when they are exposed to rodents or the burrows they live
in.
In cats, almost all cases of
Ringworm are caused by Microsporum canis. In dogs the majority of
cases are caused by Microsporum canis. Which of these 3 main
dermatophytes causes the Ringworm in dogs depends on geographic
location.
Symptoms
The skin lesions that appear with Ringworm
are variable, and do not necessarily form a ring.There
will be hair loss, usually in small patches at first. As time goes on the patches
may disappear or appear at other locations on the skin. There might be scratching
due to itchiness. If the hair loss occurs on the face or feet there is a chance
it is due to digging habits or exposure to rodents.
This patch is typical of the lesion seen in Ringworm. A diagnosis
of this disease can not be made based just on the appearance of this lesion
because other skin conditions (Demodex
for example) can show similar lesions.
Diagnosis
There are several different ways to diagnose
Ringworm. All require some type of test because it is impossible to make the
diagnosis just by looking at the skin. This concept holds true for all skin
conditions; making a diagnosis of a skin disease requires all of the aspects
of the diagnostic
process.
If a person in a household has
been positively identified with Ringworm by their physician it is
possible they obtained it from their pet, even if their pet has no
symptoms of the disease. This is especially important in multiple cat
households. We will culture these pets using the culture technique we
describe below, but in this case, we might run a new toothbrush over
the hair coat to obtain a sample for culture.
One of the simplest ways to
diagnose Ringworm is with the Woods lamp, which is an ultraviolet
lamp, also know as a black light. 50% of the Microsporum canis
species will fluoresce when the Woods lamp is placed near the area of
hair loss.
The lamp emits a
purple/blue glow from the tube, and when there is fluorescence on
the skin, it has a greenish appearance. Other material on the skin
(dander, medication, etc.) can also fluoresce, so interpretation
is important.
Since only 50% of a certain species of Ringworm
fluoresces under the glow of the Woods lamp, a culture is used to
verify the diagnosis:
The first step in the culture
process is to gently remove hair follicles in the area of the
lesion
These hairs are cultured in a
special media that inhibits bacterial growth and enhances fungal
growth. This culture can be sent to our outside lab or done in house.
Since a fungus is a slow growing organism it can take up to several
weeks to determine if there is growth or not.
The positive
culture on the right, from our in house lab, demonstrates two
findings that are needed for a positive diagnosis. The first is
the cottonish fungal growth, and the second is the reddish color
of the culture media. This color change must occur at the same
time the fungal growth appears.
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The culture media prior to the
start of the test.
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Positive fungal growth after 10
days of incubation at room temperature.
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Treatment
Topical shampoo therapy is used in
almost every case, especially in longer haired pets. It is common to
clip some or all of the hair in some pets to make it more effective.
These baths will also remove infected hairs that can be the source of
an infection to people or other animals.
Specific anti fungal cremes are
also used when a pet is infected in an area that already has sparse
hair growth, or there are small, discrete lesions.
Oral anti fungal medications are
also used in select cases. They have the potential to cause side
effects, so their use is confined to specific situations.
In some pets the disease may
resolve by itself.
Prevention
Since fungi are everywhere it is
almost impossible to prevent exposure. Pets that chase rodents,
especially into burrows, might be at an increased risk.
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