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Feline Heartworm Infection
By Wendy C. Brooks DVM, DABVP Feline Heartworm Disease
Section of normal cat lung.
White areas would be full of air.
Section of lung from a cat with circulating larval
heartworms. The cells of inflammation have
thickened the tissues so that oxygen absorption
is challenged and there is far less room for air.
Thanks to the American Heartworm Society for these pictures. Images courtesy of Dr. Ray
Dillon and Dr. Byron L. Blagburn, Auburn University College of Veterinary Medicine
Do Cats get Heartworm?
The answer is an unequivocal yes but the feline situation is vastly different from the canine
situation. While it is true that the feline infection is not as common as the canine infection,
the feline infection has recently been found to be a much more widespread problem than
previously believed. In the past, a common statistic was that within a given geographic
area, the feline heartworm infection rate was approximately 10% of the canine infection
rate. Recent research indicates this is not so; in heartworm endemic areas, the incidence of
feline heartworm infection rivals or surpasses that of feline leukemia virus or feline
immunodeficiency virus . An incidence of 2% to 14% of all cats has been reported for
endemic areas, making heartworm a concern for any cat living where there are mosquitoes.
The Parasite and its Migration
•
• The cat is not a natural host
for the heartworm, which
means the migrating larval
heartworm is not likely to
complete its life cycle. The
migrating worm uses
molecular sign posts to tell it
how to get to its host’s
pulmonary arteries. The
worm is prepared to read
CANINE directions and may
get lost in the feline body,
ending up who knows where.
Most of the larvae that
actually make it to the
pulmonary artery die soon
afterwards due to the
massive immune attack from
the feline body. Very few
larval heartworms survive to
adulthood in cats.
• Whereas a moderate
heartworm infection in a dog
would involve 25 to 50 adult
heartworms, infected cats
typically have less than six
adult worms. Because the
feline heart and blood
vessels are so small, these
few worms can wreak havoc.
In a dog, six worms or fewer might not be considered worth treating. In a cat, a
single worm could easily represent a lethal infection.
• Whereas worms found in the canine heart can reach lengths up to 14 inches, the
average length of worms found in feline hearts is only 5 to 8 inches long.
• While an adult heartworm can expect to live 5 years in a dog, it will only live 2 to 3 years in a cat, probably due to the cat's strong immune reaction.
Heartworm disease in cats is caused by the inflammatory reaction generated by
the worm’s presence.
In dogs, heartworm disease is mostly about the obstruction of blood flow from the
physical size of the worms.
See an animated depiction of how heartworm infection causes disease in a cat, courtesy of Pfizer Animal Health.
Symptoms of Disease
The cat's immune system is extremely reactive against heartworms. For this reason, it is
virtually impossible to detect microfilariae in an infected cat. (The cat's immune system
removes them too quickly.) Also, symptoms of infection tend to be more immune-related
than heart-failure related. Cats develop more of a lung disease, complete with respiratory
distress, and chronic coughing or vomiting. Feline heartworm disease is often misdiagnosed as feline asthma. Sudden death may occur just as it may occur in infected dogs.
In cats there are two phases where the disease can exert symptoms. The first is when
immature worms reach the lung and pulmonary arteries, as early as 75 to 90 days after
infection. Even small worms are inflammatory and disruptive to the circulation. Cells of
inflammation infiltrate the lung and interfere with the cat’s ability to breathe. The second
phase where problems can occur is when the worm dies. Since cats are not the natural host
for this parasite, most immature worms that make it to the lung are killed. The presence of
the dead worm is extremely inflammatory. (Imagine your body trying to remove or digest the dead body of another animal inside your lung and circulation!)
The effects of this kind of widespread inflammation can reach far beyond the lung and
circulatory system. The kidney can be affected as well as the gastrointestinal tract and even
the nervous system.
Heartworm disease is primarily a lung disease in cats, not a heart disease.
Diagnostic Testing
In dogs, diagnosis is usually not complicated. A blood sample is tested for proteins that can
only be found in the body of the adult female heartworm. In cats, disease is not dependent
on adult worms so this kind of testing has limited applications. Furthermore, there may be
too few adult worms to generate a positive antigen test.
In dogs, testing for microfilariae (off-spring of adult heartworms born in the host’s body) is
also commonly performed. Unfortunately, in cats microfilaria testing is virtually worthless.
First of all, infected cats usually do not have enough adult worms to produce off-spring.
There may be only a few adult worms and single sex infection is common. Microfilariae, if
any, are simply cleared too quickly to be found reliably. As mentioned, in a cat heartworm
disease stems at least in part from migrating immature larvae. No adult worms (and thus
no off-spring) are necessary for disease so testing for microfilariae is not worthwhile in a cat.
Antibody testing may be more sensitive but is not adequate by itself. A negative antibody
test is good evidence that a cat is not infected; however, a positive antibody test may
indicate several things. It could indicate a mature infection. It could indicate immature
worms in the body. It could also indicate a past infection. (Antibody levels will remain
somewhat elevated after the heartworms have long since died of old age.) This means that
a positive antibody test should be accompanied by some kind of signs of heart disease
(either symptoms or radiographic or ultrasonographic evidence) or with a positive antigen
test before making a diagnosis of active heartworm infection in a cat.
So if no single test is reliable, what are we supposed to do for testing? The American
Heartworm Society currently recommends using both an antigen test and an antibody test
for screening apparently healthy cats. If a cat is sick and heartworm disease is suspected,
both these tests are recommended, plus chest radiographs and/or echocardiography to
assess heart and lung disease.
Treatment
Since the major signs of disease in cats are due to inflammation and immune stimulation, a
medication such as prednisone can be used to control symptoms. In general, if the cat does
not appear sick, the American Heartworm Society recommends attempting to wait out the worm's 2 to 3 year life span and simply monitor chest radiographs every 6 months or so.
The same heartworm adulticide therapy used in dogs is best not used in cats as it is
extremely dangerous to do so and is considered a last resort. There may not be a choice,
however, depending on the degree of illness from the heartworm disease. Approximately
one third of cats receiving heartworm adulticide therapy will experience life-threatening
embolic complications when the worms die suddenly (generally an unacceptable statistic).
One month of cage confinement is typically recommended to control circulatory effort after adulticide treatment.
Prevention
The good news is that feline heartworm infection is 100% preventable and there are currently four products on the market that are reliably effective.
The dose of ivermectin (active ingredient of Heartgard) needed to prevent heartworm
infection in cats is about 4 times higher than that in dogs. Heartgard was the first FDA-
approved heartworm prevention medication available for cats. It is a monthly flavored
chewable available by prescription. The American Heartworm Society recommends testing
prior to administration.
Interceptor® also makes a monthly chewable for cats with the same active ingredient
(milbemycin oxime) as Interceptor for dogs. Interceptor for cats also protects against hookworms and roundworms.
Revolution® entered the anti-parasite scene in 1999. This product covers fleas,
roundworms, hookworms, and ear mites in addition to preventing heartworm in cats.
Uniquely, this product is applied topically rather than orally. Read more information direct
from the manufacturer of Revolution.
Advantage Multi® is a new product from Bayer which combines imidacloprid for flea control
and moxidectin for heartworm preventive in one product. Read more information direct from the manufacturer of Advantage Multi®.
The American Heartworm Society recommends monthly preventive for ALL
CATS in heartworm endemic areas. Read their feline guidelines.
Other helpful sites include:
www.knowheartworms.org
www.petsandparasites.org/cat-owners/heartworms.html
Date Published: 1/1/2001
Date Reviewed/Revised: 1/11/2008
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