Download Terry Animal Hospital

Terry Animal Hospital
Drs Perrier, Foy, Ng, & Funk
3 Washington Street, Rockville Centre, NY 11570
516-764-2880
www.TerryAnimalHospital.com
Feline Heartworm Disease
Heartworms are 9 to 11-inch long worms that live in a cat’s heart or in the
arteries leading to the lungs (pulmonary arteries). Although heartworms occur
commonly in dogs, most people do not consider them a problem for cats.
However, recent studies of cats with heart & respiratory diseases have found
an incidence of heartworms far greater than previously thought.
How are heartworms transmitted to a cat?
Heartworms are transmitted by mosquitoes.
When an infected mosquito bites a cat, it
deposits baby heartworms (larvae) into the
bloodstream that migrate & mature for
several months, ending up in the pulmonary
arteries & right side of the heart. They
mature into adult heartworms about 6 months
from the time they enter the cat. Shortly
thereafter, they begin to release immature
heartworms known as microfilaria.
Microfilariae live in the cat’s blood for about
one month. They are ingested by mosquitoes
feeding on the cat. However, most
mosquitoes originally acquire microfilaria by
feeding on heartworm-infected dogs.
Because of their life cycle, a cat must be
bitten by a mosquito in order to be infected
with heartworms. Heartworms are not
transmitted directly from cat-to-cat or dog-to-cat. (see “Life Cycle of the Heartworm”)
What signs will a heartworm-infected cat show?
One of the difficult things about diagnosing heartworms is that there are no consistent clinical
signs. The most common signs are coughing & rapid breathing however, both can be caused by
many other diseases. Other common signs include weight loss & vomiting which are also
common in other diseases. Some cats seem to be normal before dying suddenly. This occurs
secondary to a reaction within the lungs to the young heartworms or when heartworms (living or
dead) enter the pulmonary arteries & obstruct the flow of blood to the lungs.
October 2011
How are heartworms diagnosed in the cat?
There are several methods used in diagnosing heartworms but unfortunately, none are 100%
reliable by themselves. Therefore, a combination of tests is often required. Depending on the
case, we will recommend some or all of the following procedures.
Heartworm antibody test: This is the first of two relatively new tests that are proving very
helpful in diagnosing heartworms. This heartworm antibody test determines that a cat’s immune
system has been exposed to heartworms. A positive test indicates an active infection or a cat
whose heartworms have died, leaving behind antibodies for an unknown period of time. We
currently believe the antibodies persist for 2-4 months after the heartworms have died. Cats with
late-stage larvae that are not yet adults & cats with adult heartworm in places other than the heart
may also test positive with the antibody test. This test is very sensitive, so it is used first as a
screening test. However, if it is positive the next test is performed.
Heartworm antigen test: This test detects the presence of adult female heartworms. It is very
specific, but not as sensitive meaning a positive test indicates that heartworms are present (but a
negative test does not mean that they are absent). Because the cat must have at least two adult
female worms present to make this test positive, a negative test may mean that the cat may only
have a small number of worms or all the worms are male.
In summary, a diagnosis of heartworms is confirmed if both the antibody & antigen tests
are positive. We specifically carry the feline heartworm tests to test cats for heartworm disease.
The canine heartworm antigen test should not be used to test cats for heartworm disease as it is
not as sensitive to feline heartworm disease.
Cytology/Blood Smear examination for microfilariae: Blood can be
tested for the presence of microfilariae however, less than 10% of cats
with heartworms have microfilariae in their blood. The microfilariae
are only present for 1-4 weeks. A negative test means little. (see
“microfilaria & adjacent red blood cells ”).
Blood chemistries: A Complete blood count (CBC) can test a patient’s blood for circulating
eosinophils. This type of white blood cell will oftentimes be markedly elevated when parasites,
such as heartworms, are present. Eosinophils will be increased only for a few months following
heartworm infection. In addition, cats with intestinal parasites (intestinal worms) as well as
allergies can also have elevations in circulating eosinophils.
Radiographs (x-rays): permits us to view the size & shape of the heart. They also allow us to
measure the diameter of the pulmonary arteries. Many cats with heartworms have larger
pulmonary arteries. Some vessels may be obstructed with worms & appear to suddenly come to
an apparent stop. Even so, many cats with heartworms have no abnormal findings on the
radiographs, especially early in the infection.
Angiogram: A “moving x-ray” study in which contrast material (dye) is injected into the
circulatory system, highlighting the heart, veins, & arteries. With the blood tests above & the
moderate level of risk this procedure involves, this diagnostic tool is not used often.
Echocardiography: An ultrasonic examination that allows us to
see into heart chambers & even, on occasion, visualize actual
heartworms. However, worms are not visualized in many cats.
This procedure does not involve radiation & has become
increasingly common practice in veterinary medicine.
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Can heartworms in cats be treated?
There are no drugs approved for treating heartworms in cats. One of the drugs
for treating dogs has been used in cats, but there are potential side-effects.
Another problem is that dying heartworms pass through the pulmonary arteries
to the lungs & can cause sudden death. Thus, we have a dilemma when a cat is
diagnosed with heartworms. One of two choices must be made:
1. Treat with the drug designed for dogs, knowing it has been shown to have side-effects in
cats. These side-effects include acute pulmonary (lung) failure & death in a small percentage
of cats.
2. Treat the symptoms of heartworm disease & hope the cat outlives the worms. Since
heartworm infections in cats last about 2 years, several months of treatment are needed.
When cats are in a crisis, they are treated with oxygen & corticosteroids (cortisone) to relieve
the reaction occurring in the pulmonary arteries & lungs. If needed, drugs to remove fluid
from the lungs (diuretics) are also provided. If & when stable, they are treated continuously
or periodically with corticosteroids. The threat of acute crisis or sudden death always exists.
Is there a way to prevent heartworms in cats?
All cats & dogs should be on appropriate monthly heartworm preventative. Prevention of
heartworm disease in cats is safe & easy. The reasons you should consider protecting your cat
from heartworm disease include:
1. Diagnosing heartworms is not done as easily in cats as it is in dogs. A simple & reliable inhospital blood test is not yet available & the tests that are most reliable must be sent to an
outside laboratory. Often, radiographs or ultrasound studies are needed to confirm the
diagnosis. Many cats are diagnosed with an autopsy following sudden death.
2. Heartworms are not nearly as common in cats as they are in dogs however, they are more
common than we realize. As we look more aggressively for heartworms in cats with
increasingly better tests, we expect to find the incidence is greater than our current statistics.
3. There are no good treatment options for heartworm-infected cats. Effective drugs are not
available & cats that seem to be doing well may die suddenly. Treating heartworm infections
in cats is risky & not treating these cats is just as risky. If they are cured of the disease, it
takes about two years.
4. Prevention is easy: Cats given heartworm prevention drugs have shown no signs of toxicity.
There is a wide margin of safety even in kittens at least 6 weeks of age.
5. Indoor cats get heartworms too: Exposure to mosquitoes is required for transmission. Cats
do not have to be exposed to heartworm-infected cats or dogs. Obviously, cats that go
outdoors are more likely to be exposed however, about 25% of cats diagnosed with
heartworms are reported by their owners to be exclusively indoors. This simply means that
mosquitoes that come into the house are just as dangerous as the ones outside.
Have you read about “Flea Control”?
Client information sheet based on material written by Ernest E. Ward Jr., DVM.
© 2002 Lifelearn Inc. Used with permission under license. 4/30/2010.
October 2011