This article is part one of a two-part series. This month we’ll discuss what heartworms are, which cats
generally get infected, and what damage heartworm disease can cause. We will also identify how
heartworm disease differs in cats from the disease in dogs. Next month we’ll examine some of the
treatments for the problems resulting from heartworms, and most importantly, how heartworm infection
can be prevented.
What is heartworm disease?
Heartworm disease is the infection of a cat by adult heartworms. If the larval (baby) heartworms in their
most immature stage are present in the bloodstream, the condition is called microfilaremia. A cat can have
both heartworm disease and microfilaremia at the same time.
What are heartworms?
The species of worms that we refer to as heartworms is Dirofilaria immitis. The worms can grow up to 5 or
6 inches long. They live inside the pulmonary arteries (the arteries that go from the heart to the lungs).
Worms can sometimes be found inside the heart’s right ventricle and/or right atrium. In addition, worms
can occasionally be found in aberrant locations, that is, in other arteries besides the pulmonary arteries.
The occurrence of worms in aberrant locations is not found in dogs.
Dirofilaria immitis go through several stages before they reach adulthood. In order to mature, the larvae of
heartworms must spend time inside a mosquito. Therefore, mosquitoes are absolutely necessary for the
spread and development of heartworm disease. Moreover, cats are not considered hosts of the disease.
This fact means that there must be a population of infected dogs in the area in order for cats to be infected.
The first stage of the heartworm’s development occurs when an adult female worm releases microfilariae
into the blood stream of a dog. The next stage happens after a female mosquito bites the dog and takes in
some microfilariae. These microfilariae continue developing inside the female mosquito (and can only
grow to the next stage in the mosquito). They reach the infective larval stage in about 2 - 2 ˝ weeks. If
the mosquito then bites a cat, it will probably infect the cat -- but cats are more resistant to infection than
dogs, so not all cats bitten will be infected.
The infective larvae enter the cat through the bite wound. The larvae migrate through the body tissues and
continue developing for the next 4 to 7 months. (The development time of the larvae is slowed down in
cats compared to dogs.) Then young adult worms enter the vascular system. Cats have very few
heartworms, generally less than 6 compared to tens, or even hundreds, of worms in a dog. Some
microfilariae are released by the fully mature, adult female worms, but cats have smaller concentrations of
microfilariae and the duration of the microfilaremia is shorter than in dogs. Adult worms only live 2 to 3
years in a cat and when the worms die the cat may then be free of the disease. But despite cats’ greater
resistance to heartworm disease, they can have more serious reactions to the heartworms, as we’ll see.
In what regions does heartworm disease occur?
Cats are at risk for heartworm disease if there are dogs in the area with heartworms; consequently,
heartworm disease can occur anywhere. But, because mosquitoes play a vital role in the maturation of the
heartworm, areas where mosquitoes are prevalent have the highest occurrence of heartworm disease. An
average of 45% of non-protected dogs (dogs that are not on a preventative program) living within 100 miles
of the Atlantic Ocean from Texas to New Jersey have been found to have heartworm disease. Some studies
have shown infection rates for dogs of over 60%. Areas along the Mississippi River and its tributaries have
a high occurrence. Most of the rest of the United States and Canada have infection rates of about 5% in
dogs. Heartworm disease occurs in other countries as well. For example, certain areas of Japan and
Australia have high (that is, endemic) rates of heartworm disease. Therefore, the areas where dogs are
most likely to get heartworm disease are areas where cats are most likely to be infected. But the infection
rates for cats will be much lower. For example, a survey of over 700 cats and 800 dogs in Florida found
that nearly 60% of the dogs were infected, but only about 4% of the cats. A study in Louisiana found that
about 10% of the cats were infected.
Which cats get heartworm disease?
Any cat can get heartworm disease, but outdoor cats are more at risk. In addition, male cats seem to be
more susceptible than female cats. Cats as young as 6 months have been diagnosed with heartworm
disease. Most cats are between 3 and 6 years old when diagnosed, but cats as old as 17 have had
heartworms. In areas where there is a high occurrence of heartworm infection in dogs, and because the
consequences of heartworm disease in cats can be dire, all cats in endemic areas should be considered for a
What damage do heartworms cause?
Live worms damage the interior lining of the pulmonary arteries. This damage can cause swelling, loss of
parts of the lining, and adhesion of leukocytes (white blood cells) and platelets to the damaged areas. .
Furthermore, the leukocytes and platelets can cause the smooth muscle cells inside the arteries to begin
dividing excessively. These extra cells can cause villi, or little fingers, to grow and project into the arteries.
In addition, the damage to the interior lining can allow fluids to leak into the perivascular interstitium (that
is, the area surrounding the outside of the arteries).
The arteries to the lungs can dilate, but the enlargement may disappear over a period of several months.
The pulmonary arteries may also get aneurysms, and the smaller arteries can become blocked by the
worms. The arteries can become tortuous, although tortuosity occurs less often in cats than in dogs. The
presence of worms, villi, and other material in the pulmonary arteries increases resistance to the blood flow,
raising the blood pressure in these arteries. A cat may have right-sided congestive heart failure, although
this is relatively rare. In addition, fluid may build up inside the lungs between the airways, a condition
called pulmonary interstitial edema, making breathing more difficult.
These signs of damage from live heartworms are similar to those in dogs, but in cats they can be more
severe. In particular cats tend to have more inflammation in response to the presence of the live worms.
Furthermore, worms can occur in aberrant places in cats and can cause damage to other areas of the body.
The problems live worms can cause are bad enough but the worst problems can often be seen after the
heartworms die. The heartworm fragments are swept away from the heart. The pulmonary arteries can
develop villi and thrombi (blood clots that have broken free) that are even worse than in response to live
heartworms. Serious inflammation can also occur. These problems all cause blood flow to be restricted,
sometimes severely. The flow to the caudal (lower) lung lobes can actually be completely blocked,
preventing these lobes from performing their normal function of obtaining oxygen and releasing carbon
dioxide. Severe coughing or even coughing blood can occur.
It is very difficult to identify heartworm disease in cats because most cats will be asymptomatic; that is,
they will show no symptoms. A cat may cough or vomit when the larvae first infect the cat. Or, sometimes
a cat will experience some coughing or vomiting when the worms first attach themselves inside the
pulmonary arteries. But even these clinical signs may stop after a while, and the cat will become
Sometime a cat will vomit and have coughing spells. Occasional difficulty breathing (dsypnea) and
coughing of blood may occur. Another complaint may be lethargy, although cats don’t seem to exhibit
exercise intolerance the way dogs do. Occasionally the cat may have fainting spells. Also, infected cats
may lose weight. These symptoms could be confused with other diseases, like bronchial asthma, making
the identification of heartworm disease even more difficult.
Usually, a physical examination of a cat by the veterinarian reveals no symptoms, maybe a an abnormal
rhythm of the heart or possibly a systolic murmur. Sometimes harsh lung sounds can be heard
(parenchymal disease), although these don’t have to be present. Further testing by the vet may uncover
other conditions. Some conditions are PIE (pulmonary infiltrates with eosinophilia, that is, increased
eosinophilia (a particular kind of white blood cell in the lungs), hyperglobulinemia (an excess of globulins
(proteins) in the blood), allergenic pneumonitis (inflammation of the lungs), and pleural effusions (fluid
around the outside of the lungs).
To further confuse matters, if the worms are located somewhere other than in the pulmonary arteries, the
cat could show symptoms appropriate for the affected area. For example, blindness and seizures have
occurred as a result of an aberrant infection. But, these situations are relatively rare.
Unfortunately, the first symptom of heartworm disease may be the sudden death of the cat when one or
more worms die. The cat can die because of circulatory collapse and respiratory failure due to obstruction
of the pulmonary arteries and/or lung injury as the dead worm fragments are swept in the pulmonary
arteries to the lungs. There may be acute thromoembolism, which is the obstruction of a blood vessel from
a clot that has broken loose in addition to the problems caused by the fragments themselves. Even the death
of one worm can cause severe symptoms.
As mentioned above, a cat often shows no signs of heartworm disease when a physical exam is performed.
Routine complete blood counts (the identification of the number of red and white blood cells, and the type
and percentage of white blood cells) might reveal a mild anemia, as about 1/3 of infected cats will have
anemia. An excess of certain kinds of white blood cells (eosinophilia and basophilia) may be present, or it
may not be. The absence of eosinophilia does not mean there is no heartworm disease. If basophilia are
found, however, it is highly likely that heartworm disease is present, but the presence of basophilia is rare.
In addition to complete blood counts, a serum chemistry test, and urinalysis may be performed; but these
are usually normal. The serum is the fluid left over from a blood sample after the solids are removed. The
serum chemistry test reports the electrolytes, blood sugar, and kidney and liver enzymes. This report gives
an idea of how the body is functioning. Urinalysis can identify if there is something in the urine that
shouldn’t be there, like proteins, blood, etc.; the test can, therefore, add information as to how well the
kidneys are functioning. Occasionally a tracheal wash, which involves the injection of a little fluid into the
lungs that causes the cat to cough up fluid, is performed. The fluids are examined for signs of
inflammation or other abnormal cells.
The most useful test is one that examines whether the cat has developed antibodies to heartworm antigens.
A blood sample from the cat would be examined for the antibodies. Antibodies are proteins produced in
response to foreign proteins or sugars (that is, to antigens) in the body. The antibodies neutralize the
antigens and provide immunity to the invading organism. The antibody being detected is produced by the
cat in response to the infective larvae; therefore, a positive result to the test can be found as early as 2 – 3
months after infection by the larvae. A consistently positive result occurs 5 months after infection.
But there are some problems with this test. The death of adult heartworms can also produce a strong
antibody response. Consequently, the test can return a positive result just when the infection may be
ending if all the heartworms (and there may only be a few) have died. Or, infective larvae may have been
introduced into a cat (by a mosquito bite), the larvae may have started the antibody response, and then been
killed by preventative medication. The antibody test would return positive, indicating an infection although
none was present. Also, a positive result only shows that the cat has been infected by larvae, and that they
have lived at least 2 - 3 months. It does not confirm that the larvae have developed into adult worms. And
it is the adult heartworms that cause the most damage. To make the situation more confusing, there are
different antibody tests performed that are triggered by different antigens. (One test is performed by Heska
Co., the other by Animal Laboratories.) Because these tests look for different antigens, one test may return
positive and the other negative. And, there are known cases where cats with confirmed heartworm disease
have had negative test results from all antibody tests.
Another test looks for antigens in the cat’s blood to the adult female worm. This test works very well in
dogs. But, because there are only a few worms in a cat, there may not be enough adult female worms for
the test to register positive; most cats with heartworm disease, therefore, get negative test results.
Consequently, while a positive test will mean that heartworm disease is present, a negative result cannot
rule out the existence of the disease. Because of the likelihood of a negative result, the antigen test is not
A radiographic test (X-ray) may be done to show if and how much the pulmonary arteries are enlarged.
The most distinctive radiographic sign is the enlargement of the pulmonary arteries, most prominently in
the caudal lung lobes. Heartworm disease is the primary cause of enlarged pulmonary arteries. If enlarged
pulmonary arteries are seen, another test, like the antibody test, would be run to confirm the diagnosis of
heartworm disease. However, sometimes fluid buildup in the lungs (pulmonary edema) or around the
arteries in a cat affected with acute thromboembolism may make visualization of the pulmonary arteries
difficult. These conditions may make it necessary to take another X-ray.
In addition, the severity of parenchymal (inside the lung) disease can be evaluated with a radiographic test.
Cats may have lung lesions with clusters of white blood cells and proteins. They may even have a
collapsed lung -- one that due to pressure or fluid outside the lung cannot fill up with air. But, although a
radiographic test can reveal changes consistent with heartworm disease, some cats with heartworm disease
have normal radiographic results.
A radiographic test may be useful after an acute episode. In this situation a lung lobe may appear
consolidated. A consolidated lung lobe is different from a collapsed one. A consolidated lobe has shrunk
in size from a problem inside the lung (as opposed to outside). A consolidated lobe may not be able to be
fixed (while a collapsed lung can be). In both cases, breathing may be very difficult. Furthermore, repeated
radiographic tests may be performed to track the changing nature of heartworm disease for handling
An echocardiagram, or cardiac ultrasound, may also be done to further identify the extent of the problems
caused by the heartworms to the heart and pulmonary arteries. The cardiac ultrasound test can identify the
presence of worms in the pulmonary arteries and the right ventricle. It may also indicate the worm burden
(that is, how many worms are present). In the hands of an experienced individual, echocardiography in cats
correctly diagnoses heartworm disease in over half the cases.
Part of the challenge of the diagnosis is to differentiate heartworm disease from other diseases in cats with
respiratory signs. The vet may have to rule out various diseases before deciding upon a diagnosis of
heartworm disease. The other diseases may be asthma, cardiomyopathy (damage or abnormal change in the
heart muscle), FIP (feline infectious peritonitis, where an infection of the lining of the abdomen or thorax
makes breathing very difficult), lymphosarcoma (cancer of the lymph system), or infections by other
we will look at how the problems caused by heartworms are treated, and, we’ll discuss the prevention
program and why it is effective.
"Clinical Significance of Feline Heartworm Disease", by Ray Dillos, DVM, MS, Volume 28, Number 6,
November 1998, pp. 1547-1563.
Veterinary Internal Medicine, Eds. Ettinger and Feldman, 1995, Chapter 98.
Many thanks also to Dr. Oltman of Countryside Veterinary Clinic (410-461-0517) in Ellicott City, MD, for
her patient explanations and answering of questions. Any errors in this article are strictly the responsibility
of the author.