This article is part one of a two-part series. This month we’ll discuss what heartworms are, which dogs
generally get infected, and what damage the heartworms can do if an infection occurs. 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 dog by adult heartworms. If the larval (baby) in their most
immature stage heartworms are present in the bloodstream, the condition is called microfilaremia. A dog
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 to 5 or 6
inches long. They live inside the pulmonary arteries (the arteries that go from the heart to the lungs). If the
heartworm infection is severe (over 50 worms in a 55 pound dog), the worms will also be inside the right
ventricle of the heart (the chamber of the heart that pushes blood to the lungs). If the worm burden is high
-- and the number of worms can be over 250 -- worms can be found in the right atrium (the chamber that
receives blood from the body) or even in the vena cava (the veins that bring blood into the right atrium).
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. The first stage of the heartworm’s development occurs
when an adult female worm releases microfilariae into the blood stream. 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 another dog, it will infect the dog. The
infective larvae enter the dog through the bite wound. The larvae migrate through the body tissues and
continue developing for the next 100 days. Then young adult worms enter the vascular system. About 3
months after that time, microfilariae are released by the fully mature, adult female worms and the cycle
begins again. This second dog is now a host for heartworm disease. If a mosquito bites this dog, it will
spread the infection. The microfilariae are present in the dog’s blood stream in high concentrations for
about 6 months, then the number of microfilariae decreases.
In what regions does heartworm disease occur?
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 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%. 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.
Which dogs get heartworm disease?
Any dog can get heartworm disease, but some dogs are more likely to get the disease than others are. Dogs
living outdoors have 4 - 5 times the likelihood of infection. Male dogs are about 4 times more likely to be
infected than female dogs. Large dogs are more susceptible than small dogs. In endemic regions (like
within 100 miles of the Atlantic Ocean in the southeast of the United States), dogs as young as one year old
have been found with heartworms. Most dogs are between 3 and 8 years old when diagnosed with
heartworm disease, but dogs as old as 15 have had heartworms. Although some breeds, like Boxers, have a
higher rate of infection than others do, all dogs in endemic regions should be on a surveillance/preventative
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 and platelets to the damaged areas. Furthermore, the
leukocytes (white blood cells) 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 begin to dilate, become tortuous, get aneurysms, and the smaller arteries can
become blocked by the worms. The presence of worms, the villi, and other material in the pulmonary
arteries increases resistance to the blood flow, raising the blood pressure in these arteries. The pressure can
be one reason for the dilation of the arteries. The increased pressure in the pulmonary arteries can affect
the heart, causing swelling of the right ventricle. The right ventricle, which pumps the blood to the lungs
for oxygenation, has trouble doing its job; these problems can lead to congestive heart failure, especially of
the right side of the heart. Congestive heart failure is a condition where fluid accumulates in the lungs
because the heart can’t properly circulate the blood. Fluids that would normally be carried away remain in
the lungs causing difficulty with breathing.
The problems live worms can cause are bad enough but the worst problems can often be seen after the
heartworms die, either naturally or from treatment. The heartworm fragments are swept away from the
heart. The pulmonary arteries can develop even worse villi and thrombi (blood clots that have broken free)
in response to the fragments than in response to the live heartworms. 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, difficulty with breathing (dyspnea), or
coughing blood can occur. And the increased resistance to blood flow can cause right heart failure.
Usually the clinical signs, or symptoms, are a reflection of the number of worms, the length of the
infection, and the individual response of the dog. Most dogs are asymptomatic, that is, they show no
symptoms. Otherwise, coughing and difficulty breathing are the most common signs an owner would see.
If examined by a vet, parenchymal disease (wheezing, crackling, and other abnormal sounds) of the caudal
lung lobes may be heard.
Heartworm disease is usually focused on the pulmonary arteries, including increased pressure and fluid
buildup. The inflamatory response, especially to dead heartworms, surrounds the small airways in the
lungs. The combination of restricted blood flow, the heart’s decreased ability to pump blood due to the
increased pressure, and restricted airways makes the dog unable to exercise or be normally active. There is
also occasionally coughing of blood from the lungs.
Before your dog would be treated your vet would carry out an evaluation to determine if there is heartworm
disease. Usually an immunodiagnostic test called ELISA is performed on a blood sample. This type of test
takes adult female uterine components (the antigen) and a monoclonal antibody produced in response to the
antigen. It then looks for the antigens for adult female heartworms in the dog’s blood. The presence of the
antigen in the dog’s blood will cause a color change in the sample. This test is highly accurate. Only if few
adult female worms are present will a false-negative test occur, but in dogs that is rarely a problem.
In addition, a radiographic test (that is, an X-ray) will be done to show if and how much the pulmonary
arteries are enlarged and/or blocked. Also, this test can determine if the right side of the heart is expanding
and can evaluate the severity of parenchymal lung disease.
A complete blood count (CBC), serum chemistry profile, and urinalysis will also be done. The CBC
identifies the number of red and white blood cells, and the types of white blood cells and their percentage
amounts. It will show if there is inflammation or if an infection is present. The presence of particular white
blood cells, eosinophilia and basophilia, is more evidence in favor of a diagnosis of heartworm infection.
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.
These evaluations form the minimum database, that is, the least amount of data that will be collected when
evaluating a dog to see if heartworm disease is present. Additional tests will be performed if any of the
minimum tests reveal other abnormalities. A 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 also estimate the number of worms. High heartworm burdens are associated with many
heartworm echoes in the main pulmonary artery and right heart chambers. It can also measure the
thickness of the heart walls and watch the heart valves operating to see if any abnormalities are present.
We will look at how the problems caused by heartworms are treated and how heartworms are eliminated
from the dog. In addition, we’ll discuss the prevention program and why it is effective. Read next month's article,
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.