What is Heartworm Disease?

What causes heartworm disease?

Heartworm disease or dirofilariasis is a serious and potentially fatal disease in dogs. It is caused
by a blood-borne parasite called Dirofilaria immitis. Heartworms are found in the heart and adjacent large blood vessels of infected dogs. The female worm is 6 to 14 inches long (15 to 36 cm) and 1/8 inch wide (5 mm). The male is about half the size of the female. One dog may have as many as 300 worms.

How do heartworms get into the heart?

Adult heartworms live in the heart and pulmonary
arteries of infected dogs. They have been found in
other areas of the body, but this is unusual. They live
up to five years and, during this time, the female
produces millions of offspring called microfilaria.
These microfilariae live mainly in the small vessels of
the bloodstream. The immature heartworms cannot
complete their life cycle in the dog. The mosquito is
required for some stages of the heartworm life cycle.
The microfilaria are not infective (cannot grow to
adulthood) in the dog – although they do cause

As many as 30 species of mosquitoes can transmit heartworms. The female mosquito bites the infected dog and ingests the microfilariae during a blood meal. The microfilariae develop further for 10 to 30 days in the mosquito and then enter the mouthparts of the mosquito. The microfilariae are now called infective larvae because at this stage of development, they will grow to adulthood when they enter a dog. The mosquito usually bites the dog where the hair coat is thinnest. However, having long hair does not prevent a dog from getting heartworms.

When fully developed, the infective larvae enter the bloodstream and move to the heart and
adjacent vessels where they grow to maturity in two to three months and start reproducing,
thereby completing the full life cycle.

Where are heartworms found?

Canine heartworm disease occurs all over the world. In the United States, it was once limited to
the south and southeast regions. However, the disease is spreading and is now found in most
regions of the United States and Canada, particularly where mosquitoes are prevalent.

How do dogs get infected with them?

The disease is not spread directly from dog to dog. An intermediate host, the mosquito, is
required for transmission. Spread of the disease therefore coincides with mosquito season. The
number of dogs infected and the length of the mosquito season are directly correlated with the
incidence of heartworm disease in any given area.

It takes a number of years before dogs show outward signs of infection. Consequently, the
disease is diagnosed mostly in four to eight year old dogs. The disease is seldom diagnosed in
a dog less than one year of age because the young worms (larvae) take up five to seven
months to mature after infection.

What do heartworms do to the dog?

Adult heartworms: Adult heartworms cause
disease by clogging the heart and major blood
vessels leading from the heart. They interfere
with the valve action in the heart. By clogging
the main blood vessels, the blood supply to
other organs of the body is reduced,
particularly blood flow to the lungs, liver and
kidneys, leading to malfunction of these

Most dogs infected with heartworms do not
show any signs of disease for as long as two
years. Unfortunately, by the time clinical signs
are seen, the disease is well advanced. The
signs of heartworm disease depend on the
number of adult worms present, the location of
the worms, the length of time the worms have
been present, and the degree of damage to
the heart, lungs, liver, and kidneys from the
adult worms and the microfilariae.

The most obvious signs are a soft, dry cough, shortness of breath, weakness, nervousness,
listlessness, and loss of stamina. All of these signs are most noticeable following exercise, when
some dogs may even faint.

Listening to the chest with a stethoscope will often reveal abnormal lung and heart sounds. In
advanced cases, congestive heart failure may be apparent and the abdomen and legs will swell
from fluid accumulation. There may also be evidence of weight loss, poor condition, and

Severely infected dogs may die suddenly during exercise or excitement.
Microfilariae (Young heartworms): Microfilariae circulate throughout the body but remain
primarily in the small blood vessels. Because they are as wide as the small vessels, they may
block blood flow in these vessels. The body cells being supplied by these vessels are deprived
of the nutrients and oxygen normally supplied by the blood. The lungs and liver are primarily

Destruction of lung tissue leads to coughing. Cirrhosis of the liver causes jaundice, anemia, and
general weakness because this organ is essential in maintaining a healthy animal. The kidneys
may also be affected and allow poisons to accumulate in the body.

How is heartworm infection diagnosed?

In most cases, diagnosis of heartworm disease can be made by a blood test that can be run in
the veterinary hospital or by a veterinary laboratory. Further diagnostic procedures are essential
to determine if the dog can tolerate heartworm treatment. Depending on the case, we will
recommend some or all of the following procedures before treatment is started.
Serological test for antigens to adult heartworms: This is a test performed on a blood
sample. It is the most widely used test because it detects antigens (proteins) produced by adult
heartworms. It will be positive even if the dog does not have any microfilaria in the blood. This
occurs in about 20% of the cases. Dogs with less than five adult heartworms will not have
enough antigen to give a positive test result, so there may be an occasional false negative result
in dogs with early infections. Because the detected antigen is only produced by the female
heartworm, a population of only male heartworms will also give a false negative. Therefore,
there must be at least five female worms present for the most common heartworm test to
diagnose heartworm disease.

Blood test for microfilariae: A blood sample is examined
under the microscope for the presence of microfilariae. If
microfilariae are seen, the test is positive. The number of
microfilariae seen gives us a general indication of the
severity of the infection. However, the microfilariae are
seen in greater numbers in the summer months and in the
evening, so these variations must be considered.

Approximately 20% of dogs do not test positive even
though they have heartworms because of an acquired
immunity to this stage of the heartworm. Because of this,
the antigen test is the preferred test. Also, there is another
blood parasite that is fairly common in dogs that can be
hard to distinguish from heartworm microfilariae.
Blood chemistries: Complete blood counts and blood
tests for kidney and liver function may give an indication of
the presence of heartworm disease. These tests are also
performed on dogs diagnosed as heartworm-infected to
determine the function of the dog’s organs prior to

Radiographs (X-rays): A radiograph of a dog with heartworms will usually show heart
enlargement and swelling of the large artery leading to the lungs from the heart. These signs
are considered presumptive evidence of heartworm disease. Radiographs may also reveal the
condition of the heart, lungs, and vessels. This information allows us to predict an increased
possibility of complications related to treatment.

Electrocardiogram: An electrocardiogram (EKG or ECG) is a tracing of the electric currents
generated by the heart. It is most useful to determine the presence of abnormal heart rhythms.
Echocardiography: An ultrasonic examination that allows us to see into the heart chambers
and even visualize the heartworms.

How are dogs treated for heartworms?

There is some risk involved in treating dogs with heartworms, although fatalities are rare. In the
past, the drug used to treat heartworms contained arsenic so toxic effects and reactions
occurred more frequently. A newer drug is now available that does not have the toxic sideeffects,
allowing successful treatment of more than 95% of dogs with heartworms.

Some dogs are diagnosed with advanced heartworm disease. This means that the heartworms
have been present long enough to cause substantial damage to the heart, lungs, blood vessels,
kidneys, and liver. A few of these cases will be so advanced that it will be safer to treat the
organ damage rather than risk treatment to kill the heartworms. Dogs in this condition are not
likely to live more than a few weeks or months.
Treatment to kill adult heartworms: An injectable drug to kill adult heartworms is given. It kills
the adult heartworms in the heart and adjacent vessels. These injections may be divided and
given thirty days apart.

Complete rest is essential after treatment: The adult worms die in a few days and start to
decompose. As they break up, they are carried to the lungs, where they lodge in the small blood
vessels and are eventually reabsorbed by the body. This can be a dangerous period so it is
absolutely essential that the dog be kept quiet and not be allowed to exercise for one month
following treatment. The first week after the injections is critical because the worms are dying. A
cough is noticeable for seven to eight weeks after treatment in many heavily infected dogs.
Prompt treatment is essential if the dog has a significant reaction in the weeks following the
initial treatment, although such reactions are rare. If a dog shows loss of appetite, shortness of
breath, severe coughing, coughing up blood, fever, and/or depression, you should notify us.
Response to antibiotics, cage rest, and supportive care and intravenous fluids is usually good in
these cases.

Treatment to kill microfilaria: Approximately
one month following treatment to kill the
adults, the dog is returned to the hospital for
administration of a drug to kill the baby
heartworms or microfilariae. Your dog needs
to stay in the hospital for the day. Your dog is
started on heartworm preventive after this

Other treatments: In dogs with severe heartworm disease, it may be necessary to treat them
with antibiotics, special diets, diuretics to remove fluid accumulations, and drugs to improve
heart function prior to treatment for the heartworms.
Dogs with severe heart disease may need lifetime treatment for the heart failure, even after the
heartworms have been killed. This includes the use of diuretics, heart drugs, and special low
salt, low protein diets.

Response to treatment: Dog owners are usually pleasantly surprised at the change in their
dog following treatment for heartworms, especially if the dog had been showing signs of
heartworm disease. The dog has a renewed vigor and vitality, improved appetite, and weight

Are changes made in the treatment protocol for dogs that have severe heartworm

Yes. The state of heart failure is treated as described above. However, we also treat the adult
heartworms in a two-stage process. Only one treatment with the drug to kill the worms is given
initially. This causes the death of approximately half of the worms. One month later, the full
treatment is given to kill the remaining worms. By killing them in two stages, the severe effects
on the lungs are much less likely to occur. This protocol is also used in moderate cases to
provide a safer treatment.

How can I prevent this from happening again?

When a dog has been successfully treated for heartworms, it is essential to begin a heartworm
prevention program to prevent future recurrence. With the safe and affordable heart preventives
available today, no pet should ever have to endure this dreaded disease.