How do Heartworms affect you and your dog?
causes heartworm disease?
Heartworm disease (dirofilariasis) is a serious and potentially
fatal disease in dogs. It is caused by a worm called Dirofilaria
Heartworms are found in the heart and large adjacent vessels of infected
dogs. The female worm is 6 to 14 inches (2.3 to 5.5 cm) long and 1/8
inch (5 mm) wide; 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 survive up to 5 years and, during this time, the female
produces millions of young (microfilaria). These microfilaria live in
the bloodstream, mainly in the small blood vessels. The immature
heartworms cannot complete the entire life cycle in the dog; the
mosquito is required for some stages of the heartworm life cycle. The
microfilaria are therefore not infective (cannot grow to adulthood) in
the dog, although they do cause problems.
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 mouth parts 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 bites the dog where the haircoat 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 2
to 3 months and start reproducing, thereby completing the full life
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 the 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 4 to 8 year old dogs.
The disease is seldom diagnosed in a dog under 1 year of age because the
young worms (larvae) take up to 7 months to mature following
establishment of infection in a dog.
What do heartworms do to the dog?
Adult worms: Adult worms 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 the lungs, liver and kidneys, leading to malfunction of
Most dogs infected with heartworms do not show any signs of disease for
as long as two years. Unfortunately, by the time 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, chronic cough, shortness of
breath, weakness, nervousness, listlessness, and loss of stamina. All of
these signs are most noticeable following exercise, when some dogs may
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 anemia.
Severely infected dogs may die suddenly during exercise or excitement.
Microfilariae (Young worms): 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 affected.
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. Further
diagnostic procedures are essential, in advanced cases particularly, 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 about 20% of the time. Dogs with less than five adult
heartworms will not have enough antigen to turn the test positive, so
there may be some false negative results in early infections. Because
the antigen detected is produced only by the female worm, a pure
population of male heartworms will give a false negative, also.
Therefore, there must be at least 5 female worms present for the most
common test to be positive.
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
microfilarial parasite which is fairly common in dogs; on the blood
smear, these can be hard to distinguish from heartworm microfilariae.
Blood chemistries: Complete blood counts and blood
tests for kidney and liver function may give an indirect 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 treatment.
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) is a tracing of
the electric currents generated by the heart. It is most useful to
determine the presence of abnormal heart rhythms.
Echocardiography (Sonogram): An echocardiogram allows us to see
into the heart chambers and even visualize the heartworms themselves.
Although somewhat expensive, this procedure can diagnose heartworms when
other tests fail.
How are dogs treated for heartworms?
There is some risk involved in treating dogs with heartworms,
although fatalities are rare. The drug that is used contains arsenic.
The amount of arsenic is sufficient to kill heartworms without undue
risk to the dog. However, dogs with poor liver or kidney function may
have difficulty breaking down and eliminating the arsenic. In spite of
this we able to treat more than 95% of dogs with heartworms
We see some dogs 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 far advanced that it will be safer to just treat the
organ damage rather than risk treatment to kill the worms. Dogs in this
condition are not likely to live more than a few weeks or months.
Treatment to kill adult worms: An injectable drug to
kill adult heartworms is given for two days. It kills the adult
heartworms in the heart and adjacent vessels over a period of about 30
Complete rest is essential after treatment: Some adult
worms die in a few days and start to decompose; the remainder will die
within a month. 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 is a dangerous period, and it is absolutely essential
that the dog be kept quiet and not be allowed to exercise for 1 month
following treatment. The first week after the injections is very
critical because the worms are dying. A cough is noticeable for 7 to 8
weeks after treatment in many heavily infected dogs.
treatment is essential if the dog has a significant reaction in the
weeks following the initial treatment, although such reactions are not
common. 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, such as
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 microfilariae. Your dog
needs to stay in the hospital for the day. Seven to ten days later a
test is performed to determine if microfilariae are present. If they
have been all killed, the treatment is complete. If there are still some
present in the blood, treatment for microfilariae is repeated.
In some cases, the heartworm infection is "occult," meaning that no
microfilariae were present. In this case, a follow-up treatment at one
month is not needed.
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
failing heart, even after the heartworms have been killed. This includes
the use of diuretics, heart drugs, aspirin, and special low salt, low
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 gain.
How can I prevent this from happening
When a dog has been successfully treated for heartworms, you
cannot sit back and relax because dogs can be reinfected. Therefore, it
is essential to begin a heartworm prevention program. There are three
drugs which can be used to prevent heartworm infection. One is a daily,
chewable tablet; the others are chewable tablets that are given only
once monthly. All these products are very safe and very effective. Their
costs are essentially identical. One of these should be started
immediately after the treatment is completed.