What causes heartworm disease?
Heartworm disease (dirofilariasis) is a serious and potentially fatal disease
in dogs in many parts of the world. It is only found in the United Kingdom in
imported dogs. It is caused by a worm called Dirofilaria immitis.
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
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 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 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 these organs.
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 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 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 by the veterinary practice. Further diagnostic procedures are
essential, in advanced cases particularly, to determine if the dog can tolerate
heartworm treatment. Depending on the case, some or all of the following procedures
will be recommended 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
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.
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 your veterinarian to predict an increased possibility
of complications related to treatment.
Electrocardiogram: An electrocardiogram (ECG/EKG) is a tracing of the
electric currents generated by the heart. It is most useful to determine the
presence of abnormal heart rhythms.
Ultrasonography (Ultrasound scan): An echocardiogram allows your veterinarian
to see into the heart chambers and even visualize the heartworms themselves.
Although somewhat expensive, this procedure can diagnose heartworms when other
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 95% of dogs with heartworms are treated successfully.
Some dogs can have 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 days.
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.
Prompt 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, treatment will be necessary. 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 then undergoes 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 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 gain.
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