What causes heartworm disease?
Heartworm disease (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 (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.
it was once limited to the south and southeast regions. However, the disease is
spreading and is now found in most regions of the
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 if 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 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 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
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 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 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.
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
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
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
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 treatment.
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.
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
side-effects. We can now successfully treat more than 95% of dogs with
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
kill adult heartworms: An injectable drug to kill adult heartworms is given for two days. It
kills the adult heartworms in the heart and adjacent vessels. In more serious
infections, these injections may be divided and given thirty days apart.
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
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.
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
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.
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.
Are changes made in the treatment protocol for dogs
that have severe heartworm disease?
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.
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.