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 baby worms(microfilaria).
These microfilaria live in the bloodstream, mainly in the
small blood vessels.
Heartworms are transmitted by mosquitoes. 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 mosquito bites the another dog and transmits
the disease to that other dog.
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 most prevalent year round.
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 from the lack of air
passing through their lungs.
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 (baby 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.
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) 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?
Treatment can cost around $500 TO OVER $1000
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 successfully.
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 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, you should notify your veterinarian immediately.
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 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.
How can I prevent this from happening ? Prevention
cost vary by the weight of the dog but run between $25 - $100
Prevention - There are currently several brands of heartworm
prevention available to dog owners: Heartguard Plus, Interceptor,
and Sentinal. Prevention should be given on the same day of
EVERY month. It is usually easier to remember if you give
it on either the first or the last of each month. Beyond popular
belief, there are no months in the year when your dog is naturally
protected. They can get infected just as easily in December
as in June.
---portions of this document were copied with permission
and the courtesy of animalclinic.com