Terry Animal Hospital
Drs Perrier, Foy, Ng, & Funk
3 Washington Street, Rockville Centre, NY 11570
516-764-2880
www.TerryAnimalHospital.com
Canine Heartworm Disease
What causes heartworm disease?
Heartworm disease (dirofilariasis) is a serious & potentially fatal disease in dogs. It is caused by
a blood-borne parasite called Dirofilaria immitis.
How do heartworms get into the heart?
As many as 30 species of mosquitoes can transmit heartworms. The female mosquito bites an
infected dog & ingests microfilariae (pre-larval stage) during a blood meal. The microfilariae
further develop for 10-30 days inside the mosquito
before migrating to the mouthparts. The microfilariae
are now infective larvae because at this stage of
development, they will grow to adulthood once they
enter a dog. The mosquito usually bites the dog where
the hair coat is thinnest however, long hair does not
prevent a dog from getting heartworms. The fully
developed infective larvae enter a dog’s bloodstream &
migrate to the heart & adjacent large blood vessels
(pulmonary artery) where they grow to mature worms
in 2-3 months. Here they begin reproducing
microfilariae. This completes the life cycle.
(see “Life Cycle of the Heartworm”)
Adult heartworms dwell in the heart & pulmonary
arteries & are rarely found elsewhere in the body. The
mature female worm is 6-14 inches long (15-36cm) & 1/8-inch wide (0.5cm). The mature male
worm is only about half the size of the female. One dog may have as many as 300 worms. Adult
heartworms can live up to 5 years during which a female worm can produce millions of
microfilariae. Although they can cause problems, microfilariae are not infective in the dog
therefore cannot grow to adulthood. The microfilariae live mainly in the small vessels of the
bloodstream (capillaries) awaiting a mosquito bite in order to continue to the next stages in the
heartworm life cycle.
Where in the United States do you find heartworm disease?
Canine heartworm disease occurs all over the world. Originally, heartworm disease was limited
to the southern & southeastern regions of the United States. Now the disease is spreading & is
found in most regions of North America, particularly where mosquitoes are prevalent.
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How do dogs become infected with heartworms?
The disease cannot be spread directly through dog-to-dog contact. An intermediate host is
required for transmission; in this case it is a mosquito. Spread of this disease coincides with
mosquito season. Thus, the number of dogs infected & the length of mosquito season directly
correlates with the incidence of heartworm disease for any given area(s). It takes a few years
before dogs show outward signs of infection. Consequently, the disease is diagnosed mostly in
dogs 4-8 years old. The disease is seldom diagnosed in dogs less than 1 year of age mainly
because the heartworm life cycle usually requires 5-7 months to mature to infection.
What does heartworm disease do to the dog?
Two life stages are found within the dog:
- Adult heartworm
- Microfilaria (non-infective heartworm)
Adult Heartworms – cause disease by obstructing the
flow of blood through the heart & major vessels of the
heart. They interfere with the normal opening/closing of
the heart valves. The blood supply to organs is
compromised by clogging the major heart vessels. This
leads to the malfunction of organs, particularly affecting
the lungs, liver, & kidneys.
Most infected dogs show no signs of disease until at
least 2 years of age. Unfortunately, the disease is well
advanced by the time clinical signs are seen. Clinical
signs relate to the location, length, & amount of worms
causing infection as well as the degree of damage to the
heart, lungs, liver, & kidneys caused by both adult
worms & microfilariae.
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The most obvious signs of heartworm disease include a soft, dry cough, shortness of breath,
weakness, nervousness/listlessness, & loss of stamina. All of these signs are most noticeable
following exercise (exercise intolerance), when some dogs may even faint. Severely infected
dogs may die suddenly during exercise or excitement. Listening with a stethoscope will often
reveal abnormal heart &/or lung sounds. In advanced cases congestive heart failure may be
apparent & the abdomen & legs may swell secondary to fluid accumulation. Weight loss,
anemia, & poor condition may also occur.
Microfilariae – are young, non-infective, pre-larval, heartworms that circulate throughout the
body, residing primarily in capillaries. Since they are only as wide as a capillary, a microfilaria
can get stuck in these tiny vessels, obstructing the flow of blood to the surrounding organs &
tissues. Consequently, this obstruction of nutrients & oxygen to the tissues leads to organ
dysfunction; the lungs & liver are primarily affected.
Destruction of lung tissue leads to coughing. Cirrhosis of
the liver causes jaundice, anemia, & general weakness
since this organ is crucial in maintaining a healthy
animal. If the kidneys are affected, metabolic poisons will
accumulate in the body because they cannot be excreted
by the kidneys.
How is heartworm infection diagnosed?
In most cases, diagnosis of heartworm disease can be made in a veterinary hospital by a
blood test. After confirming heartworm disease, further diagnostics are essential in determining
if a dog can tolerate the heartworm treatment. Depending on the case, we will recommend some
or all of the following procedures before treatment is started.
Adult heartworm antigen serology: This test is performed on blood. It is the most widely used
test because it detects antigens (proteins) produced by adult heartworms & will be positive even
if the dog does not have any circulating microfilariae (which occurs 20% of the time). Dogs with
less than 5 adult heartworms will not have enough antigen to give a positive test so a false
negative result may occur in very early infections. Since the detected antigen is only produced by
female heartworms, an infection with only male heartworms will also yield a false negative.
Therefore, there must be at least 5 female worms present for the most common heartworm test to
diagnose heartworm disease.
Cytology/Blood Smear examination for microfilariae: A blood
sample is examined under the microscope for the presence of
microfilariae. If seen, the test is positive. The number of microfilariae
seen gives a general indication of the severity of infection (see
“microfilaria & adjacent red blood cells ”). However, greater numbers are
seen in the summer months as well as in the evening so these
variations must be taken into consideration. Some dogs do not test positive even though they
have heartworms because approximately 20% of dogs infected with heartworms have an
acquired immunity to the stage of the heartworm. It is because of this fact that the antigen test
(above) is the preferred screening test. Secondly, there is a blood parasite common in dogs that
can be hard to distinguish from the heartworm microfilaria.
Blood chemistries: Complete blood counts (CBC) & Serum biochemical assays are utilized to
determine the presence of heartworm disease by letting veterinarians evaluate blood composition
as well as liver/kidney function. These tests are also performed on heartworm-positive dogs prior
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to treatment in order to determine the presence of any organ dysfunction.
Radiographs (X-rays): A radiograph of a dog with
heartworm disease will usually show heart
enlargement & engorged arteries leading from the
heart
to the lungs. These signs are considered presumptive
evidence of heartworm disease. Radiographs may also
reveal
additional information about the condition of the
heart,
lungs, & vessels. This information allows us to
predict an increased possibility of complications
related to treatment.
Electrocardiogram: An electrocardiogram (ECG or EKG) is a tracing of electric currents
generated by the heart & is most useful in diagnosing abnormal heart rhythms.
Echocardiography: An ultrasonic examination that allows
us to
see into heart chambers & even, on occasion, visualize actual
heartworms.
How are dogs treated for heartworms?
In the past, the drug used to treat heartworms contained arsenic so toxic side-effects & reactions
occurred more frequently. Today, a newer drug is available that does not produce those toxic
side-effects. We can now successfully treat more than 95% of dogs with heartworm disease.
Still, treating dogs with heartworm disease is not without risk although fatalities are very rare.
Treatment to kill adult heartworms: An injectable drug is given once daily for 2 consecutive
days to kill adult worms in the heart as well as adjacent vessels. These injections may be divided
& administered 30 days apart in more critical infections. The first injection will kill
approximately half of the worm burden. One month later, the second injection kills the remaining
worms. The lungs are much less likely to be affected by killing the worms in two stages.
Complete rest is critical after treatment! The adult worms die & begin to decompose
in a few days following injection(s). As they break down, worm fragments are carried via blood
circulation to the lungs where they lodge in small vessels & eventually get reabsorbed by the
body. It is absolutely critical that your dog be kept quiet & not be allowed to exercise for one
month following treatment. The first week is the most dangerous period of the recovery phase
because the largest number of worms are dying & decomposing. Severely infected dogs may
have a noticeable cough for up to 2 months following treatment.
Seek immediate veterinary care if your dog has a significant reaction in the weeks
following treatment. Such reactions are rare but worth noting. Notify us immediately if your dog
shows loss of appetite, shortness of breath, severe coughing, coughing up blood, fever, &/or
depression. The response to antibiotics, cage rest, intravenous fluids, & supportive care is
usually good in these cases.
Treatment to kill microfilariae: Approximately one month following the treatment to kill adult
worms, the patient returns to the hospital for administration of a drug to kill the baby, pre-larval
heartworms (microfilariae). Your dog needs to stay in the hospital for this day. Dogs with severe
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heartworm disease may require additional treatments such as antibiotics, special low salt/low
protein diets, diuretics to remove accumulated fluid, &/or cardiac medication(s).
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 & vitality, improved appetite, & gains back lean muscle
mass.
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. With the safe & affordable heartworm preventatives available today, there
is no excuse a pet should ever have to endure or succumb to this dreaded disease.
Have you read about “Flea Control”?
Client information sheet based on material written by Ernest E. Ward Jr., DVM.
© 2002 Lifelearn Inc. Used with permission under license. 4/30/2010.
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