Heartworm Disease

Heartworm is a preventable, but serious and potentially fatal, parasite

that primarily infects dogs, cats and ferrets. It can also infect a variety
of wild animals, including wild canids (e.g., foxes, wolves, coyotes), wild
felids (e.g. tigers, lions, pumas), raccoons, opossums, and pinnipeds (e.g.,
sea lions and seals), as well as others. There have been documented
human infections, but they are thought to be rare and do not usually
result in signs of illness.

How is heartworm transmitted and what does it cause?

Where are heartworms found?

Geographically, heartworms are a potential threat in every state as
well as in many other countries around the world. All dogs, regardless
of age, sex, or living environment, are susceptible to heartworm
infection. Indoor, as well as outdoor, cats are also at risk for the disease.
If you plan to travel with your dog or cat to a different part of the
country, or another country, ask your veterinarian about the risk of
heartworm infection in the area where you are going to relocate or

What pets should be tested for heartworm?

Because heartworms are spread by mosquitoes, any pet exposed to
mosquitoes should be tested. This includes pets that only go
outside occasionally.

How can I tell if my pet has heartworm infection or disease?

DOGS: If your dog has been recently or mildly infected with
heartworms, it may show no signs of illness until the adult worms
have developed in the lungs and signs of heartworm disease are
observed. As the disease progresses, your dog may cough, become
lethargic, lose its appetite or have difficulty breathing. You may notice
that your dog seems to tire rapidly after only moderate exercise.
Numerous diagnostic tests are available for your veterinarian to
detect the presence of adult heartworm infection (> 6 month old
infections) in your dog. Antigen tests detect the presence of adult
female heartworms, and antibody tests determine if your pet has
been exposed to heartworms. The antigen test is most commonly
performed, and is very accurate in dogs. Further tests, such as chest
radiographs (x-rays), a blood profile and an echocardiogram (an
ultrasound of the heart), may be necessary to confirm the diagnosis,
to evaluate the severity of the disease, and to determine the best
treatment plan for your dog.
CATS: Signs of possible heartworm disease in cats include coughing,
respiratory distress, and vomiting. In rare cases, a cat may suddenly
die from heartworms.
The diagnosis of heartworm infection in cats is more difficult than
it is with dogs. A series of different tests may be needed to help
determine the likelihood of heartworm infection as the cause of your
cat’s illness and, even then, the results may not be conclusive. In
general, both antigen and antibody tests are recommended for cats
to give the best chances of detecting the presence of heartworms.

How can my pet be treated?

DOGS: As with most medical problems, it is much better to prevent
heartworm infection than to treat it. However, if your dog does
become infected with heartworms there is an FDA-approved treatment
available. There is substantial risk involved in treating a dog for
heartworms. However, serious complications are much less likely in
dogs that are in good health and when you carefully follow your
veterinarian’s instructions.
The goal of heartworm treatment is to kill the adult worms and
microfilariae present in your dog, as safely as possible. However,
when a dog is treated it is important to consider that heartworms
are dying inside the dogs lungs. While your dog is treated, it will
require complete rest throughout hospitalization and for some time
following the last treatment. Additionally, other medications may
be necessary to help control the body’s inflammatory reaction as the
worms die and are broken down in the dog’s lungs.
CATS: There is currently no effective and safe medical treatment
for heartworm infection or heartworm disease in cats. If your cat is
diagnosed with heartworms, your veterinarian may recommend
medications to reduce the inflammatory response and the resulting
heartworm disease, or surgery to remove the heartworms.

Can heartworms be surgically removed?

Surgical removal of heartworms from dogs and cats is a high-risk
procedure and is typically reserved for severe cases. However, in
many cases surgical removal of heartworms may be necessary to
afford the best opportunity for the pet’s survival.
Heartworms can only be transmitted from animal to animal by
mosquitoes. When a mosquito bites an infected animal, young
heartworms called microfilariae enter into that
mosquito’s system. Within two weeks, the
microfilariae develop into infective larvae
inside the mosquito; these infective larvae can
be transmitted to another animal when this
mosquito takes its next blood meal. Unlike dogs,
infected cats do not often have microfilariae circulating in their blood,
and an infected cat is not likely to transfer the heartworm infection
to another mosquito.
The infective larvae mature into adult heartworms in approximately
six months. During the first three months, the larvae migrate through
the animal’s body, eventually reaching the blood vessels of the lungs.
During the last three months, the immature worms continue to
develop and grow to adults, with females
growing to lengths of up to 14 inches.
The worms damage the blood vessels,
and reduce the heart’s pumping ability,
resulting in severe lung and heart disease.
When the animal shows signs of illness
due to adult heartworm infection, it is
called heartworm disease.
If adult worms (5-7 months postinfection)
of both sexes are present,
they will mate and produce new
microfilariae. The microfilariae can
cause the animal’s immune system to
mount a reaction; this immune reaction can actually cause damage to
other organs. This life cycle continues when a mosquito bites the
infected animal and becomes infected by the microfilariae. After
development of the microfilariae to infective larvae within the
mosquito (10 days to 2 weeks later) the infective heartworm larvae
are capable of infecting another animal. Adult heartworms can
survive for 5 to 7 years in dogs and several months to years in cats.

American Veterinary Medical Association
1931 North Meacham Road, Suite 100
Schaumburg, Illinois 60173 – 4360
Phone: 847-925-8070 • Fax: 847-925-1329
Revised 9/08
Printed in the U.S.A.
For more information, contact the American Heartworm Society

Pets age more rapidly than people and can develop disease conditions that
can go unnoticed, even to the most attentive pet owner. Veterinarians are
skilled in detecting conditions that have gradual onset and subtle signs.
Early detection allows problems to be treated most easily and affordably.
Help foster early detection and treatment by scheduling regular examinations.
And Now A Note On Your Pet’s
General Good Health
• Lumps or swelling
• Reduced or excessive appetite or water intake
• Marked weight loss or gain
• Limping, stiffness, or difficulty getting up or down
• Difficult, discolored, excessive or uncontrolled waste
elimination (urine and feces)
• Abnormal discharges from any body opening
• Head shaking, scratching, licking, or coat irregularities
• Changes in behavior or fatigue
• Foul breath or excessive tartar deposits on teeth
A healthy pet is a happy companion. Your pet’s daily well-being requires
regular care and close attention to any hint of ill health. The American
Veterinary Medical Association suggests that you consult your veterinarian
if your pet shows any of the following signs:
Can heartworm disease be prevented?
Heartworm infection is almost 100% preventable in dogs and cats.
There are several FDA-approved heartworm preventives available
in a variety of formulations. Your veterinarian can recommend the
best method of prevention based upon your pet’s risk factors and
lifestyle. Of course, you have to remember to give your pet the
preventive in order for it to work!
The preventives do not kill adult heartworms, and will not eliminate
heartworm infection or prevent signs of heartworm disease if adults
are present in the pet’s body. Therefore, a blood test for existing
heartworm infection is recommended before beginning a prevention
program to assess the pet’s current heartworm status. Because it is
more difficult to detect heartworms in cats, additional testing may
be necessary to make sure the cat is not infected.

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