Heartworm disease is a serious disease that results in severe lung disease,
heart failure, other organ damage, and death in pets, mainly dogs, cats, and
ferrets. It is caused by a parasitic worm called Dirofilaria immitis.The
worms are spread through the bite of a mosquito. The dog is the definitive
meaning that the worms mature into adults, mate, and produce offspring while
living inside a dog. The mosquito is the intermediate host, meaning that
the worms live inside a mosquito for a short transition period in order to
become infective (able to cause heartworm disease). The worms are called
because the adults live in the heart, lungs, and associated blood vessels of
an infected animal.
In the United States, heartworm disease is most common along the Atlantic
and Gulf coasts from the Gulf of Mexico to New Jersey and along the
River and its major tributaries, but it has been reported in dogs in all 50
The Heartworm Lifecycle in Dogs
In an infected dog, adult female heartworms release their offspring, called
microfilariae, into the dog’s bloodstream. When a mosquito bites the
dog, the mosquito becomes infected with the microfilariae. Over the next 10
to 14 days and under the right environmental conditions, the microfilariae
become infective larvae while living inside the mosquito. Microfilariae
must pass through a mosquito to become infective larvae. When the infected
bites another dog, the mosquito spreads the infective larvae to the dog
through the bite wound. In the newly infected dog, it takes about 6 to 7
for the infective larvae to mature into adult heartworms. The adult
heartworms mate and the females release their offspring into the dog’s
completing the lifecycle. See a graphic of the
heartworm lifecycle in dogs.
Heartworm disease is not contagious, meaning that a dog cannot catch the
disease from being near an infected dog. Heartworm disease is only spread
the bite of a mosquito.
Inside a dog, a heartworm’s lifespan is 5 to 7 years. Adult heartworms look
like strands of cooked spaghetti, with males reaching about 4 to 6 inches
in length and females reaching about 10 to 12 inches in length. The number
of worms living inside an infected dog is called the worm burden. The
worm burden in dogs is 15 worms, but that number can range from 1 to 250
How is a Dog Tested for Heartworms?
A veterinarian uses blood tests to check a dog for heartworms. An antigen
test detects specific heartworm proteins, called antigens, which are
by adult female heartworms into the dog’s bloodstream. In most cases,
antigen tests can accurately detect infections with one or more adult female
earliest that the heartworm proteins can be detected in a dog’s bloodstream
is about 5 months after it is bitten by an infected mosquito.
Another test detects microfilariae in a dog’s bloodstream. Microfilariae in
the bloodstream indicate that the dog is infected with adult heartworms
only adult heartworms can mate and produce microfilariae). The earliest
that microfilariae can be detected in a dog’s bloodstream is about 6 months
it is bitten by an infected mosquito (because it takes about that long for
the heartworms to develop from infective larvae into adults that mate and
When Should a Dog Be Tested for Heartworms?
The timing and frequency of heartworm tests depend on many factors. Some of
these factors include:
list of 5 items
.. The dog’s age when heartworm prevention is started;
.. If the owner forgot to give heartworm prevention and for how long;
.. If the dog is switched from one type of heartworm prevention to another;
.. If the dog recently traveled to an area where heartworm disease is more
.. The length of the heartworm season in the region where the dog lives.
Dogs that are 7 months of age and older should be tested for heartworms
before starting heartworm prevention. A dog may appear healthy on the
but on the inside, heartworms may be living and thriving. If a
heartworm-positive dog is not tested before starting a preventive, the dog
infected with adult heartworms until it gets sick enough to show symptoms.
Heartworm preventives do not kill adult heartworms. Also, giving a heartworm
preventive to a dog infected with adult heartworms may be harmful or deadly.
If microfilariae are in the dog’s bloodstream, the preventive may cause the
microfilariae to suddenly die, triggering a shock-like reaction and possibly
Annual testing of all dogs on heartworm prevention is recommended. Talk to
your dog’s veterinarian about the best time for your dog’s annual heartworm
What are the Symptoms of Heartworm Disease in a Dog?
The severity of heartworm disease is related to how many worms are living
inside the dog (the worm burden), how long the dog has been infected, and
the dog’s body is responding to the presence of the heartworms. The dog’s
activity level also plays a role in the severity of the disease and in when
symptoms are first seen. Symptoms of heartworm disease may not be obvious
in dogs that have low worm burdens, have been recently infected, or are not
very active. Dogs that have heavy worm burdens, have been infected for a
long time, or are very active often show obvious symptoms of heartworm
There are four classes, or stages, of heartworm disease. The higher the
class, the worse the disease and the more obvious the symptoms.
list of 4 items
.. Class 1: No symptoms or mild symptoms such as an occasional cough.
.. Class 2: Mild to moderate symptoms such as an occasional cough and
tiredness after moderate activity.
.. Class 3: More severe symptoms such as a sickly appearance, a persistent
cough, and tiredness after mild activity. Trouble breathing and signs of
failure are common. For class 2 and 3 heartworm disease, heart and lung
changes are usually seen on chest x-rays.
.. Class 4: Also called caval syndrome. There is such a heavy worm burden
that blood flowing back to the heart is physically blocked by a large mass
worms. Caval syndrome is life-threatening and quick surgical removal of the
heartworms is the only treatment option. The surgery is risky, and even
surgery, most dogs with caval syndrome die.
Not all dogs with heartworm disease develop caval syndrome. However, if
left untreated, heartworm disease will progress and damage the dog’s heart,
liver, and kidneys, eventually causing death.
Is There a Treatment for Heartworm Disease in Dogs?
Melarsomine dihydrochloride (available under the trade names Immiticide and
Diroban) is an arsenic-containing drug that is FDA-approved to kill adult
in dogs. It’s given by deep injection into the back muscles to treat dogs
with stabilized class 1, 2, and 3 heartworm disease. Another drug, Advantage
Multi for Dogs (imidacloprid and moxidectin), is FDA-approved to get rid of
microfilariae in the dog’s bloodstream. Advantage Multi for Dogs is a
solution applied to the dog’s skin.
The treatment for heartworm disease is not easy on the dog or on the owner’s
pocket book. Treatment can be potentially toxic to the dog’s body and can
cause serious complications, such as life-threatening blood clots to the
dog’s lungs. Treatment is expensive because it requires multiple visits to
veterinarian, blood tests, x-rays, hospitalization, and a series of
The Best Treatment is Prevention!
Many products are FDA-approved to prevent heartworms in dogs. All require a
veterinarian’s prescription. Most products are given monthly, either as a
topical liquid applied on the skin or as an oral tablet. Both chewable and
non-chewable oral tablets are available. One product is injected under the
skin every 6 or 12 months, and only a veterinarian can give the injection.
Some heartworm preventives contain other ingredients that are effective
certain intestinal worms (such as roundworms and hookworms) and other
parasites (such as fleas, ticks, and ear mites).
Year-round prevention is best! Talk to your dog’s veterinarian to decide
which preventive is best for ayour dog.