What is heartworm?
Heartworms belong to the same class of worms as roundworms. Adult heartworms
produce very tiny larvae. These larvae can live up to three years in the dog’s
blood stream, but usually only a month in cats. When a mosquito sucks blood from
an infected animal it also sucks up larvae and transmits it to other animals. The
larvae live in the animal's skin until it gets bigger and moves to the heart where
it continues to grow. It takes about 6 months between the time the animal is
bitten until the larvae moves to the heart. Severely infected dogs can have up to
several hundred heartworms in their hearts and vessels. Infected cats, on the
other hand, usually have only 1-4 worms.
What does a heartworm do?
The worms block the heart chambers as well as the blood vessels. If the larvae
or worms die, they can flow through the animal's blood system and block blood
vessels along the way, including those in the lungs.
In severe infections, the worms can also block the large vein (vena cava)
bringing blood to the right side of the heart. As the blood backs up, the liver
becomes enlarged and damaged.
Heartworm Symptoms
Lethargy - lack of endurance during exercise
Coughing - sometimes with blood
A history of not receiving heartworm preventative medicine
Decreased appetite
Loss of weight
Some will accumulate fluid in their abdomen that makes them look pot-bellied
Vomiting
Blindness
Seizures
Detecting Heartworm
Blood tests detect most cases of heartworm. In rare cases, heartworm infections
can occur without the larvae in the bloodstream. This occurs if only male worms
are present or if the females are not laying larvae at the time of the test.
There are special tests designed to detect heartworm if this is the case. Most
cases are detected early in a routine exam and can be treated successfully.
Heartworm Prevention
Dogs should be placed on heartworm preventative medicine on a schedule prescribed
by your veterinarian. Daily and monthly forms of medication are available.
Heartworm preventative medication should not be given until the dog has been
tested. These medications will not kill the adult worms, only the growing larvae.
Special medicines will kill the adult worms. Like all medications, follow the
recommendations and guidelines of your veterinarian and the manufacturer of the
drug. Check with your veteranian for the best drug to use.
Special Notes for Dogs
The best program for prevention of heartworm infection in dogs includes having
routine heartworm testing, using preventative drugs and reducing exposure to
mosquitoes. Remember that mosquitoes can get indoors, so even though your dog
may not go outside, the dog is still susceptible.
Your dog should be tested at least once a year. Depending on the prevalence of
mosquitoes and of heartworm infection in your area, it may be necessary to test
more often. Your local vet will be able to direct you more specifically.
Special Notes for Cats
Cats, like dogs should be tested before they are started on a preventative
medicine. Since mosquitoes can come inside all cats should be tested, even the
indoor cats. Before any treatment for heartworm infection in a cat is started,
it must be verified that the cat actually has an active infection. It is often
more difficult to detect heartworm in cats than dogs but there are
specially-designed tests. If a cat has a confirmed heartworm infection, but is
not showing marked signs of the disease, it is generally recommended that no
treatment be given. Generally, treatment is only attempted in cats that have
clinical signs that do not respond to certain drugs. Treatment in cats is even
riskier than in dogs. The possibility of the dead worms obstructing the vessels
to the lung is much greater in cats. This complication can be expected in 1/3 of
the cats treated. Surgical removal of heartworms from some cats may be necessary.