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.

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