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Learn about heartworm infection

Heartworm is a large worm, up to 30cm long, that lives in the heart and major arteries of an infected dog. Dogs become infected through bites from mosquitoes that carry immature heartworms from infected dogs.

Lifecycle of Heartworm
Adult Heartworms

The adult heartworm lives in the blood vessels that flow from the heart to supply the lungs (pulmonary arteries). The worm is nourished by the blood passing round it. A worm in these arteries will cause many serious problems

  • The heart has to work harder to pump blood through the blocked arteries
  • Blood tends to clot within the blood vessels.
  • Blood clots can break off and block the smaller arteries in the lungs
  • A strong inflammatory response that can cause damage to other, distant organs

If the worm infection is heavy the worms will also grow inside the heart, occupying a significant amount of space and decreasing blood flow to the lung.

Microfilariae (First Stage Larvae)

Female heartworms give birth to live baby worms called microfilariae. These flow around in blood until they are ingested by a mosquito taking a blood meal from the dog. This infected mosquito then transfers the microfilariae to another dog. Microfilariae can live up to two years in a dog but will eventually die. They can also be transmitted to puppies in the womb.

Inside the mosquito

The larva continues to develop in the mosquito’s body, so that it can pass onto a new dog. When another dog is bitten the transformed larva will circulate through the blood vessels, eventually migrating to the heart and out into the pulmonary arteries, where it may mate, approximately 6 months after first entering the new host.

Signs a dog may have heartworm
  • Many dogs may show no symptoms of disease
  • Coughing or difficult or rapid breathing, blood may be coughed up
  • Weakness or tiring easily with exercise, collapse or feinting
  • Bleeding from the nose due to clotting defects.
  • If heart failure develops fluid may accumulate in the chest and the abdomen causing a "pot-bellied" appearance
  • If the dog develops a marked inflammatory response the eye, kidney, blood vessels and joints can be badly damaged
  • In chronic cases dogs will lose weight

Heartworm infection can be diagnosed using a simple test kit at the clinic. The test takes 8 minutes to produce a reliable result, so this can be done while you wait. The test detects proteins (antigens) from the heartworms. However it can take 6 months after infection for a dog to test positive for heartworm infestation and an infection with a single male worm may be missed.


Heartworm disease is a significant problem. Treatment is available, but prevention is far better. Heartworm prevention will be discussed here

Heartworm in Cats

Cats can also be infected with heartworm. This often does not cause any signs but can result in problems such as coughing, signs like asthma or even sudden death. We are so far unaware of any confirmed diagnoses of heartworm in cats in Hong Kong. As most cats here live 100% of their time indoors the risk level for infection is probably low. Heartworm in cats can be prevented by monthly treatment with Milbemax tablets.

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