Heartworm disease (dirofilariasis) is a serious and potentially
fatal disease in dogs. It is caused by a worm called
Dirofilaria immitis.
Heartworms are found in the heart and large adjacent vessels of
infected dogs. The female worm is 6 to 14 inches (2.3 to 5.5 cm) long and 1/8
inch (5 mm) wide; the male is about half the size of the female. One dog may
have as many as 300 worms.
How do heartworms get into the heart?
Adult heartworms live in the heart and pulmonary arteries of
infected dogs. They have been found in other areas of the body, but this is
unusual. They survive up to 5 years and, during this time, the female produces
millions of young (microfilaria). These microfilaria live in the bloodstream,
mainly in the small blood vessels. The immature heartworms cannot complete the
entire life cycle in the dog; the mosquito is required for some stages of the
heartworm life cycle. The microfilaria are therefore not infective (cannot grow
to adulthood) in the dog, although they do cause problems.
As many as 30 species of mosquitoes can transmit heartworms. The female
mosquito bites the infected dog and ingests the microfilariae during a blood
meal. The microfilariae develop further for 10 to 30 days in the mosquito and
then enter the mouth parts of the mosquito. The microfilariae are now called
infective larvae because at this stage of development, they will grow to
adulthood when they enter a dog. The mosquito bites the dog where the haircoat
is thinnest. However, having long hair does not prevent a dog from getting
heartworms.
When fully developed, the infective larvae enter the bloodstream and move to
the heart and adjacent vessels, where they grow to maturity in 2 to 3 months
and start reproducing, thereby completing the full life cycle.