Glossary
Heartworm disease
A serious parasitic disease spread by mosquitoes in which worms grow in a dog's heart and lungs, and which monthly prevention can stop.
Also called: heartworm, dirofilariasis, Dirofilaria immitis, heartworm prevention
Heartworm disease is caused by Dirofilaria immitis, a worm passed on by mosquito bites. Larvae mature over about six months into adult worms up to a foot long that live in the heart and the blood vessels of the lungs, where they damage the vessels and strain the heart. Early on dogs may show nothing. Later signs include a mild cough, tiring easily, weight loss and, in advanced cases, heart failure.
It occurs in every US state and in many warm and temperate countries, and it does not pass directly from dog to dog. It is far easier to prevent than treat, since treatment is long, costly and hard on the dog.
Vets test with a blood test, usually once a year, and prescribe prevention. Never use a prevention product without a negative test.
Last reviewed November 8, 2026