Heartworm Disease in Dogs

Heartworms are a serious and potentially deadly parasite. Adult worms are found in the heart and in the major blood vessels leading from the heart.



Heartworms are not spread directly from dog to dog. The mosquito is required for transmission. The female worms produce millions of young called microfilaria. The microfilariae circulate throughout the dog’s bloodstream. The immature worms cannot fully develop in the dog; the mosquito is required for some stages of the heartworm lifecycle. Microfilariae are not infective at this stage in the dog but do cause problems.

The mosquito bites an infected dog and ingests the microfilariae. The young worms continue to develop in the mosquito for 10 to 30 days. The microfilariae are now infective larvae and can now be transmitted to another dog through the mosquito’s next blood meal. The infective larvae now circulate through the bloodstream and move to the heart where they mature and reproduce thereby completing the life cycle.

Effects on the Dog

Adult heartworms cause damage by clogging the heart and major blood vessels leading from the heart, therefore causing changes to the heart valves as well as depriving other organs of oxygen. The degree of damage to the heart, lungs, liver, and kidneys depends on the number of adult and microfilariae present and the length of time the worms have been present.


It can take several years before an infected dog starts showing any signs of having the disease. Symptoms may include coughing, exercise intolerance, vomiting, and lethargy.


Heartworm disease can be diagnosed by a blood test that Heartworm testcan be run in our office or sent to an outside laboratory. The test detects for antigens (proteins) produced by the adult female worms. If a dog is under 6 months of age and has been infected, it will not generate a positive test because it takes the infective larvae 6 to 7 months to mature, therefore the test is unable to detect the antigen.


Heartworm preventative is started at 12-16 weeks of age. There are several brands of heartworm prevention. Most are chewable pills given once a month. Many brands also help treat and control some intestinal parasites. All are very effective if given accurately. Our doctors recommend Heartworm preventative be given year round. A blood test is recommended every year.


For more information on Heartworm Disease and prevention go to the American Heartworm Society home page.