The heartworm is a parasitic roundworm that is spread from host to host through the bites of mosquitoes. The parasitic worm is called a "heartworm" because the parasite, in the final reproductive stage of its life cycle, resides in the heart of its host where it may kill through congestive heart failure. The best defense against heartworm is the use of preventative treatment given regularly during the mosquito season. A course of heartworm prevention begins with a blood test to see if the parasite is present. If the dog is parasite free, a prophylactic medication can be used for the prevention of a heartworm infection. A positive test result, on the other hand, usually requires prompt treatment to eradicate the worms. Even treated dogs must go through a long period of treatment (sometimes requiring surgery) to kill the worms and remove them from the body

Symptoms of infestation

Dogs show no indication of heartworm infestation during the 6 month long prepatent period prior to the worms' maturation, and current diagnostic tests for the presence of microfilariae or antigens cannot detect prepatent infections. Rarely, migrating heartworm larvae get "lost" and end up in unusual sites such as the eye, brain, or an artery in the leg, which results in unusual symptoms such as blindness, seizures and lameness.

Many dogs will show little or no sign of infection even after the heartworms have matured. To some degree these dogs may be described as seeming to age slightly faster than normal as the worms slowly damage the lungs, kidneys and liver. These animals usually have a light infection and live a fairly sedentary lifestyle. However, active dogs and those with heavier infections will quickly show the classic symptoms of heartworm disease. Early symptoms include a cough, especially on exercise, and early exhaustion upon exercise. More advanced cases progress to severe weight loss, fainting, coughing up blood, and, finally, congestive heart failure.