Heartworm disease is a serious and potentially fatal disease in pets caused by foot-long worms. Heartworm disease causes lasting damage to the heart, lungs, and arteries, and can affect the dog's health and quality of life long after the parasites are gone. In the early stages of the disease, many dogs show few symptoms or no symptoms at all. The longer the infection persists, the more likely symptoms will develop.
Heartworm medications are effective, but dogs can still become infected. If you miss just one dose of a monthly medication - or give it late - it can leave your dog unprotected. Even if you give the medication as recommended, your dog may spit out or vomit a heartworm pill - or rub off a topical medication. Heartworm preventives are effective, but not 100 percent effective.
(Source: American Heartworm Society)