Heartworm disease is caused by a parasite called Dirofilaria immitis. It is transmitted by mosquitoes and primarily infects dogs, but can infect cats as well. The adult heartworm lives in the blood vessels of the heart and lungs and can cause severe disease or even death. Fortunately, there are effective preventive medications. We recommend that all dogs be on a monthly heartworm preventative year-round. Even indoor dogs can be exposed to mosquitoes and we do occasionally see mosquitoes in the winter when the weather gets warm. An added advantage of heartworm preventatives is their ability to control some of the intestinal parasites.
Source: http://www.petsandparasites.org/about-capc
Although heartworm preventatives are very effective, no medication protects 100%. For this reason, we follow the American Heartworm Society’s recommendation to test dogs for heartworm disease each year. In most cases infections can be treated successfully when caught early. If an infection has been present for a long time, treatment may be more difficult. Our heartworm test also checks for several tick-borne diseases including Lyme, Ehrlichia, and Anaplasma.