What Is Heartworm?

HeartwormHeartworm (Dirofilari aimmitis) is a worm that infects many animals including dogs and cats. Adult heartworm lives in the pulmonary artery, where they release microfilaria (baby form of the parasite) into the blood stream. Heartworm is transmitted from animal to animal by a wide variety of mosquito species. When a mosquito bites an infected animal, they take in microfilaria which develops into infected larva within the mosquito. The mosquito will then pass the larva onto the next animal they feed from. Once an animal becomes infected with heartworm larvae as a result of a mosquito bite, the larvae begin to migrate through the skin into blood stream. While migrating through tissue, the larvae mature into immature worms. Within 4 months of, migration immature worms can grow from less than 1 inch to as long as 11 inches.The damage caused by heartworms can result in progressive pulmonary disease and secondary heart problems. Exercise makes matters worse for the pet, such that an unsuspecting pet owner can worsen his or her pet’s heartworm disease during play. The extent of illness caused by heartworms varies among pets, but dogs and cats can die from heartworm disease.

Geographic Distribution

Heartworms are found throughout most of the United States and southern Canada. Human development, with its irrigated lawns, golf courses and planted trees, has created areas that are highly favorable to mosquitoes. Transmission of heartworm within these areas can result in the infection of wild and domestic dogs, which may lead to an endemic infection. There has been a 60% increase in the number of dogs with heartworm between 2002 and 2010. In a recent study, 564 dogs tested positive in Canada in 2010, 431 of them were from Ontario (over 75%), and 80% of them were not treated with heartworm preventive. On average,1 to 3 cats are diagnosed in Ontario annually with heartworm.

Heartworm Disease Prevention

Fortunately, our veterinary hospital does have excellent pharmaceuticals that offer very effective and safe protection from heartworm disease by killing larval heartworms before they reach the pulmonary blood vessels. All of these are currently available, and are highly effective. In order to be successful,these drugs must be administered according to manufacturers’ recommendations. It is critical that doses not be skipped or intervals between doses be extended because this will diminish the success of the prevention. All of the heartworm preparations are given monthly.

Annual heartworm testing is recommended in accordance with existing guidelines. Blood testing is required to diagnose heartworm disease. The approach to heartworm prevention has been advocated by the American Heartworm Society (AHS), the Companion Animal Parasite Council (CAPC) and Canadian Parasitology Expert Panel (CPEP). The same recommendation is found in the AAHA Canine and Feline Preventive Health Care Guidelines.