Love May Not Be the Only Thing in Your Dog’s Heart

Heartworms in Dogs – What You Should Know

Heartworm disease is caused by a parasite that is becoming more common. While dogs and other members of the canine family (wolves, foxes, coyotes, etc.) are the primary hosts, other species can also be infected including members of the cat family, muskrats, raccoons, bears, ferrets, and sea lions. The parasite is carried from one host to another by mosquitoes.

Heartworm only exists in geographic areas with a supply of both primary hosts and mosquitoes. Although infection in humans has been reported, such cases are rare, and people can’t pass the parasite on. Doctors do not consider human infection a serious condition.

A dog who is a host or carrier of heartworms may be completely free of symptoms or may show signs of severe illness. The earliest sign is usually a cough. Further signs include tiring with exercise, collapsing spells, and an enlarged abdomen due to heart failure. Severely infested dogs can die from heartworm disease.

The Heartworm Life Cycle

1. The host dog has adult heartworms in the right side of the heart and the lung arteries.

2. Fertile female heartworms release first stage, microscopic larvae called microfilariae into the host dog’s blood.

3. A mosquito bites the host dog and ingests microfilariae.

4. In the mosquito, the microfilariae undergo two stages of development and become third-stage or infective larvae.

5. The mosquito bites another dog, injecting the infective larvae into the bite.

6. In the second dog, these larvae undergo two more developmental stages, called moults, and migrate to the right side of the heart and lung arteries where they mature to adults. Some microfilariae, or first stage larvae remain in the blood, making this dog a host. The entire life cycle takes from 7 to 9 months.

Diagnosis of Canine Heartworm Disease

Heartworm disease is usually diagnosed by detecting microfilariae in a sample of the dog’s blood or by detecting antigens associated with heartworm in the dog’s serum.

Treatment of Canine Heartworm Disease

Dogs who test positive for heartworm are first treated with injections to kill the adult worms. At a later date, additional medication is required to eliminate the microfilariae. Check with your veterinarian about different types of treatments available.


Medication to prevent heartworm is available. It is safe for dogs who test free of the disease. Young puppies can be given the medication without having a blood test, but dogs six months or older must be tested first. Testing is very important, as five to 10 percent of infested dogs will have serious or even fatal reactions if they are given preventative medication. Different types of heartworm medication can be given on a daily or monthly schedule. Ask your veterinarian which type is best for your dog. When used properly this medication is nearly 100 percent effective in preventing heartworm disease.