Heartworm Life Cycle

Pet Tales
by Barry B. Burtis D.V.M.

Adult females that measure 27 cm (10.6 inches) in length and males, who are a bit shorter at 17 cm (6.6 inches) long, are living in the pulmonary arteries and right ventricles of the heart. The pulmonary arteries are the blood vessels that carry blood from the heart to lungs. This blood having completed its journey through the body is depleted of oxygen and is being returned to the lungs to take on a new load before once again departing to supply distant body tissues that require it for vital life processes. The right ventricle of the heart is the chamber whose muscular contractions will propel the blood on this short trip from the heart to the lungs. The presence of the adult parasites in these locations does make the task a bit more difficult to accomplish. However, they are living here without causing a major occlusion or blockage of the blood flow. The story is just about to begin and it would not be in their best interest to cause a major problem for their host at this stage of the affair.

The story is about Dirofilaria immitis, more commonly called heartworm parasites. The host is some unfortunate member of the canine species, perhaps a dog living on your street. The location for the story is Halton Region or somewhere nearby in south-western Ontario. It is happening right now as you read about it.

The offspring produced by these adult females and males are called microfilaria. They are discharged into the blood stream that surrounds their parents. Microfilaria can survive in the blood as it circulates through the body for 1 to 3 years. The number of microfilaria in dogs is increased in warm ambient temperature, after the dog has eaten a meal and late at night. This is the reason for these changes. If the story is to continue the microfilaria must be ingested by a mosquito during its feeding from the dog. The increase in numbers of microfilaria mentioned will increase the odds of a mosquito ingesting microfilaria.

When ingested by a mosquito, microfilaria now called the infective larvae stage (L1), migrate to the stomach. After some development in the mosquito, the larvae (L3) move to the mouthparts. The rate of development can be as short as 8 days at an air temperature of 30C or as long as 28 days at 18C. After a mosquito acquires the microfilaria (L1) there must be adequate exposure to warm temperatures during the relatively short lifespan (1 month) of most mosquitoes. This fact determines the length of the heartworm season in various geographical areas. In south-western Ontario that season when infection can be spread runs from June to November.

When an infected mosquito feeds again, infective larvae (L3) are deposited on the skin of an animal and enter its body through the bite wound. A maximum of 10-12 L3 larvae can be transmitted by a single mosquito. The L3 stages moult to L4 and L5 (adult) and migrate to the pulmonary arteries. On arrival, the L5 stages are about 1-2 cm (.4-.8 inches) long and it has taken them approximately 75 to 90 days, after infection, to reach this part of the body. Once inside the pulmonary artery it will take the L5 stage another 2 to 3 months to reach sexual maturity. If both sexes are present, microfilaria will be produced about 6 to 7 months after infection with the L3 stage. At this time, microfilaria can be detected in the blood in an infected dog, but rarely in a cat. There are other blood tests that veterinarians can use to detect the presence of adult worms. These are typically positive about 6 to 7 months after infection. High enough quantities of the glycoprotein to be detected, in these tests, are only associated with fully mature adult female heartworms.

After reading this story, the homework assignment for any caring, concerned pet owner will be to make sure their pet plays no role in it. Ask your veterinarian how to protect your dog or outside cat from the danger of heartworm disease.