In dogs infected with heartworms (Dirofilaria immitis) the adult heartworms live in the large blood vessel feeding the lungs – the main pulmonary artery. This artery exits the right ventricle (pumping chamber) of the heart and branches to carry blood to each lung. This large artery and its tributaries are home to the twelve inch long adult heartworms.
The adult heartworms are swimming in the blood and are not attached to the heart or blood vessel walls. Heartworms consume nutrients directly from plasma. They live and mate within the main pulmonary artery. The female heartworms give birth to live larva (L1) which is microscopic, hence their name – microfilaria.
The L1 larva circulates in the blood stream waiting for a mosquito to transport them to their new host. The L1 larva cannot complete their development into adult heartworms without first being passed through a mosquito. Therefore, these heartworm babies cannot grow up in the host dog in which they were born. A mosquito will pick up the circulating microfilaria (L1) from an infected dog during a blood meal. The gastric juices in the stomach of the mosquito stimulate the L1 larva to molt to the L2 stage.
The L2 larva then migrates to the salivary glands of the mosquito where they molt to infective L3 larva. The process of molting within the mosquito takes 10 to 20 days. The migration of heartworm larva can be fatal to the mosquito if too many are ingested at once. The heartworm larva will only continue their development after they have infected a new host dog.
The L3 larvae are deposited into the salivary (hemolymph) bleb that surrounds the mosquito’s mouthparts during feeding. After the beak of the mosquito is pulled out, the heartworm larva swims down the hole into the area under the skin (subcutaneous tissue) on the new host dog. The L3 larva will molt to L4 larva in 7 to 14 days, and then to L5 larva in 45 to 60 days as further development continues.
During this time the L4 larva travels through the tissues first to the abdomen then passes through the diaphragm to arrive at the lungs. The L5 larva will penetrate the small arteries in the outer parts of the lungs on their way to the heart. This L5 larva will eventually become the adult heartworm. As the heartowrm adults grow and develop they are forced to move to progressively larger arteries of the lungs until they finally arrive in the main pulmonary artery. These heartworms will then continue to mature, mate and give birth to L1 larva.
The L1 larvae (microfilaria) are carried in the blood throughout the body. The number of circulating microfilaria may vary during different times of the day. The number of circulating microfilaria is greatest during the morning and evening hours coinciding with mosquito activity. The L1 larva ingested by the new mosquito are ready to start the development cycle again ready for the next dog to be biten in a never ending cycle.
- An infected dog is bitten by a mosquito. The mosquito sucks up blood containing heartworm larva.
- The microscopic larva (L1 stage) from the dog mix with gastric juices in the mosquito stomach. This stimulates the larva to develop to the next stage (L2).
- The L2 larva now migrate to the mosquito salivary gland where they undergo their next stage of development. This process takes 10 to 20 days. At the end of the development or molt the larva are now classed as L3 and are at the infective stage.
- The infected mosquito bites a dog injecting the infective L3 larva into the dog.
- The larva are now in the swelling under the skin where the mosquito bites the dog. Another development takes place. In 7 to 14 days L4 larva emerge.
- The L4 larva travel through the body tissues to the abdomen then penetrate through the diaphragm into the lungs. As they travel the L4 larva are developing into L5. This migration takes 45 to 60 days to complete.
- The L5 larva arrive at their destination, the small arterioles (blood vessels) of the lungs. Here they develop into adult heartworms.
- As the young adult grow the absorb nutrients from the blood and swim against the current in the blood vessels to remain in position. As they grow they are forced to move to bigger and bigger blood vessels until they arrive in the pulmonary artery.
- The adult heartworms in the pulmonary artery mate and produce new baby heartworms – the microscopic L1 larva or microfilaria.
- The baby heartworms (microfilaria) are ready to be picked up by new mosquitoes and transferred to new hosts