Heartworm In Dogs – Intro & Lifecycle

What is Heartworm?


Heartworm (Dirofilaria immitis) is a thin worm, called heartworm since the adult stage of the worm lives in and around the heart. The female adult worms are up to 2 1/2 feet long and the male adult worms can be 2 feet long and have a coiled tail.

Heartworm can be found in the hearts of other animals, but dogs and some of their wild relatives are the main hosts. Other animals that have been found to have heartworm include coyotes, wolves, foxes, cats, ferrets, sea lions and on very rare occasions humans.

Find out about heart worm in dogs;

Lifecycle Of Heartworm – Heartworm Infection & How It Spreads

In order to understand how to prevent and treat heartworm, it is useful to understand the heartworm lifecycle.

Mosquitoes are an intermediate host for heartworm

Adult female heartworms, after mating with male heartworms give birth to live young called microfilariae. (Microfilaria is used for one, microfilariae for more than one.) The microfilariae circulate in the blood waiting for a mosquito to feed on the blood. It is thought that the microfilariae can live in the blood for up to 2 years.

When a mosquito ingests blood infected with microfilariae, they start to develop. They grow and molt (shed skin). After at least 2 weeks they are in the third larval stage, which is an infective stage and move to the salivary glands in the mouth of the mosquito.

Warm temperatures are required for development in the mosquito and hence spread

The development of the heartworm larva in the mosquito is temperature dependent. The lowest temperature at which the larvae can develop is about 56°F. Below this temperature the larvae will not develop. The optimal temperature for development is 80°F and above.

The temperature dependence means that heartworm will not be transmitted during the winter in many areas, since it is too cold. However, in areas that have warm winters heartworm can spread, and dogs can be infected year round.

Maturation of heartworm in dogs after infection

When a mosquito bites an animal, it injects some saliva into the wound to stop the blood clotting while they feed. If a mosquito has infective larvae of heartworm in its salivary glands then some of the larvae will be injected with the saliva, into the dog.

The larvae initially stay under the skin at the site of mosquito bite, feeding and growing. After about two weeks they molt to become the fourth larval stage. The larvae move to the muscle of the chest and abdomen. After two to three months they molt again to the fifth stage, immature adult.

The immature adult heartworms then enter the blood stream and migrate to the heart. Here they grow, develop into adults and mate. The worms usually first locate in the pulmonary artery (the artery from the heart to the lungs) and migrate into other local blood vessels. They then migrate to the lungs, and then to the right ventricle of the heart. If the worm infestation is heavy the worms can start to fill up the heart.

About 6 to 7 months after the mosquito bite, microfilariae start to be born, and the lifecycle starts again.

Adult worms can live up to 7 years in the blood vessels, lungs and heart.

Next page – Heartworm In Dogs – Symptoms & Prevention
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See also Heartworm – Table of Common Preventative Medicines