Dog Owner's Home Veterinary Handbook.pdf - Mr. Walnuts

Sexual reproduction occurs if worms of both sexes are present. Females
give birth to live young called microfilaria; 5,000 microfilariae can be produced
in one day by a single worm. Microfilaria are able to remain alive in the
dog’s circulatory system for up to three years.
Before the microfilariae can become infective to another dog, the L i larvae
must go into a secondary host, the mosquito. This occurs when the mosquito
bites the dog. The L I larvae in the mosquito molt to L 3 larvae. In warm southern
climates this process takes less than 10 days; in northern climates it can
take up to 17 days. The L 3 larvae then move to the mouthparts of the mosquito
and are ready to infect a new host.
HEARTWORM DISEASE
THE CIRCULATORY SYSTEM • 343
This model shows heartworms
in the right atrium
and right ventricle.
When there are fewer than 50 adult worms in the average-size dog, the worms
live primarily in the pulmonary arteries and the right ventricle of the heart.
When the numbers are greater than 75, the worms usually extend into the
right atrium. With a heavier infestation, the worms may migrate into the
superior and inferior vena cavae and the veins of the liver.
Worms in the lungs can migrate into the terminal branches of the pulmonary
arteries, where they obstruct the flow of blood and cause the vessels to
clot. This is known as pulmonary thromboembolism. Even after treatment,