What Is Heartworm Disease?

Canine heartworm disease is a potentially deadly infection, caused by worms (Dirofilaria immitis) that may grow to be 14-inch-long adults. These worms live in the right side of the heart and arteries of the lungs. Dogs of any age and breed are susceptible to infection. Heartworm infection can cause potentially serious damage to these arteries, eventually leading to heart failure, and in severe cases, damage other organs such as the liver and kidneys. In extreme cases, a dog can be infected with several hundred heartworms. Cats are also susceptible to the disease.

Coinciding with mosquito season, heartworm disease is spread by mosquitoes that become infected with microfilariae while taking a blood meal from an infected dog. The microfilariae mature into the infective larval stage within the mosquito. When the mosquito then bites another dog, cat, or susceptible animal, it then passes the larvae into the animal’s blood stream through the bite wound, resulting in heartworm infection. It then takes a little over six months for the infective larvae to mature into adult worms that can live for five to seven years in the dog.

Heartworm Life Cycle

Heartworm infection is spread from animal to animal by mosquitoes. Dogs, cats, ferrets, coyotes, foxes, wolves, sea lions and even humans have all been found to be infected by heartworm.

Heartworms are classified as nematodes (roundworms) and are but one of many species of roundworms.

Adult female heartworms release their young, called microfilariae, into the animal’s bloodstream. Mosquitoes then become infected with microfilariae while taking a blood meal from an infected animal. During the next 10 to 14 days, the microfilariae mature to the infective larval stage within the mosquito.

When the mosquito bites another dog, cat, or other susceptible animal, the infective larvae enter through the bite wound. In dogs, it then takes a little over six months for the infective larvae to mature into adult worms that may live for five to seven years in dogs. In cats, it takes about eight months to mature into adult worms that live from two to three years.

Microfilariae cannot mature into adult heartworms without first passing through a mosquito.

How Do You Detect Heartworm Disease?

Detection of heartworm infection in apparently healthy animals is usually made with blood tests for microfilariae or a heartworm substance called an "antigen," although neither test is consistently positive until about seven months after infection has occurred.
Heartworm infection may also be detected through x-ray and/or ultrasound images of the heart and lungs, although these tests are usually used in animals that are known to be infected.

Microfilaremia, the presence of heartworm offspring in the blood of the host, is relatively common in dogs. However, not all heartworm infections result in such offspring circulating in the blood.

These are known as occult heartworm infections and may be the result of a number of factors such as single sex heartworm infections, host immune responses affecting the presence of circulating offspring (microfilariae) and most significantly, the administration of heartworm preventives.