The Time Frame for Heartworm Larvae to Develop in Dogs

Preventing your dog from contracting heartworm is cheap, but treatment isn't.
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Heartworm is a cruel disease. Dogs might not show symptoms until years after they've been infected, but by then, it may be too late for successful treatment. The process starts with microscopic larvae transmitted from the blood of one infected mammal to another by an intermediary host -- the mosquito. Seven months later, adult worms will be firmly entrenched in the chambers and blood vessels of the dog's heart and lungs, choking the oxygen supply to other vital organs while expelling millions of new larvae into the bloodstream.

Heartworm: A Worldwide Problem

More than a century ago, when Dirofilara immitis -- heartworms -- were first discovered, the parasites were believed to be restricted to southern climes. However, these nematodes, found in all 50 states of the U.S., have a global presence. Heartworms aren't just a scourge of domestic dogs -- they also infest more than 30 other warm-blooded species, including wolves, foxes, coyotes, domestic and wild cats, ferrets and, rarely, humans. The females of more than 70 mosquito species can transmit the disease.

In the Beginning

After a mosquito ingests first-stage heartworm larvae, called microfilariae, they settle in the insect's gut until they've grown enough to shed their skin twice, typically about two weeks. When the time is right, the third-stage larvae make their way up to the mosquito's mouthparts. When she alights on a dog and sinks her proboscis into his skin, the larvae invade his body through the bite wound.

The Next Stage: Young Adulthood

The larvae still have some growing to do, so they rest in tissue beneath the skin until they molt again, one to three days later. For the next couple of months, the fourth-stage larvae tour around the dog's body on the bloodstream, finally settling in the chambers of the heart, the pulmonary arteries, or both. There, they molt for the final time, becoming sexually mature adults. By 6 1/2 months after the worms first invade the dog's body, females will be about 10 to 12 inches long, and males will be about half that length. Your dog could begin experiencing symptoms such as a cough, difficulty breathing or abnormal lung sounds.

Future Generations of Larvae

Six to seven months after infection, the second generation of larvae are detectable in blood tests. The average lifespan of a heartworm, five to seven years, means that the numbers occupying a dog's heart and lungs mount steadily -- in severe cases, as many as 250 to 300 adults. By then, he's at risk of congestive heart failure. The overcrowded worms may then start migrating. If a mass of them lodges in the large vein that conveys blood to the heart's right atrium, the vena cava, the dog dies within two to three days.

Treatment: Usually Successful but Costly

When heartworm disease is caught in time, it can be successfully treated in about 95 percent of cases. However, this doesn't reflect the true mortality rate because vets won't treat cases so advanced that the dogs have no chance of survival. According to Texas A&M University, treatment can cost up to $6,000 as of 2013 and is hard on the dog, since it entails a series of injections of highly toxic drugs to kill both adult worms and larvae. The course of treatment can last four months, during which time the dog's activity levels must be restricted. In contrast, Texas A&M University estimates that the yearly cost of anti-heartworm medication ranges from $30 to $100. After ensuring the dog is free of heartworms, vets prescribe preventative options including monthly pills, chewable tables and injections that last six months. These prevent larvae from developing into adult heartworms.