Understanding the canine heartworm lifecycle

March 18, 2016 Category: Parasite education

Heartworm disease can be a deadly issue for your dog, and more than 1 million pets in the United States have heartworm disease.1 So understanding the lifecycle of a heartworm is a critical part of helping to keep your dog safe. Learn more below about the heartworm lifecycle and what you can do to protect your dog.


Microfilariae are actually microscopic larvae that can be found in the blood of heartworm-infected dogs. When a mosquito feeds on an infected animal, it ingests these microfilariae, which develop into infective larvae inside the mosquito in about two weeks.2


The mosquito’s mouthparts pass the infective larvae to your dog’s skin, where they burrow into the tissue through the bite wound.2 The larvae begin to mature within three days and continue migrating through your dog’s tissue for several weeks.2

Juvenile worm

About two months after initial infection, the larvae develop into juvenile worms and enter your dog’s blood.2 Immature adult worms that range from 1-1.5 inches in length arrive at your dog’s heart and lungs as early as 67 days after initial infection.2 After arriving, they continue to mature into adulthood.

Adult worm

Heartworms generally mature into mating adults in the large blood vessels of the lungs and, in more severe infections, the right ventricle of the heart.2 These areas can become clogged with worms, resulting in a number of health issues, including but not limited to liver and kidney failure, heart failure and even death.2 An infected dog typically carries microfilariae within six to seven months after infection from the initial mosquito bite.2 And the cycle continues.

What should you do?

A routine, yearly blood test performed by your veterinarian is an accurate way to detect heartworm infection in your dog.2 Start by talking to your veterinarian, and ask about protecting your dog from heartworm disease year-round with Trifexis® (spinosad + milbemycin oxime). Trifexis is approved for use in dogs and puppies 8 weeks of age and older and 5 pounds of weight or greater. 

Learn more about heartworm disease