Are Heartworms Contagious in Dogs?

Laura Dayton, DVM
By Laura Dayton, DVM on Aug. 11, 2020

Heartworm disease is not a new problem for dogs and cats, but it certainly has lots of myths and misunderstandings that surround it. These are a few of the major questions: “Can humans get heartworms from dogs? Are heartworms contagious to other dogs?”

This article will help clarify how heartworms are contracted, whether heartworms are contagious to other dogs or people, and how they can be prevented.

How Do Dogs Get Heartworms?

Let’s say a dog is infected with mature heartworms. These mature heartworms produce "baby" heartworms called microfilariae. The heartworm life cycle starts when a mosquito bites the infected dog and picks up the microfilariae as it feeds on the infected dog’s blood.

The microfilariae go through several larval stages in the mosquito, until they are “infective” or “L3” stage larvae. When the mosquito bites another dog, it transfers the L3 larvae to the new dog.

Once in the new dog, these L3 larvae become L4 larvae. The length of this stage can vary but it’s roughly 45 to 60 days. Only L3 and L4 larvae are killed by heartworm preventatives. This is why it’s so important to give the medication on time as prescribed.

The heartworms are now mature adult worms and have been present in your dog for 60 days. If the vet gave your dog a heartworm test, it would still come up negative. It takes an additional 120 days for the heartworm to show up on a standard heartworm test run at your veterinarian's office.

During this period, any doses of heartworm prevention you administer will not be effective in killing the worms.

Are Heartworms Contagious to Other Dogs or People?

Since the mosquito is needed to carry the microfilariae, heartworm disease is not contagious from one dog to another dog. People also cannot get heartworms from dogs.

Dogs and humans can only get heartworms from infected mosquitos. That said, the likelihood of coming across a positive mosquito increases dramatically with just one heartworm-positive dog in the area.

How to Prevent Heartworm Disease in Dogs 

All heartworm preventatives have the ability to kill L3 and L4 larvae in dogs. There are many different formulations available, so you should ask your vet which one is best for your dog.

The most important part of any heartworm preventative medication is that you give it as prescribed. Heartworm resistance to our preventative medications is a growing problem, and it’s caused by pet parents not being consistent with their dog’s heartworm medication. The unintended result has been exposing adult worms to doses of medication that cannot kill them.

And if your dog does get heartworm disease, once they show symptoms, it’s already in a more advanced stage. For dogs, heartworm treatment is costly and involves several months of confinement and activity restriction, three painful injections, and the potential for long-term effects. The clear choice when it comes to your pet’s best interest is preventing heartworms to begin with.

Featured Image:

Laura Dayton, DVM


Laura Dayton, DVM


Dr. Laura Dayton is a small animal practitioner in Charlotte, NC. She graduated in 2010 from the NC State College of Veterinary Medicine....

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