Heartworm in Dogs: What You Need to Know!

September 17, 2019 5 min read 0 Comments

Corgi running through grass on a sunny day

Everyone has heard about heartworms and may even have your dog on monthly prevention. Not everyone actually knows what heartworms are and how they can affect your dog. This article is everything you need to know about heartworms and what you can do to keep your pet healthy and happy.

What are heartworms?

Heartworms are parasites that live in the blood vessels, heart, and lungs. The worms can cause very deadly heart and lung problems.

How do dogs get heartworm?

Dogs and cats can only get heartworms from being bitten by a mosquito that contains baby heartworm (microfilaria).

Mosquito bites infected animal transmits microfilaria (baby worms)

Husky curled up on the couch next to a blanket

The life cycle of heartworm can be very complicated. The simplified version of this complex life cycle is a mosquito bite an animal that has heartworms and during their blood meal ingests baby heartworm. This baby heartworm grows up to teenage heartworm in the mosquito. The mosquito then bites another animal and injects this teenager's heartworm into the new animal where the heartworm can grow up to be adult heartworm. The adult heartworms can mate and reproduce in the animal.

7 months gestation period (average)

The life cycle from reproduction until adult heartworm takes an average of 7 months. In the adult stage, a heartworm can live in the heart lungs and blood vessels. Sometimes heartworms are found in unusual places such as skin, eyes, and internal organs. Anywhere that blood is heartworm can be. Microfilaria can be found in the blood. Usually, when a dog that has heartworms has their blood drawn, you can see these baby heartworms swimming around in the blood under a microscope.

Can live 5-7 years in dogs

Adult heartworms can live for 5 to 7 years in a dog. When these worms move around, they are causing damage to the heart, lungs, and vessels. This damage causes all the signs associated with heartworms.

Signs your dog has heartworm

White Chihuahua Sitting in a car with its paws on the steering wheel

There are many signs that your dog may have heartworms. Some are very mild, and some can be very severe and even life-threatening. Working with your veterinarian, you can figure out if heartworms are for sure, causing the symptoms that you are noticing in your dog and what to do about them.

Early-onset may have no signs

Early heartworm disease you may notice little to no signs as all. Usually, young dogs who have heartworm will not show symptoms.

Active dogs will show signs earlier as the vessels spread the worms

If your dog is very active and always running and playing, they will show symptoms earlier than lazy dogs. When a dog is very active, the blood vessels will dilate, allowing the heartworms to get into what was a smaller space.

Persistent cough

King Charles spaniel laying on a bed under a blanket

Dogs with mild to moderate signs of heartworms will have a cough that usually no matter the treatment will not go away.

Swollen belly

Heartworms disease progresses to a more severe stage will show up with a swollen belly. The swollen belly is due to fluid accumulation in the abdomen. The worms are taking up space in the vessel, which limits the amount of fluid that can circulate in the body. This buildup of fluid will get pushed out of the vessels and into the abdomen.

Caval syndrome (cardiovascular collapse)

In end-stage heartworm disease, they will have a condition known as caval syndrome. In caval syndrome heartworms fill-up the vena cava, the vessel that brings blood back to the heart severely limiting blood flow to the body. During caval syndrome, a dog will have labored breathing, pale gums, and dark bloody or coffee-colored urine Caval syndrome is an emergency and should be taken to the veterinarian immediately. Many times, even with prompt veterinary intervention, dogs in caval syndrome will die.


Pit bull panting on a sunny day in tall grass

Dogs with heartworm disease can also be exhausted. They do not have the proper amount of blood circulating in their body. Blood is what carries oxygen to all the cells in the body, and just like when we are at high altitudes with less oxygen, cells also have a hard time functioning.

Decreased appetite

Dogs with heartworm may also have a reduced appetite, especially as the disease progresses.

When should my dog be tested?

All dogs should be tested at least every few years, even if they are on heartworm prevention. Nowadays, no prevention is 100%. There are a few resistant strains of heartworms.

Heartworm prevention (puppies under 7 months)

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Puppies under 7 months of age can be started on heartworm prevention without testing. Since the life cycle of the heartworm is about 6 to 7 months, it is very rare to see a dog under 6 months of age test positive for heartworms.

Adult dogs that never had heartworm prevention or had methods switched

Adult dogs that currently are not on any heartworm prevention or have missed dosages of heartworm prevention should be tested before starting prevention. If you have changed from one type of prevention to another, you should also have your dog checked. This is to make sure that the prevention that you are using is adequately protecting your dog.  

What to do if your dog has heartworms?

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There are many different options for heartworm treatments. If your dog has been diagnosed with heartworms, discuss all your different options with your veterinarian.

Limit exercise

Dogs who are positive for heartworms should limit the amount of exercise that they do to help prevent them from getting the side effects that come with heartworm disease.

Veterinarian-recommended treatment

Many veterinarians follow the heartworm protocol put out by the American Heartworm Society. This protocol is a 3 to 4-month treatment where your dog will receive injections in their back muscles to slowly kill the adult worms. This can cause pain and stress so if your veterinarian says it is safe you can use CBD oil to help ease the process.

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Retest for confirmation

Dogs who show up as a light positive or who have been on heartworm prevention should be retested for confirmation. Usually, this retest is sent off to a lab to use a different method than in-clinic testing uses.

While heartworms can be a terrifying disease, by giving month prevention, this can be eliminated. If your dog does come up positive for heartworm, discuss with your veterinarian all the different treatment options for keeping your dog healthy and happy

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Heartworm in dogs By Dr. Sara Ochoa

Dr. Sara Ochoa DVM
Since she was a little girl, she knew that her dream was to become a veterinarian. With a tremendous passion and love for animals that makes her a great source of knowledge for others. She lives happily with her husband Greg and her babies Ruby the Schnoodle, and Bam-Bam the bunny.