Heartworm Disease and Your Pets

By Dr. Shawn Finch

When was the last time you saved a life?

You have probably saved a life within this very month.

You give your pets their heartworm preventative medication when you see our monthly reminder on Facebook or Twitter, or see the sticker on your calendar.

You bring your pets for their yearly heartworm test and pick up their heartworm preventative medication as you leave.

You stop by Gentle Doctor for more Heartgard or Revolution as soon as you run out or run low.

You are protecting your pets from heartworm disease, the worst parasitic disease of companion animals. 

Never again do we want you to think you are merely performing a boring monthly chore, keeping a routine appointment or running a simple errand. You are savings lives, and you are our hero!

Why Heartworm Disease is So Awful

Heartworm disease is caused by very long worms that start off tiny enough to live inside of a mosquito.  The mosquito bites a dog; the worms travel to the blood vessels of the lungs, growing all the while into a huge mass of wriggling pasta, causing considerable damage to the cardiac and respiratory systems, including coughing, lethargy, and often, sudden death.


Larva – immature form, from the Latin for “ghost” or “mask.”  The larval form “masks” the adult form.

Microfilaria – micro – small + filaria – worm.  Microfilariae are the first of five stages of heartworm larvae.

Adult – the final stage of heartworm development.

Why We Give Medication Every Month, Yearround

The violent, destructive heartworm lifecycle has a somewhate variable timeframe from dog to mosquito. It heartworm begins when a female mosquito bites a dog infected with adult heartworms that are producing microfilariae and releasing them into the dog’s bloodstream. 

From Mosquito to Dog

The microfilariae develop into the infective stage of heartworm inside the mosquito, where they are then ready to be passed on to a dog.  Time:  10 – 42 days.

The infective larvae are passed to the dog through a mosquito bite.  At this stage, the larvae are vulnerable to heartworm preventative medication. 

The infective larvae migrate between muscle fibers and mature into the next stage, which are also vulnerable to heartworm preventative medication, at least at the beginning of the stage.  Time:  3 – 14 days.

The heartworms migrate through the abdomen to the lungs.  Along the way, they develop into adolescent heartworms, the last larval stage.  At this point, they are, for the most part, no longer vulnerable to heartworm preventative medication.  The adolescent heartworms are between one and three centimeters long.

As they grow, the heartworms begin travelling through muscles instead of between them, and eventually right through small vessels in the outer parts of the lungs to enter the vascular system.  Time:  50 – 70 days.

As the adolescent heartworms grow into adult heartworms, they move to increasingly larger blood vessels.  Time:  90 – 120 Days.

Adult heartworms live in the main artery of the lungs, swimming against the flow of the blood and consuming nutrients directly from the blood.

If numerous worms are present, some may be found in the right ventricle of the heart and the smaller vessels of the lungs and even in the vena cava, the largest vein of the body.

Adult worms can grow up to 30 cm long.  A dog can have as many as 250 worms.

After the heartworms have been in the dog for about six months, the adult female heartworms start releasing a protein from their uteruses into the dog’s blood stream.  This protein is the substance that heartworm tests detect to determine that a dog is “heartworm positive.”

A dog with heartworm disease that is not treated can have adult heartworms in the body for five to seven years, if the dog survives that long. 

The adult heartworms are not replicating within the dog during this time, but they are continually causing damage, even as they eventually die and are removed by the dog’s body.

The adult heartworms also continuously mate and give birth to live young (microfilariae), which are released into the blood stream.

The microfilariae are carried in the blood stream throughout the body.  The number of circulating microfilariae varies during different times of the day and is greatest during the morning and evening hours, coinciding with mosquito activity.

Microfilariae can live in the dog’s vascular system for up to two years.

From Dog to Mosquito Again

The microfilariae are ingested by a new mosquito, and begin to develop into the infectious stage.  The heartworms are soon ready to infect the next dog.

Heartworm Prevention

Heartworm disease is treatable (treatable in dogs, not cats or ferrets), but prevention is much easier on your pet, much safer, completely pain free and several times less expensive!

Heartworm preventative medications do not actually prevent heartworm infection – they prevent heartworm disease.  The purpose of heartworm preventative medication is to prevent an infection from continuing to the point of adult heartworms setting up home and wrecking havoc. 

Heartworm preventative medications paralyze the mouthparts of the heartworms within your dog at the infective and next larval stage, causing the young worms to starve to death before they become adolescent or adult worms.  

Once heartworms have matured past the stages at which they are vulnerable to heartworm preventative medication, the preventative will not work - the worms will not die when the preventative is given.  We have a fairly small but manageable window in which to stop heartworms from harming our pets.

Every pet...every month.

Think of heartworm disease as a rainstorm - Heartworm preventative medications are not umbrellas outside of your dogs.  They are windshield wipers inside of the dogs’ bodies.  Once a month, we give heartworm preventative medication to kill the two vulnerable stages of heartworm larvae already growing and travelling through your dog. The mosquitoes are the clouds and the infective larvae are the raindrops...if clouds were buzzy and annoying and raindrops were potentially fatal.

Cats, Ferrets and Heartworm Disease

The good news about heartworm disease in cats and ferrets is that these pets are infected at a lower rate than dogs.  When cats and ferrets do develop heartworm disease, they usually have a lower worm burden than dogs, and the heartworm lifecycle is usually not able to complete itself within their bodies.

The bad news about heartworm disease in cats and ferrets is that when they are infected, they are even less equipped to handle the infection than dogs are.  The first sign of heartworm disease in cats and ferrets is often sudden death.  If the disease is not immediately fatal, it can be difficult to diagnose, and there is no treatment for cats or ferrets!

Every dog, cat and ferret should be on a monthly heartworm preventative medication year round.

And beyond...

Many mammals can be infected with heartworms, but the definitive host (the one that harbors them best and is best able to be used to complete the lifecycle) is the dog.  If we can eradicate heartworm in our pet dogs, we can really strike a blow to the disease overall.

California Sea Lions are another species vulnerable to heartworm disease.  By giving your pets their monthly heartworm preventative medication, you are keeping your pets safe, breaking the cycle of heartworm disease, protecting the pets of our communityY and even protecting the sea lions of Henry Doorly Zoo

Now do you believe us that you are a hero?