Dogs And Heartworm | What Are The Signs

When it comes to protecting our dogs’ from serious illnesses, heartworm is one of those diseases at the top of that list.   It is very important that we all need to be completely aware of this disease, as it can be a potentially fatal disease for our dogs’.

dogs and heartworms
Photo courtesy of Matt W. Miller, DVM, MS, Diplomate ACVIM (Cardiology)
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Over 250,000 dogs’ per year are diagnosed with heartworm.   That means 1 in every 200 dogs’ will become infected with heartworm.   Very similar to our own risk of getting cancer, which is also 1 in every 200.   The difference between the two is that dogs’ and heartworm is 100% preventable. YES you heard me right…. PREVENTABLE!

What is it?

Heartworm or “Dirofilaria immitis” is a parasitic roundworm.   This roundworm is referred to as heartworm.   These worms are small thread like worms that almost look like spaghetti.

The disease is caused when a dog gets infected with the worm.   Heartworm directly affects their lungs, heart, blood vessels and organs.

They are called heartworms because the adult worms actually live in the heart, lungs and blood vessel of the dogs’. Specifically in the heart pulmonary arterial system of the dog.

A female heartworm mates and releases their offspring called “microfilariae” into the dogs’ blood stream there by continuing to spread the disease through the body.   This completes the worms life cycle.

A male worm can grow to be 12 to 20 cm long while the female is 25 to 31 cm long and both are about 1″ in diameter. The severity of the disease strictly depends on how many worms the dog has in it’s body.   A dog can have anywhere from 15 worms up to 250 worms.   THAT IS A TON OF WORMS!

One adult worm can live in a dog for 5 to 7 years.  The mass of worms is called a worm burden, so just imagine your dog having 100 worms in their body.  You can see how that absolutely becomes a situation that is very dire for the dog.

How do our dogs’ catch it?

A dog can catch heartworm through the bite of a mosquito who is carrying the heartworm larvae.  When the mosquito bites the dog, the larvae then travels through the dogs’ body to the heart and blood vessels of the lungs.

This process of infection takes approximately 6 mos to happen.  The larvae hatch and grow into adult worms that continue to reproduce and spread throughout the dogs’ organs, blood vessels and heart.

The American Heartworm Society (AHS) believes that the US has approximately 1 million dogs’ infected with heartworm.  Areas along the Gulf and Atlantic coast are commonly known to have higher numbers of dogs’ affected by heartworm.  Tropical areas like this are prime conditions for a prevalence to heartworm.  At the same time, there is strong evidence that suggests that heartworm is in all states in the US.

In Canada there is also heartworm in the warmer provinces like: Ontario, Quebec and British Columbia.

Stats indicate that heartworm disease is on the rise throughout the US.  With an increase of more than 20% between 2013 – 2016.  The reason they give for the high increase is strictly a lack of prevention being done to protect the dogs’.

Contrary to some stories you hear, heartworm is not contagious between dogs’, the only way the disease can be transmitted is from an infected mosquito biting your dog.


I mentioned that the infection of heartworm larvae from the mosquito growing to adulthood can take up to 6 mos. During that time, the dog will show absolutely no sign or symptoms of being sick.  It is not until the worm reaches maturity that symptoms appear.

Once the worm is an adult, a dog that is highly active will show more dramatic signs than say a dog that leads a less active life.

There are 4 classes of heartworm infection in dogs’. Each class has it’s each set of symptoms associated with it.


A dog can show absolutely no symptoms at all, or what we call “asymptomatic”. If showing any sign at all, it would be a cough once in a while.


In this class, a dog would exhibit an intolerance to exercise.  They would cough before and during the exercise, making it hard for them to continue.


With class 3, a dog would exhibit a different looking appearance.  Things like a dramatic weight loss, their hair may become dry or oily looking and have a pot-bellied appearance.  They would also be unable to exercise at all due to labored breathing caused by a fluid buildup in the heart causing heart failure.


The last and highest class is 4.  A dog in this stage, would have a condition knows as “Caval Syndrome”.  This syndrome is what happens when your dogs’ heart becomes overrun by worms.  So many in fact that it restricts blood flow to the heart.  Once a dog gets to this stage, it is too late to save them.

dogs and heartworm


Prevention and treatment

The treatment for a dog with heartworms will completely depend on the stage and number of worms the dog has.  A dog will first be required to have a blood test.  They will also need to test the dogs’ organs to ensure they are functioning properly before determining the course of treatment required.

Treatment for heartworm is an arsenic containing drug called “Melarsomine”.  The brand names in the market are
“Immiticide and Diroban”.  
This drug is administered through multiple injections in the muscle of the dogs’ back.  Usually 3 injections are required over a period of 1 months time.  The reason for the multiple injections, is to ensure that all the worms are killed.  One needle is just not enough to effectively irradiate the worms in your dogs’ heart and lungs.

dogs and heartworm needle treatment

The problem with the treatment of heartworm is that the medication can have severe side effects for your dog.  One of the most severe side effects is that it can cause blood clots in your dogs’ lungs.  This is a life-threatening issue for your dog.

All treatments are giving at the Vet or hospital and may require your dog to stay overnight to help monitor for any side effects.

The prognosis for dogs’ with mild to moderate heartworm disease is actually quite good.  Dogs that have reached Class 4 are ultimately just made comfortable as it is too late to actually cure them of the disease.  This is the cases where the disease becomes fatal to the dog.

For dogs’ that have a much more severe case of heartworms and are in the Caval Syndrome stage, surgery will be required to actually go in and remove the worms from the dogs’ body.  This is achieved by going through your dogs’ jugular vein.  With any surgery, the risks obviously go up, and heartworm surgery is no exception.  The damage that has been done to the dogs’ heart and lungs, usually ends in a fatality even with the surgery.  Ultimately the damage is just to catastrophic for the dog to recover.

dogs and heartworm treatment


For prevention of heartworm, your dog will need to first be tested for heartworms.  If the test is negative, then your dog can be given a monthly heartworm medication that is prescribed by your Veterinarian.

One of the most popular medications is called “Heartgard”.

Heartgard dogs and heartworm screenshot

Like any medication, it is not a 100% guarantee, so regular heartworm screening is important.

Even if your dog has mild to moderate heartworm any treatment can be very expensive as your dog will require multiple blood tests, x-rays, hospital stays and of course the medication itself.  For this reason, prevention of heartworm is by far the right decision to make, financially and for your dogs’ overall health.

An entire life of preventative heartworm medication is the same cost as one treatment if your dog actually develops heartworm disease, so please remember that.

dogs and heartworm dog say thank you

On top of the high cost, no dog parent wants their dog to be infected with this very serious disease and have to go through the treatment process.

There can be long term effects for your dog, even if they get successfully treated.   Help your dog say healthy, and they will thank you for it.

buy heartgard now


4 thoughts on “Dogs And Heartworm | What Are The Signs”

  1. Aw, this post is going to make me cry. I remember my first dog had and an issue with his heart but the vets could not find out what was wrong. At first, it was asymptomatic and then he began being very sluggish and almost looked like he kinda gave up on life.

    He never wanted to do anything and we had to put him down eventually. Although, it was a long time ago, maybe if we had better knowledge or technology at the time, we could have extended his life.

    Thanks for this post, as seeing a dog have heart or any problems for that matter is just a sad thing.

    • Hello,

      Gosh I am so sorry to hear about your dog.  We love our dogs so much, that when they are sick it really hits home for us. 

      We are so lucky now that we have so much more information and ability to find and treat things.  Thank you for taking the time to tell me your story, I appreciate you sharing.  I know it is hard to think about our past dogs.

      I still get teared up thinking of my dogs that have past on.



  2. Hello there, thanks for sharing this awesome article I know it would be of great help to the public as it has been of help to me. I have been having noticing some of these signs in my young dog and I have been wondering what’s going on.

    I think I would take her to the vet immediately. Thanks once more you just saved my dog from pain and discomfort.

    • Hi,

      I am so glad that you found this article so helpful. Heartworm is something that can be easily missed early on, so it is great that you are going to check with the Vet just to be sure.

      Best of luck with your dog, and I hope that everything is going to be OK.  Thanks again for sharing your thoughts on my post and for sharing how helpful it is.



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