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What Is Heartworm?

Unlike the most confusingly named ‘ringworm,’ which is actually caused by a fungus and has nothing to do with worms whatsoever, heartworm disease is genuinely caused by a disgusting, wriggling worm, and it goes by the name of Dirofilaria immitis. Its life appears on the surface to be pretty complex, and is divided into so many stages they are simply known as L1. L2, L3, L4, L5, and adult.  However, the breakdown is fairly simple and I’m going to share it with you.  Why?

Because it’s important for making decisions about your dog’s health.

Don’t worry though, I’ll leave out all of the boring bits.  Or at least most of them.

Let’s make a deal okay.  If you make it all the way to the end, let me know what you thought on a scale of so-boring-I-wanted-to-die to hmmph-I-guess-it-was-mildly-interesting.  Feedback is good!

Anyway, knowing just a tiny bit about the fascinating trials and tribulations experienced by these unpleasant little parasites helps us immensely to understand some very important points regarding infection of our pets.  So I shall describe it for you briefly in a story I like to call –

A Heartworm Called Laverne

**Disclaimer: This tale is completely fictitious and is not based on any specific heartworm, living or dead.  Any resemblance to any worms you know is entirely coincidental.  Also, the terribly drawn cartoons come with no money-back guarantee if you find them a bit shit.**

  •  So Laverne’s mother, Gertrude, was out dancing one evening in the pulmonary artery of an unfortunate dog by the name of Butch.  Gertrude was all about that bass.  There were a lot of worms there that night, but one special worm caught Gertrude’s eye.  He was squirming enthusiastically to the beat among scores of others, but there was something about him.  She couldn’t put her finger on it, because she didn’t have fingers.  That night, Laverne was conceived.

heartworm lifecycle dog

  • When Laverne was born, she was just a tiny little microfilaria, also known as an L1.  Her mother wiped away a tear, kissed her goodbye, and pushed her out into the big, wide, world that was Butch the dog’s circulation.  Here she would spend quite some time, floating aimlessly around, waiting for a hungry mosquito to come visiting.  Laverne hoped desperately to be the chosen one, but she knew in her heart of hearts that the odds were stacked against her.  After all, there were so many just like her, all knowing that without the help of a bloodsucking mosquito they could never grow up, never date, never fall in love, and never get the opportunity to create the next generation of little heartworms.
  • Then one day, it happened!  She was quietly swimming along a capillary when SLURRRRRP, a greedy mosquito called Gary gobbled her up in its blood meal.  She excitedly waved goodbye to all her L1 friends and to Butch the unfortunate dog, and began the next exciting phase of her life.

heartworm lifecycle dog

  • Laverne lived inside the mosquito for a few weeks, as it buzzed from place to place, annoying anyone in it’s path.  During this time she matured into an L2, and then into an L3.

heartworm lifecycle dogs mosquito

  • This was it.  Laverne had come of age and it was her time to shine.  The time had come to do her mother proud and infect another dog.  The next dog Gary decided to take a blood meal from got more than just an itchy mozzie bite.  This dog, Bruce, got the beginnings of a case of heartworm disease.

heartworm lifecycle dogs

  • Laverne was ecstatic!  Who would have thought it! Laverne! In the skin of a dog!  Like all L3s, she took up residence in the skin for around three months, where she matured through stage L4 to L5 – she had finally become a young adult.  If only her mother could see her now.
  • It was show time.  She put on her game face and struck out into Bruce’s bloodstream.  She battled her way to his heart, and out into his pulmonary arteries.
  • Around 5-7 months after she was injected into Bruce’s skin by Gary the mosquito, Laverne met a delightfully slimy young buck called Ryan, and together they started a family…

heartworm lifecycle dogs mosquito

The Heartworm Life Cycle – Creature Clinic Style!

I’m sure there are probably people out there, somewhere, who prefer their nematode life cycles to be displayed in a more “accurate” or “scientific” manner.  I’ve always found these depictions difficult because I feel bored before I’ve even looked at them, but there is actually a really excellent one by the American Heartworm Society.

Why is the Heartworm Life Cycle Important?

It’s more than just an excuse for me to draw worms in love, that’s for sure!

  1. Microfilariae (L1s) CANNOT mature without living inside a mosquito. This means –
    • Puppies born with microfilariae (remember baby Laverne) from their mothers (passed through the placenta) will not develop heartworm disease.
    • However, they will test positive to heartworm on certain tests.
    • They can pass heartworm disease to other dogs if they are bitten by a mosquito.
    • Very importantly, dogs do not need to be in contact with other dogs to develop heartworm disease.  They only need to be bitten by a mosquito.
  2. It takes 5-7 months after a dog is infected before adult heartworms are present. This means –
    • There is no point whatsoever in the whole world doing a heartworm test on a puppy less than 5-7 months old.
    • Any infected dog that has been infected for less than 5-7 months will test negative.

What is the Risk of Heartworm?

There are a few requirements for heartworm to become established in a geographical area.

  • The right types of mosquitoes need to be present.
  • The weather has to be warm enough to allow heartworm larval development within the mosquito (usually temperate and tropical climates, but global warming is changing things).
  • There needs to be some infected animals around (dogs, foxes, coyotes…)
  • There must be vulnerable host dogs in the area. i.e. dogs that are not on any form of heartworm prevention.

When all these conditions are met, we have the perfect heartworm storm.  An interesting short video on heartworm prevalence can be found here.

Factors that support the spread of heartworm disease include dogs moving in and out of areas where it exists, lack of mosquito control, and presence of a ‘reservoir host’ such as foxes or coyotes.

The world map below shows in blue areas where heartworm can be found.

heartworm distribution world

Did You Know Heartworm is a Zoonosis?

A zoonotic disease is something that can be passed between animals and humans.  Some you may have heard of include ringworm, giardia, rabies, anthrax and Ebola, although there are many, many others.

Although not common, heartworm (Dirofilaria immitis) can cause disease in humans.


The Symptoms of Heartworm Infection

The majority of dogs infected with heartworm have no symptoms.  This is why heartworm testing is really important.  The severity of disease depends on a few things:

  • The number of adult worms living in the dog’s pulmonary arteries (this could be anywhere from one worm up to 250!)
  • The amount of exercise the dog does (more exercise = more severe disease as the heart can’t cope with the extra work)
  • The duration of infection (the longer a dog is infected, the more damage the worms cause)

Possible signs of heartworm disease include:

  • Weight loss
  • Tiring easily
  • Lethargy
  • Poor condition
  • Coughing
  • Difficulty breathing
  • A big round belly
  • Fainting
  • Death

Heartworm Prevention – So So Much Better Than a Cure!

Heartworm prevention is really easy.  All of the preventative options we have available to us act by killing the L3 and L4 larval stage in the skin (although some can also kill young L5s).  If you remember Laverne’s adventures, this occurs for around the first 2-3 months after a dog is bitten by an infected mosquito.  After this, the preventatives will not work.

Once adult worms are present, the only way to kill them is with an arsenic-containing drug called melarsomine.

The most effective way to avoid heartworm disease is to use a heartworm preventative every month (or here in Australia there is also a once-yearly injection).  Heartworm preventatives are available as tablets or spot-ons that are placed on the skin, and are often used in combined products that also cover intestinal worms +/- fleas.

There are many options available, and it’s a great idea to discuss them with your veterinarian and come to an agreement about what will work best for your pet.

puppy dog cute

Source: Shutterstock


An Little Bit About Heartworm Treatment

I hope your dog never needs this.

There is more than one way to go about it, but in general the safest method is as follows.

  1. Start with a monthly heartworm preventative for a few months to prevent further infection, reduced numbers of circulating microfilariae, and kill any larval stages that are too young to be susceptible to adulticide treatment.
  2. Use the heartworm adulticide melarsomine dihydrochloride.  Give one dose, then another one month later, then another 24 hours after that.  This drug contains arsenic.

There are a couple of problems with heartworm treatment.  While live heartworms cause progressive damage to arteries and potentially block up the heart causing heart failure, dead worms have nasty consequences too.  Dead worms can lodge in all sorts of places with the potential to cause fatal thromboembolism.  They can also stimulate a very strong immune response that causes damage in itself. Oh, and arsenic is toxic.

Strict cage rest during treatment is imperative to minimise the risks, and over a couple of months this can make treating heartworm very expensive.

If there are so many heartworms present they have filled the right side of the heart, the only possible treatment is surgical removal by passing forceps down the jugular vein and attempting to pluck them out.  As you can imagine, this carries immense risks.

It is actually possible for some of the preventatives to kill adult heartworms, but using them for this purpose is not recommended because it can take around two and a half years to do the job, during which time the worms are causing more and more internal damage.

The Take Home Message

Because we don’t see many cases of heartworm disease, it’s easy to become complacent and think heartworm prevention is expensive and unnecessary.

It’s not.

The fact is, heartworm disease exists, it’s deadly, and the treatment itself can be deadly too.  And it’s spread by mosquitoes so your dog does not have to be in contact with other dogs to become infected.

If your dog isn’t on heartworm prevention, please, discuss it with your vet today, and if you have any questions please leave a comment below;

I LOVE questions and I LOVE comments!

heartworm heart dog

This Post was Sponsored by Bayer

I’m sure it’s glaringly obvious, but the good people at Bayer cannot be held responsible for any of the nonsensical awesome content in this post, which came entirely from the space between my ears.  It’s also not their fault that I can’t colour inside the lines.

If you’re after some more information about parasite prevention for pets (as well as many other interesting topics!) please have a browse around their very helpful and informative website, Friends Furever.


Bowman D, Clarke E. Heartworm Biology, Treatment, and Control. Veterinary Clinics of North America. 2009;39:6:pp1127-1158
Ettinger S, Feldman E. Textbook of Veterinary Internal Medicine 7th Ed. 2010;1353-1380
Nelson R, Couto C. Small Animal Internal Medicine 5th Ed. 2014;173-184

Joanna Paul

Dr. Joanna Paul BVSc (hons) BSc

Jo is a practicing small animal veterinarian based in Melbourne, Australia. Working in partnership with loving pet owners to ensure their fur-kids remain happy, healthy family members life-long is what brings her joy. Well, that and taking naps. Jo strongly believes that helping to maintain the wonderful bond between a pet and their human is reason enough for a happy dance.

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Showing 25 comments
  • Rachel

    Ha, what an awesome post! I love the way you explained what can be a dry subject in such a novel and entertaining way. And I love the pictures too!

    • Joanna Paul

      Hey Rachel!
      Lol, it is definitely dry subject material, but nonetheless something I think is still worth knowing a little about 🙂 I’m so glad you liked it – You’ve made my day!

  • Eloise

    I love this Jo! I didn’t read it at first, since heartworm can be such a dull topic, but it really is brilliant, so well explained! I will share it far and wide…

    • Joanna Paul

      Thank you so much Eloise! It sure can be a dull topic, but it looks like you made it all the way to the end without falling asleep so I think my work here is done 🙂

  • Belinda Parsons

    Love it! Another great article Jo!

  • Hugzilla

    LOL those pictures are epic! You need your own book deal. Thanks for reminding me I need to do this again with my two dogs.

    • Joanna Paul

      Haha, I’m blushing!!
      I’m so glad after that unhelpful feline diabetes post I was able to provide something relevant, haha. Unless you’ve since purchased a cat, in which case you will be totally onto it if they ever show signs of diabetes!

  • Cindy Jones

    Hi Joanna, great article :)!! Thumbs up for your drawings too :). I have always been lead to believe that heartworm isn’t in Tassie, can you tell me if that’s right or not? I love the way you get your message across so simply but with all the info we need..thank you

    • Joanna Paul

      Hi Cindy,
      Thank you so much for bringing the Tasmania issue up! I have always been under the impression heartworm disease (and foxes who are a reservoir) were a mainland problem. I recently saw some data from the Heartworm Surveillance Project that I think suggested cases have been seen in Tassie, but I will have to go back and double check! Watch this space 🙂

  • Rosi

    Dr.My daughter love hug my beautiful dog Do you think is good give to her cap Our treat her to prevent not have worm.In Australia what’s is good to give to children to prevent from worm and what’s good to give to my puppy dog please tell me I meed look after my dog my family .Thank you is very important our undeswoold what’s important look after the dog

    • Joanna Paul

      Hi Rosi,
      It’s beautiful that your daughter and your dog have such a lovely relationship. You should be able to get something appropriate for your puppy from your local vet – they will be able to talk to you about the best option. If you are concerned about your daughter I would suggest speaking to a pharmacist. They will stock worming treatments for humans and should be able to tell you if it is necessary or not. I hope that helps 🙂

  • Kristy | The Pug Diary

    What a great article. I have always given Ref the full worming treatment that our vet recommends which includes heart worm. I knew it was deadly but never really knew much more than that. Thank you for writing this. It is so informative. And of course, I love your way of explaining it all. The little cartoons are awesome.

  • Kerry

    Great article Dr Jo! Love the hand drawn illustrations and story for explaining. Highly educational too – I didn’t realise the treatment was arsenic. Thats scary for dogs.

    • Joanna Paul

      It is scary, isn’t it! I’m glad you gleaned some info from my ramblings 😉

  • Vicki M

    That was a great post! I know I shouldn’t laugh and I did out loud a number of times (and I do feel terribly sad for Butch and Bruce, I do) but you have done a super job explaining the whole heart worm life cycle and I can say quite honestly, I knew what heart worms were but had no real idea of how they developed and were passed on. Thank you for the demystification … and the super cute drawings!

    • Joanna Paul

      Thanks Vicki! I don’t think we totally entirely need to know the life cycle in detail, I just got swept away with the romance that was Laverne’s life! I do think it’s important for people to understand the need for Gary or one of his mates though, because I often see people who say, ‘no, we don’t need heartworm prevention, our dog doesn’t play with other dogs.’
      And I love that you laughed 🙂

  • Rachel Sheppard

    I love the illustrations! This is a great post!

    • Joanna Paul

      Thank you so much Rachel 🙂 I really appreciate you coming by.

  • Amy

    LOVED your cartoons! Explained it so perfectly and in a less confusing way.

    Favourite part was the Ryan Gosling worm who was all like ‘Hey Girl!’. No, but seriously the idea of my dogs getting heartworm is terrifying and it is one of the take home things that I remember after taking Chowski to his early vet appointments as a puppy.

    It has always been part of my parasite protection for both dogs and I’d never dream of leaving it out because it’s expensive. 🙂 Great post. Is that last photo spaghetti inside a heart. Heurgh! :p

    • Joanna Paul

      Thanks Amy!
      The last pic is a fake heart with fake heartworms in it, haha. Does look suspiciously like spaghetti though!

    • Spark

      Hello Doctor,
      A Nice and Perfect explanation about heart worms.
      Any vaccination is there for heart worms?

      • Joanna Paul

        Hello Spark,
        Thank you! While there is no vaccine for heartworms, in some areas there is a one yearly injection available that protects dogs for the full year. Here it is called SR-12, and must be given by a veterinarian.

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