Dog Health & SafetyDogs in General

Heartworm symptoms in dogs. What is a worm burden?

Heartworm Maturing Inside a Blood Vessel

Warning: this article does contain some graphic pictures to help explain the disease and its symptoms.

What is heartworm?

Heartworm disease is a terrible affliction for dogs, and is basically as it sounds “worms in the heart”. Although worms can also travel to the lungs and major blood vessels – i.e. anywhere that is blood-rich and close to the heart. They are parasites that feed on the blood and the disease can spread to other organs such as the liver and kidneys. If it’s left untreated, it is usually lethal either through heart failure or long term damage to other body organs.

It’s incredible to think these worms can grow up to a foot in length and live as long as 7 years, but this does go some way to marking this disease as a really bad one for dogs. If there are a lot of worms living in the dog, then they are described as a “worm burden”. An average number in a burden is about 15 worms, but there have been recorded cases of 250!

Worm Burden Inside a Dog's Heart
By Alan R Walker (Own work) [CC BY-SA 3.0 (], via Wikimedia Commons
So let’s take a closer look at what the disease means for dogs and what the symptoms of heartworm are.

How do dogs get heartworm?

Mosquitoes carry heartworm diseaseFirstly, it’s important to note that disease is not contagious between dogs and other animals. It takes a pesky little insect to carry and transmit the disease from one dog to another – the Mosquito (They have a lot to answer for!)

As a result, it’s an obvious correlation that wherever there is a high concentration of mosquitoes the risk of heartworm disease is greater. In the USA, the disease is most prevalent along the coasts (Atlantic and Gulf), plus all along the Mississippi River and it’s tributaries. All the sorts of places that mosquitoes like, where it’s warm with a high concentration of water. However, you shouldn’t be complacent if you don’t live in these areas, because the disease has been reported in all 50 States as well as other places around the world.

Heartworm Maturing Inside a Blood Vessel
By Joelmills (en wikipedia) [GFDL (, CC-BY-SA-3.0
Unfortunately though, it’s in dogs that the worms grow and mature into adults. It’s here that the adults then mate and produce offspring – let’s call them “mini-worms”., although they are a very small form of larvae at this stage. These then get sucked up by the next mosquito that bites the infected dog, and then passed onto a new canine host.

The larvae live inside the mosquito for some time, and it takes 10-14 days plus the right conditions for them to mature to a point where they are infectious.

Once passed onto a new dog host, the time span for the worms to grow into the adult form takes much, much longer – 6-7 months in fact!

So all in all, the mosquitoes are none the worse for the ordeal, but the dogs suffer experiencing some truly horrible symptoms.

What are the symptoms?

It’s a fairly basic rule, but a true one – how quickly and severely the disease manifests itself in a dog will depend on the number of adult worms present (or the worm burden, as mentioned earlier). Also, as the disease progresses in a dog the symptoms will become more apparent – i.e. the longer they have the condition, the more they will be affected by it.

Some dogs are likely to show symptoms earlier than others, particularly if they are very active dogs. In this case the heart muscle is being used more often and at a higher load, so if heart worms are present the whole circulatory system can become rapidly distressed.

Many vets will use a 4-stage classification system relating to the severity of symptoms. For example:

Stage One: The dog may exhibit hardly any symptoms, or sometimes none at all. This would be the case if a dog has only recently been infected or if the worm burden is very low. If there are symptoms at this stage, it’s probably only an infrequent coughing. (Dogs can cough for a number of reasons, but any cough should be checked out by a vet to rule out any serious illnesses)

Stage Two: Any coughing is still likely to be relatively infrequent, but at this stage a dog might appear more tired than usual after a medium level of exercise.

Note: At stage two and three, the effects on the heart and lungs can often be seen clearly on chest x-rays.

Stage Three: There will be noticeable effects at this stage, when both breathing difficulties and heart failure can occur.

  • Heartworm could mean emergency surgeryan overall lack of condition in a dog. It’s hardly surprising, because the parasitic worms are feeding on the dog’s blood and therefore taking away valuable nutrients and oxygen from the dog. The greater the worm burden, the more strain the dog’s body will be under.
  • the dog will be coughing frequently.
  • there will be obvious tiredness, even with the shortest or mildest levels of exercise.

Stage Four: At this point there is likely to be a huge worm burden in the dog, to the point that the worms can actually clump together and cause blockages in the blood vessels. This is sometimes known as “Caval Syndrome” and urgent surgical intervention is the only option left to try and save a dog. Unfortunately the surgery is very often unsuccessful, resulting in the death of the dog.

What are the treatment options?

A typical way to determine whether a dog has heartworm disease or not, is through a specific blood test carried out by a vet. A positive test finds specific types of protein in the dogs blood, called antigens, that are only released by female worms. However, there is a catch which makes it slightly more difficult – the adult female has to be at least 7 or 8 months old. If younger than this the test becomes less accurate and it’s almost impossible to detect the disease if the worm is less than 5 months old.

Blood tests for dogs, including heartworm
By Airman 1st Class William Johnson ( [Public domain], via Wikimedia Commons
There is a drug used to treat the disease, but’s it’s not very pleasant for the dog. To be effective it has to be injected very deeply into the back muscles, where it can travel through the blood system back towards the heart and the worms. It does contain arsenic, so it’s not exactly a “good for your dog” injection either.

There is also another drug which can be used to kill the adult offspring or larvae, while they are still travelling around the dog’s circulatory system.

In short, the treatment is unpleasant for the dog and the associated costs for the owner are likely to be very steep. The situation will involve multiple visits to the vet and include blood tests, admission to hospital, a series of toxic injections along with multiple x-rays. All of this is very tough on a dog.

Why prevention is better than cure

Firstly, I don’t think any dog owner would want their dog to go through the treatment regime above, so it’s far better to consider how to prevent your dog getting the disease in the first place.

There are many more options for prevention. However, these are not available over the counter and need to be prescribed by a vet, who will know the best option for each individual dog.

However, it’s really important to note that a dog should be tested for the disease BEFORE prevention medication is used. So particular care should be taken if you are moving and changing your vet. This is super important if you live in a mosquito-laden area, as your dog could be infected but in the early stages, which means to your eye they might appear completely normal, but they could already have the disease.

Heartworm can killThe testing can be done as soon as your dog is older than 6 months. The reason it’s so important is preventive medication WILL NOT KILL ADULT HEART WORMS. Also, if there are already larvae present some of the preventive medications can kill them so suddenly that they cause your dog to go into shock, which is occasionally fatal.


Because of these factors, it’s recommended that all dogs who are taking preventive medication are tested for the disease on an annual basis, just to be sure nothing sinister slips through the treatment net.

I’m not going to say I hope you enjoyed this article, as it’s not that type of information. However, I do hope it’s been educational and helps dog owners who either live in disease-prone areas or who may have concerns about symptoms their dog is showing.

If you do have any comments or would like to share any information of your own, please do so below. Anything we can do to increase awareness of this disease is a good thing.



6 thoughts on “Heartworm symptoms in dogs. What is a worm burden?

  1. Hi, Mara,

    Wow – I wonder how many dog owners actually realize how serious a condition that this can be?… Thanks so much for giving us the low-down about heartworms!

    I’ve got a 17 year old, 20 lb dog that has a coughing condition, which I’ve heard is typical of small dogs as they get on the old side. The vet has previously, a few years ago, figured it was due to a build-up of fatty tissue around the heart and lungs, putting pressure on those organs. I was wondering if their was any differentiation symptoms that I could recognize? Or is this something that I should only rely on a vet to figure out?

    1. Hi Mike,
      What a ripe age your dog is! It’s really hard to give you any differentiation between different types of coughing, because just as illnesses manifest differently in humans, the same occurs for dogs. Severity of heartworm also depends on how long a dog has been infected. The fact your dog has been living with this cough for several years would suggest that it’s not heartworm. Did your vet test for the condition at the time? I have attached a link to a video of a dog coughing that is heartworm positive – it is a bit distressing to watch, but it would give you an idea. My advice would always be if you have concerns about any condition, see a vet, and if you are still concerned go back again or even get a second opinion.
      I hope that has given you some reassurance, but if not, please do go back to your vet. Mara

  2. Wow, these worms are really terrible. And the images are also… quite frightening haha. I know you’ve added them just to explain, don’t worry 🙂
    I would have never believed a mosquito could transmit this disease, so this means it’s a bit harder to prevent.
    But I’m really glad you offered some treatment options, they’re a great info for people who don’t know too much about this, like me.
    Thanks a lot for this great tips 🙂

    1. Hi Ashley,

      Thanks for reading – I know the images are a bit off-putting, so I’m glad you didn’t make me feel bad about it! It is a terrible disease, so the more awareness I can help create the better it will be for dog owners.

      Hope to see you on here again,


  3. Very interesting article. I have two dogs and they both get the heartworm shot to keep them safe. I know some pet owners who really don’t do any of this, or keep up on any shots for that matter. These people “claim” to love their dogs. Loving them also means taking care of them. Heartworm is a serious thing and, as you said can kill them. I have one question, if a dog is an outdoor dog, are they more susceptible to heartworm? I keep pestering my friend to get the heartworm shot for her dog and I don’t think she realizes how deadly it can be.

    1. Hi and thank you for your interesting question. The answer is a huge YES – dog’s who are outdoors are much more susceptible, as by default they are more exposed to mosquitoes. It is sad to hear you know of some owners who aren’t taking the right precautions – I think in the long run if they lost their dog to the disease they would really regret it.

      Thanks for sharing your feedback – it’s greatly appreciated 🙂

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