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Written by Vet Evidence Based

Heartworm in Dogs – Symptoms, Treatment and Prevention

Margarita Boyd
Written by: Dr. Margarita Boyd, BVSc MRCVS
This disease transmitted by mosquito bites can be potentially fatal which is why a lot of dog owners wonder what are the best way to prevent it. The problem with it is that some factors that make this disease more likely are completely external and out of your control. However, there are things you can learn about heartworm that will make occurrence of this disease less likely.

Heartworm disease is also known as dirofilariasis, and it is a potentially fatal parasitic disease found in many countries worldwide. The adult heartworms live in the heart and nearby blood vessels of dogs infected with the disease and often cause the dog to have a soft cough, shortness of breath and weakness.

What is frustrating about this disease is that it can be a silent killer, as the disease can take years before the dog starts showing signs, with the worms slowly growing inside the dog’s heart. It sounds pretty frightening, right?

In this article, we will discuss all about the parasite that causes this serious disease and learn about the symptoms you need to watch out for in your own dog. Thankfully heartworm can be treated, but there are some major risks involved. We will also cover the diagnosis options, possible treatments and how heartworm can be prevented too.

What Is Heartworm Disease?

Heartworm disease is caused by the parasite Dirofilaria immitis. This parasite lives in the bloodstream and is spread by mosquitoes from dog to dog.

Adult heartworms tend to live in the dog’s heart or the large blood vessels nearby. The female worms produce thousands of tiny parasitic offspring known as microfilaria which are released into the bloodstream. The female worm is bigger than the male and can measure between 23-31cm(1). Some infected dogs may have tens to hundreds of worms in their heart!

So, you are probably wondering if your dog is at risk and how do dogs become infected? First, let’s take a look at the distribution of this disease.

Where Is Heartworm Disease Found?

Canine Heartworm is a global problem, but it is found mainly in tropical and temperate regions. The highest rates of infection in dogs is seen in the USA, South America, Japan, Australia, and Italy (2).

This disease is spread by the mosquito, therefore it is found commonly where mosquitoes are more widespread, such as near waterways, standing water, and coastline.

With more dogs traveling the world with their owners for vacations, work or military purposes, there is also a risk of bringing the disease back to areas which were previously thought to be free from the disease(3)! This is why it is important to ask your veterinarian before you travel, about any risks of heartworm and potential prevention measures to take if necessary.

How Do Dogs Get Infected?

The life cycle of the heartworm requires the mosquito, and there around 30 species of different mosquito that can transmit the infection. The mosquito is known as an intermediate host, as it is necessary for the lifecycle of the worm and to transfer the infection. Mosquitos bite animals and people, as they use the blood as food. They might as well be known as little vampires.

A mosquito bites a dog with heartworm and ingests some microfilaria (the tiny heartworm offspring). The microfilaria develops inside the mosquito for up to a month, to become larvae or young heartworms. Then when the mosquito bites another dog, the larvae are transferred into its bloodstream. The larvae move to the dog’s heart, where they stay until they mature into adult heartworms, which takes around 6 months(4).

Can It Spread Directly from One Dog to Another?

Often owners are worried if heartworm can spread from one dog to another through direct contact, sharing food or water bowls or through sneezing. The answer is no, the infection always needs the mosquito as an intermediate host, so it is not spread directly from one dog to another.

When Do Dogs Start Showing Clinical Signs?

It can actually take several years after infection before dogs start showing clinical signs of heartworm, as the parasites slowly grow inside the heart. By the time the clinical signs are seen, the worms are larger and the disease is often advanced. Therefore, your dog might have heartworm growing and you will not know! If concerned talk to your veterinarian about testing, which we will discuss a little later in the article.

What Are The Symptoms Of Heartworm Disease?

The signs vary depending on the size of the dog, and the location and the number of heartworms present. Small dogs, with smaller hearts and blood vessels, tend to have more serious signs. Also, the more worms the dog has, usually the clinical signs are worse. Some dogs may just have a few worms, and not show any signs at all. The clinical signs may be mild, moderate or severe.

Adult heartworms cause disease as they take up a lot of space in the heart, and block the major blood vessels nearby. The blood supply to the rest of the body can be affected as the heart has to work extra hard to pump blood and oxygen around the body. If there is reduced oxygen reaching the organs, they might start to fail too. The organs most commonly affected are the lungs, kidneys, and liver.

The signs seen with heartworm infection may include(5):

  • Exercise intolerance
  • Shortness of breath
  • Sort, dry cough
  • Pale gums (anaemia)
  • Weight loss
  • Liver and Kidney Disease
  • Heart Failure
  • Nose bleeds
  • Collapse

In chronic cases, congestive heart failure may develop, with further signs such as swelling of the abdomen, and limbs, decreased appetite, weight loss and increased breathing rate. Often the vet may hear a heart murmur or irregular heart rate when listening to the dog’s heart with a stethoscope.

How is Heartworm Diagnosed?

In the majority of the cases, heartworm can be diagnosed using simple blood tests(6).

These blood tests work by detecting the presence of heartworm proteins. Sometimes, the vet can run a test quickly in the clinic, at other times they may need to send them off to a laboratory.

Usually, the disease can only first be detected about 6 months after initial infection. Before this time, the blood tests may come back as negative, even if the dog is infected with heartworm. Therefore, dogs may need to be tested regularly if the disease is suspected.

Let’s take a look at the methods of diagnosis:

Antigen Test

These blood tests detect the presence of adult heartworms. It is the most accurate test.

Microfilaria Test

These detect the presence of the immature heartworm offspring. However, some dogs that have adult heartworms might not have microfilaria. These tests are often used to confirm a positive diagnosis.

Are Other Tests Required?

Yes, often other tests are required to check the overall health of the dog. These check if there is any organ damage and if the dog is healthy enough to undergo heartworm treatment.

Blood tests

Further blood tests are recommended to check for things such as anaemia (red blood cell levels), dehydration, signs of infection and inflammation (white blood cell levels) and liver or kidney damage.


X-ray images can check for changes to the heart and blood vessels, which are often enlarged when the dog has lots of heartworms. There may also be changes noted in the lungs.


This is an ultrasound examination of the heart, also known as an echo. It allows the veterinarian to look at the heart in detail, including checking the valves, chamber sizes and the contractions of the heart. Heartworms can be seen on the examination too.

What Is The Treatment For Heartworm?

The treatment can vary depending on the clinical signs of the dog and the number of heartworms present. There are many protocols available, with different drugs and schedules. However, to reduce the risks of side effects which occur when the heartworms die, a variety of drugs are often given.

The American Heartworm Society recommends the following treatments(7):

  • Doxycycline: This antibiotic is given to fight a common bacteria (Wolbachia) which the heartworm carries. When the heartworm starts to die, this bacteria can cause serious problems in the dog’s body if not treated.
  • Macrocyclic Lactone: This anti-parasitic treatment is used to kill the immature stages of the heartworm. It is often given for 2 months before administering melarsomine.
  • Melarsomine: An injectable drug which kills adult heartworm. Commonly three doses are given at monthly intervals.

Your veterinarian will be able to advise the best treatment protocol available for your dog and discuss any potential risks too.

Are There Any Risks Involved?

Yes, there are risks involved when treating dogs with heartworm infection. Thankfully, death due to treatment is rare but unfortunately can happen. The risk is higher when the dog has a lot of heartworms. Thankfully, the drugs used nowadays have lower risks and side effects, and more dogs are successfully treated.

As the worms die off and decompose inside the dog’s body they can trigger a big inflammatory reaction and infection, especially if they are carrying bacteria. The dead heartworm fragments can also lodge in tiny blood vessels around the body, but mainly in the lungs. Possible side effects of treatment include severe coughing, shortness of breath, reduced appetite, coughing up blood, fever or even collapse.

Complete rest is essential after treatment, as the dog’s body is in the process of eliminating the worm burden. The dog may also require anti-inflammatories, antibiotics, cage rest, intravenous fluids or even hospitalization in some cases.

Before discovering more about heartworms, check out this great, educational video about this disease. It contains some useful tips every dog owner should know.

Can Heartworm Be Prevented?

Heartworm infection can be prevented in dogs, by using a preventive drug protocol. Common medications include ivermectin, selamectin or milbemycin(8). These can be used in dogs free from infection or dogs that have been successfully treated, to prevent reinfection.

Preventative treatment is safe, but vets usually recommend that it is given year-round in areas that are a high risk of infection with mosquitoes. Usually, it is advised to be given monthly if your dog lives in a heartworm risk area and the treatment can often cover other internal intestinal worms and skin parasites.

There are a few different ways of reducing a dog’s risk of catching this disease:

  1. Monthly preventative heartworm medication
  2. Application of a canine mosquito repellent
  3. Environmental control of mosquitoes
  4. Reducing the dog’s outdoor exposure, during mosquito feeding periods

The drugs used to prevent heartworm are strong anti-parasitic medications. Some owners are concerned about giving their dog a monthly treatment using such strong drugs. In these cases, they might opt for a more natural approach such as taking steps to boost their dog’s immune system, use mosquito control measures, natural insect repellent sprays or homeopathic remedies.


Canine heartworm is a serious and sometimes fatal disease. The main problem is you might not even know your dog is infected until he starts showing signs years later. Heartworm is found in many countries worldwide and it needs the mosquito to fulfill its life cycle and transmit the infection from dog to dog. Diagnosis is pretty straightforward using blood tests, but often at least two tests will be carried out to confirm infection.

Many dogs that live in high-risk areas will be tested regularly, often every 6-12 months. If a dog has a lot of heartworms present, then there is much more risk involved during treatment, as the worms can cause a lot of problems as they die off.

Thankfully there are safe, affordable preventative medications which can be given monthly to prevent this disease. Talk to your veterinarian if you are concerned your dog might have heartworm or how to prevent it- catching or preventing this disease early is vital!