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Heartworm Disease: Signs, Symptoms & Treatment

April is Heartworm Awareness Month around the world.

Heartworm (Dirofilaria immitis) disease is a worrying condition for dog owners: whilst many of us think it is much less common than decades ago, it is still a disease that kills pets.

Heartworm is one of those prevalent and preventable diseases.

Dr Jennifer Scott from Southern Cross Vet explains that “without prevention, your dog has roughly a 1 in 10 chance of being infected with heartworm, with infection leading to potentially fatal consequences. “

In Australia, it is recommended to use heartworm preventative all year round as it is impossible to reliably estimate a “mosquito season”. Dogs should start heartworm prevention by three months of age, but some products are safe to use on puppies from two weeks of age.

A survey of 1,600 Australian dog owners found that many were not compliant with disease protection with 64% admitting to administering monthly heartworm prevention AFTER it was due. [1]

Of most concern was the fact that, although 99% of dog owners acknowledged that heartworm could be fatal to dogs, 8% did not use any prevention and although 48% use monthly prevention, many were missing doses.

These findings support recent data from the Heartworm Surveillance Project, which found that 40% of recently-reported heartworm cases were in dogs on monthly heartworm tablets or spot-ons [2]. Vets are reminding us that it only takes one bite from an infected mosquito for a dog to catch this fatal disease.

How are dogs usually infected?

Heartworm is carried and spread by mosquitoes - just like Ross River and Dengue Fever affecting people. It is a slow, insidious disease that gradually incapacitates pets. By the time you notice the tell-tale signs of the disease, the damage that has been caused is serious.

Heartworm is a parasitic worm, transmitted to dogs and cats by mosquitoes.  

The entire lifecycle requires 6-7 months 
depending on environmental conditions
When a mosquito infected with heartworm larvae bites your dog the larvae is transferred into their system. It is here where they develop into adult heartworm and live in your dog’s heart and lungs.

Dogs get infected by being bitten by a mosquito carrying a life stage of the heartworm. It will require two dogs plus an infected mosquito to complete the disease transmission. 

After injection by a mosquito, adult worms eventually start to grow inside a pet's heart and lungs, causing very serious damage. 

Adult worms live within the heart and large blood vessels where they can grow to more than 30 cm in lengthBeing so large, they are a major barrier to the free passage of blood from the heart to the lungs. The infection slowly progresses. The heart dilates and becomes weak and in the lungs, the worms cause scarring and pneumonia.

If you want to see how large adult worms are, watch this (graphic) video here showing live worms during the autopsy of a dog that died of heartworm disease.

What are the signs of infection?

Heartworm disease damages the lining and valves of the heart, and with time this causes severe congestive heart failure and death when not diagnosed or treated.

The symptoms of heartworm include:

✔️ chronic coughing,
✔️ exercise intolerance,
✔️ reduced appetite / weight loss,
✔️ weakness / collapse and even
✔️ sudden death.

The severity of these signs depends on the number, size and location of the worms. 
If left untreated, a heartworm infected dog will progress to heart failure.
In severe cases, fluid leaks out of the blood vessels and accumulates in the lungs and the lower part of the abdomen. This fluid gives the dog's abdomen a 'pear-shaped' appearance, resembling the shape of a balloon filled with water.

Sometimes, the animal will suddenly collapse with no warning. It is associated with laboured breathing, extreme weakness, a blue appearance to the tongue, and very pale gums.

Which areas of Australia are most commonly affected?

Heartworm is more prevalent in areas with large numbers of mosquitoes and a warmer climate.

Environments where mosquitoes thrive usually share those common factors:

• High temperatures, and high humidity
• Storms, extreme rainfall and flooding events
• Urban expansion into previously rural areas
• Infected dogs (or wild dogs or foxes) acting as a reservoir of infection for other pets. Fox studies have shown that around 9% of foxes in the Sydney surrounds, and around 7% in the Melbourne surrounds carry heartworm, acting as a source of infection for city pets!

Unfortunately the increased infection rate in unprotected dogs has put everyone else’s pet at risk. Just as with unvaccinated children, the message is clear: if your dog is not properly protected, it is part of the problem!

How is Canine Heartworm diagnosed?

Early infection is the best time to diagnose the problem.

A diagnosis of heartworm is based on your dog’s current symptoms, a thorough physical examination performed by your veterinarian as well as blood tests.

If your dog is diagnosed with heartworm, your veterinarian may also recommend further diagnostics, including chest x-rays, an ultrasound and full blood tests. 

These results will assist in formulating a tailored treatment plan for your dog. It is important to note that although treatment options are available, the damage caused by heartworm can be irreversible, and treatment can be lengthy, expensive and include strict exercise restrictions for your dog.

How is Canine Heartworm treated?

It really depends on how severe the infection is but treatment usually involves a combination of the following:

✔️ a blood test (for at least two decades the most commonly used diagnostic tests have been antigen tests which detect small pieces of the female worm's body)
✔️ radiographs of the chest
✔️ a series of injections called Immiticide
® into the dog's muscles
✔️ medications to reduce reaction to the drug injections
medications to kill the microfilariae in the blood stream and reduce reactions to the heartworms dying.
✔️ medications to combat heart failure signs if indicated.
✔️ restriction of activity for 6-8 weeks minimum.

Treatment with 
melarsomine (brand name Immiticide®) in hospital is expensive and most people have not budgeted for it. However the disease gets worse with time and we encourage people to start treatment straight away.

If you're not in a position to draw on your savings to cover unexpected vet bills, maybe you need to consider a pet health insurance cover.

What are the dog's chances of survival?

The earlier the diagnosis the better. The chance of survival is excellent with a dog showing little or no clinical signs. The survival is good with dogs who are showing mild clinical signs and then decreases depending on how severe the cardiovascular system is affected beyond that.

Confining/keeping your dog quiet for 12 weeks or even up to two years (using a slow kill treatment) has been highlighted as one of the most important factors affecting outcomes in patients after treatment.

So what do I need to do know?

If your dog is NOT on heartworm prevention: get him tested now! Early infections have simple and cost effective treatment available. Even advanced disease is treatable. Don’t start prevention without knowing your dog’s heartworm status – not only does it carry serious risks, it also doesn’t fix the problem!

The good news is you can protect your dog from heartworm by stopping the heartworm larvae developing into adult worms. 
Prevention options include:

✔️ monthly tablets or chews,
✔️ monthly topical spot-on-treatments or
✔️ an annual injection.

The ideal time for your dog to receive the injection is at the time of their annual vaccination.

This heartworm prevention injection can be given to puppies as early as three months of age however due to the pup's rapid growth it will need to be repeated at six months of age.

"When considering the prevention options best for you and your dog, it is important to consider how reliably you can give your dog their medication." said Dr. Jennifer Scott. "An advantage of the annual injection, over other forms of prevention is that it provides a three month safety margin of protection for your dog. This means that if you are a one or two months late to organise the annual injection, your dog will still be protected."

Monthly heartworm medications are still very popular.

Heartworm Preventatives Comparison Chart

* This table (Credit: Budget Pet Care) gives simplified product - comparison information only. You should carefully read the individual product information to be sure the product is right for your pet.

Note: Beware of products advertised as an “Allwormer”. This term refers to treatment of intestinal worms only, and many products sold for intestinal worms do not prevent heartworm.

Speak to your local veterinarian to decide which heartworm prevention treatment will best suit your dog but don't be complacent and ensure your dog is protected!

To summarise, heartworm can be a serious and potentially fatal disease BUT easily avoidable.

Heartworm prevention is an affordable and easy way to protect your dog from this disease and ensure they live a happy, healthy, heartworm free life. Please contact your veterinarian to discuss which form of prevention will best suit you and your dog.

We’d like to thank Dr. Jennifer Scott from Southern Cross Vet for her contribution to this revised version of our article. 

Follow Southern Cross Vets on Facebook at 


1. PawClub, Australian Dog Owner Survey, Weber Shandwick, December 2015. 
2. Heartworm Surveillance Project. Available from:
3. For more information, visit

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