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Preventing Heartworm Disease in Dogs

Heartworm disease is a worrying condition for dog owners. While modern science has made this disease much less common that it was a decade ago, it is still a disease that kills pets. Heartworm prevention is the key but what are your options?

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

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.

Vets are emphasizing that it only takes one bite from an infected mosquito for a catch to catch this fatal disease.


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 telltale signs of the disease, the damage that has been caused is serious.

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. 

Being 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.

Heartworm Disease: Lifecycle


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.

Signs include coughing, weight loss, tiredness and reduced appetite, however dogs with early and minor infections show few or no signs. 

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. This occurs with no warning. It is associated with laboured breathing, extreme weakness, a blue appearance to the tongue, and very pale gums.


The disease is very prevalent in areas such as Mackay and North East Queensland and it affects up to 80% of totally unprotected dogs and 30% of dogs who have had irregular heartworm prevention. There is still further testing and data being correlated on the prevalence in other areas however what we do know is that it is spreading, and it is spreading fast. Environments where mosquitoes thrive are high risk, and 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.

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!


Early infection is the best time to diagnose the problem. A quick and simple blood test can detect infection with even a single heartworm. Testing allows us to identify your dog’s status, and determine the best course of treatment or maintain adequate prevention for your dog. 


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

  • a blood test
  • 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 microfilaria 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 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.


Depending on how advanced the disease is, treatment can vary from as little as $250 up to thousands if the dog is already showing severe clinical signs. On average,the treatment of a mildly affected dog could cost $1200-$2000 depending on its bodyweight. 

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.


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 the dog quiet has been highlighted as one of the most important factors affecting outcomes in patients after treatment.


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!

Thankfully, preventing heartworm disease is easy and all dogs (and cats) should be on some form of preventive medication and there are several choices.

Monthly heartworm medications are very popular. There are several brands available, such as Proheart, Revolution, Heartgard, Interceptor Spectrum, Advocate and RSPCA Heartwormer range. 
Many monthly preparations are available in a chewable treat form which makes them easy to administer, while Revolution and Advocate are available as a 'spot on the back of the neck' preparation. In addition, many of the monthly preparations also help to control intestinal worms.

The most recent advance in heartworm prevention is the once-a-year Heartworm Prevention injection.

The ideal time for your dog to receive the injection is at the time of its 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.

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!

1. PawClub, Australian Dog Owner Survey, December 2015

We'd like to thank Dr. Holly Goldring from Better Pet Vets for her assistance in writing this article.


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