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Heartworm (Dirofilaria immitis) is a somewhat controversial disease in Melbourne.

Should I prevent it or not worry about it?

It is a complicated disease that needs to be prevented because it is extremely hard to cure once infected. We always encourage our clients to let us know whether they want to prevent this disease or not. Here we explain how the disease works and the pros and cons of prevention:

What is heartworm?

Heartworm is a parasite which lives in the heart. It is transferred from dog to dog by mosquitoes. Once infected, after one month the worm actually changes to a stage that the treatments no longer kill, hence there is only about one month’s time frame in which we can kill new infections. For proper heartworm prevention we need to use a monthly treatment, and never miss! This is the big difference between heartworm and normal intestinal worms. Normal intestinal worms will be killed by intestinal worm tablets no matter how old the worms are. It doesn’t really matter if regular intestinal worm treatments are missed. It does matter however if routine heartworm tablets are missed. Fortunately, there is a yearly prevention that can be given in the form of an injection. This injection lasts one year therefore all new infections are killed.

What does heartworm do to my dog?

The young worms introduced via mosquitoes circulate in the blood stream until they become adults at six months of age, they then lodge in the heart and lungs. It usually takes about two years to actually start showing signs of heart disease but at this stage it’s very difficult to cure. Left untreated heartworm is often fatal.

Do I need to prevent heartworm in Melbourne?

Many clients have told our veterinarians that they’ve had dogs all their lives and never used heartworm prevention and that their dogs have never suffered from heartworm. In Melbourne we expect this to be the case because it is a rarity here. Only a few dogs get infected each year in Melbourne whereas in Queensland and the Northern Territory, heartworm is a very prevalent disease and one would be unwise not to be on prevention in these areas.

In Melbourne, heartworm prevention is a slightly tricky one for our ‘risk-reward’ ratio of medicating our pets. The chance of getting it is low but very nasty for the few that do. As an example, one of our veterinarians treated their own dog until approximately 11 years of age and then stopped. The rationalisation in doing so in this case was due to the breed of the dog being a Burnese Mountain dog. This particular breed are not generally long lived dogs.

As a generalisation we recommend treating young and middle aged dogs but suggest to our clients to consider stopping prevention in their very latter years. Ultimately the choice is yours as each animal and each case is different!

The take home message on heartworm is:

  • In Melbourne, this is a low risk disease
  • If infected and not treated, it is usually fatal
  • For owners deciding they want prevention, diligence is required – whether using monthly or yearly treatments