Macarthur Veterinary Group
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Heartworm Prevention in Dogs

The prevention of heartworm disease is one of the most important things that you must do for the health of your dog. Heartworm is the most serious of all the worms, and intestinal ‘allwormer’ tablets do not prevent heartworm infection. If you own a dog, then you must have him/her on regular heartworm prevention medication, from 6 weeks of age, for the rest of his/her life – no exceptions. Download PDF Download a PDF of this Article

All dogs are at risk of heartworm disease.

In Sydney, dogs can get infected with heartworm at any time of year, and all it takes is a simple mosquito bite. It makes absolutely no difference if your dog never leaves your yard. They’re just as likely to catch heartworm disease in your yard as anywhere else.
It’s even been shown that indoor dogs are just as likely to get heartworm as those that live outside.

Some facts about heartworm disease:

  • Heartworms are long, thin worms that live in the major arteries in the heart and lungs.
  • They are 17–30cm long! (that’s 6 – 12 inches).
  • Heartworm causes thickening and blockage of the arteries in the lungs, and can damage the major valves in the heart. Serious damage can occur before any signs of disease are noticed.
  • Heartworm breed in the heart and their tiny larvae circulate throughout the bloodstream, damaging a variety of organs including the kidneys.
  • One of the classic signs of heartworm disease is a mild cough, but the first signs can be as vague as weight loss, poor hair coat or reduced fitness.
  • Serious illness can occur without warning signs. Heartworm is a fatal disease if left untreated.
  • Heartworm infection can usually be confirmed by a blood test, but even this isn’t 100% reliable.
  • Heartworm infection is complicated, expensive and sometimes risky to treat.
  1. Larvae enter dog from infected mosquito.
  2. Larvae migrate through tissues, reaching the heart after approximately 4 months.
  3. Worms mature in the heart at 6 months, and reproduce. Microfilariae circulate in the bloodstream.
  4. Mosquito ingests blood infected with microfilariae.
  5. Microfilariae develop into infective larvae within the mosquito.


Prevention is definitely better than cure.


Heartworm really is a nasty disease. The good news is that heartworm infection is easy to prevent. All of the following products will kill the migrating larvae before they reach the heart. If your dog is on regular doses of any of these products, then they will be OK.

  • ProHeart SR-12 – once-a-year injection.
  • Revolution or Advocate – monthly liquid on the back of the neck. Also kill fleas.
  • Interceptor – monthly flavoured tablets that also control most intestinal worms.
  • Heartgard – monthly chewables or tablets.
  • Sentinel Spectrum – monthly flavoured tablets that also control fleas and intestinal worms.

Over the last few years, with so many good products, the choice has become a bit bewildering! We can help you cut through the confusion and will recommend a product that is right for you.

Note that we have not included the old daily heartworm tablets and syrups on this list. They are no longer thought to be a reliable or safe means of preventing heartworm infection. And remember that intestinal allwormer tablets do not prevent heartworm.

All of the other products are very safe and effective, providing you take the following precautions:

Heartworm Testing

  • Dogs greater than 6 months old must be blood-tested before starting on medication (to make sure that they are not already infected with heartworm). This also applies if they miss more than 6 months medication. (Heartworm tablets will not usually kill heartworm once they reach the heart, and giving tablets to a dog with adult heartworm may cause side-effects.)

    If treatment lapses for less than 6 months, it should be restarted ASAP, but a blood test will be needed 8 - 12 months later as described in the next point.
  • Dogs more than 8 weeks of age when they have their first treatment should also have a follow-up blood test 8 - 12 months later. This eliminates the possibility that your dog may have been incubating heartworm disease before being started on medication. (Heartworm tests are only able to detect heartworm after they mature in the heart).

If you have any further questions about heartworm disease, please give us a call.