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Heartworm Disease in Dogs

What is Heartworm Disease?

It is an infection by the nematode worm Dirofilaria immitis that is transmitted to dogs by a mosquito intermediate host.

What effect does it have on dogs?

The worm lives inside the right chamber of the heart as well as occasionally being found in the caudal Vena Cava and pulmonary arteries. It rarely causes problems in the heart itself, the main effect being on the pulmonary circulation and lung tissue around it, where severe inflammatory changes occur. The symptoms of Heartworm Disease will therefore mainly be associated with lung disease, such as coughing, shortness of breath and exercise intolerance. Sometimes chronic weight loss may be seen. The important point is the symptoms are gradual and chronic. Left untreated, the disease will eventually be fatal.

How long does it take to see symptoms after infection?

Generally symptoms will rarely be seen till 1 to 2 years after infection. So in other words, a healthy dog may harbour an infection for many months before disease symptoms surface.

How important is Heartworm in Perth?

The incidence in Perth is around 5% of dogs. While this has been held reasonably low because of diligent prevention in the dog population it is still considered an important disease.

Can Heartworm infections be treated?

Heartworm Disease can be treated especially if detected early in its course, before symptoms occur. The treatment is an Arsenical based injection, which is costly and may produce side effects. If a symptomatic case, treatment is more complex and costly and there is a greater risk to a dog's health. Blood tests, X-rays and medical pre-treatments will be required. In all cases post treatment Heartworm testing will be necessary. This is why prevention is used rather than relying on treatment.

How do we prevent Heartworm Disease?

Heartworm Disease can be prevented by any of the following:

1. Daily Tablets -eg Dirozine. These must be given every day or a window of opportunity for infection will occur. A severe reaction can occur if use on dogs harbouring a Heartworm infection so it is essential to test before starting dogs over 6 months of age.
2. Monthly tablets or chews - eg Heartguard. These are generally more convenient, safer and are more forgiving of lapses in treatment. We advise you stick to dosing each calender month but a lapse of a few days is not critical. A Heartworm blood test is required if more than two weeks late for a dose.
3. Combination topical applications - eg Revolution. A convenient way to effectively deal with fleas and heartworm with a monthly skin application.
4. Annual Heartworm injections - provide exactly the same protection as monthly medications but without the hassle of having to remember to dose every month. A reminder is sent to you when the injection is needed again.

How do we test for Heartworm Disease?

Heartworm disease is detected by a simple blood test that we can carry out in the consult room in a matter of minutes. It detects the presence of Heartworm antigen released into the bloodstream by adult worms. It cannot detect larval stages that are present in the first 6 months of an infection.

Why is a Heartworm blood test needed?

We advise a heartworm blood test on dogs in the following circumstances:

1. When a dog over 6 months of age is starting Heartworm prevention
2. When there has been a substantial gap in Heartworm prevention. This means that there has been more than a 24 hour gap in daily prevention, a 2 week gap in monthly prevention or a one week gap in yearly prevention.

What do I do if I am late with a Heartworm prevention dose?

This will depend on a number of factors determined by the fact that Heartworm cannot be detected till 6 months after infection has occurred.

1. If there has been a break in monthly or daily prevention within the last 6 months, you can restart prevention without testing. A blood test will be required 6 months after Heartworm prevention is restarted.
2. If a break in monthly Heartworm prevention occurred more than 6 months ago, we advise an initial Heartworm test followed by a second test 6 months later. This is so we are not waiting 6 months to find out your dog has an asymptomatic infection as prevention will only prevent further infection, not deal with a current infection.
3. If a break in daily heartworm prevention occurred more than 6 months ago, a heartworm test MUST be done prior to restarting medication to minimise the chance of a serious reaction.

If you have any queries please don’t hesitate to contact us for clarificarion.