Heartworm Disease and Its Prevention

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Heartworm disease is a silent killer of dogs and cats. It’s 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.

This is one disease that can be easily and totally prevented.

Initially, heartworm disease was a condition mainly found in subtropical/tropical areas such as Darwin and Brisbane, and northern temperate areas like Sydney. However, the disease has been gradually squirming southward and is now widespread over much of Australia, including Melbourne, Adelaide and much of Western Australia. This can be explained by the movement of untreated dogs from endemic areas and suitable mosquito vectors becoming more resistant to the cold.

What causes heartworm disease?

Heartworms are spread by mosquitoes.

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.

What effect does heartworm disease have on a pet?

In a dog, the disease initially causes a cough which progressively becomes worse. The dog becomes inactive and lethargic due to the weakening of its heart. It will not be able to tolerate exercise without coughing. 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 deep, laboured breathing, extreme weakness and a blue appearance to the tongue, and very pale gums.

In cats, heartworm disease is well recognised as a problem. Serious disease can be caused with just one worm, whereas in dogs, one or two worms are usually well tolerated. Tragically, the most common sign of the disease in cats is sudden death, but if your cat is breathless or develops a cough, you should also be concerned.

How can heartworm disease be prevented?

Thankfully, preventing heartworm disease is easy and all dogs and cats should be on some form of preventive medication. There are several choices. Daily tablets are still used but the most common form of prevention these days are the monthly medications.

There are several brands available, such as Proheart, Revolution, Heartgard, Sentinel and Interceptor, Advocate, Milbemax and Panoramis. 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.

There is also a yearly option in the Proheart yearly heartworm prevention injection. This product represents an exciting breakthrough for modern science because of the unique way it works. The active ingredient, moxidectin, is enclosed in minute beads called microspheres. After injection, the microspheres slowly release moxidectin which then diffuses into fatty tissues. From there, the moxidectin kills the immature forms of the heartworm menace for a full twelve months.

The ideal time for your dog to receive the injection is at the time of its annual vaccination. The yearly heartworm prevention injection can be given to pups as early as three months of age. Due to the pup’s rapid growth it needs to be repeated at six months of age. If your dog is currently on a monthly or daily heartworm preventive, it can be easily switched onto this new injection. While it is not suitable for cats, the yearly prevention is useful for dog owners that have difficulty remembering to give their dog its monthly or daily heartworm preventive.

It is very important that you know with certainty that your dog is free from heartworm disease before starting on any heartworm preventive medication, including yearly heartworm prevention injections. Therefore, unless your veterinarian knows that your dog is free from heartworm disease, he or she may advise that your dog is tested for heartworm infection before the medication is sold to you.

For more advice on heartworm prevention, contact your veterinarian.

Contributors: Dr Julia Adams BVSc  
By Provet Resident Vet – Last updated 1 September 2016
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