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| Heartworm Disease and Its Prevention |
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 Imagine a Steven Spielberg special - the victim writhes on the floor, breathless, gasping, coughing. Deep inside, relentlessly squirming in the pounding ventricles and auricles of the heart, a worm grows - bigger and bigger. A sudden, choking cough, a feeling of weakness, palpitations of the heart - will the victim survive this alien invasion?
It’s the stuff nightmares are made of. Sadly, it is fact not fiction. Heartworm disease is a silent killer of dogs and cats.
Initially, heartworm disease was a condition only found in the northern parts of Australia. However, the disease has been gradually squirming southward and now is seen in Melbourne, Adelaide, Western Australia and Alice Springs and is present throughout Queensland.
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What causes heartworm disease?
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Heartworms are spread by mosquitoes.
Once an infected mosquito bites a dog or cat, minute young worms that have been developing inside the mosquito, enter the pet’s body. Now inside the dog or cat, the juvenile worms travel through the muscles and tissues for a few months. Eventually, they enter the blood stream and migrate to the heart and lungs. Here they grow into adult worms that are 12 to 30 centimetres long. These worms eventually produce offspring called ‘microfilaria’ which are sucked up by a mosquito when it bites the infected dog. And so the cycle starts again.
The worms inside the heart and lungs cause very serious damage to the dog and cat. Being so large, they are a major barrier to the free passage of blood from the heart to the lungs. The infection slowly progresses. In the lungs the worms cause scarring and pneumonia and the heart dilates and becomes weak.
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What effect does heartworm disease have on a pet?
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In a dog, the disease initially causes a cough. This worsens, and the dog becomes inactive and lethargic. It will not be able to tolerate exercise without coughing and it becomes breathless. In advanced cases, fluid leaks from the blood vessels and accumulates in the lungs and 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 being 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 breathlessness and a cough should also have you concerned.
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How can heartworm disease be prevented?
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Heartworm disease is a scourge that all pet owners should want to prevent. Thankfully, this is easy.
There are several choices. A daily heartworm preventive has been available for many years. Based on the drug Diethylcarbamazine, it is available through veterinarians, pet shops and the supermarket and is very effective.
Monthly heartworm medications are very popular and are now used more commonly than daily medications. There are several brands, such as Heartgard and Proheart which are widley used and very safe products. Heartgard Plus has the advantage of also controlling roundworm and hookworm infections and is no more expensive than Heartgard on its own. Canimax is similar, but as well as preventing heartworm disease and controlling roundworms and hookworms, it also controls whipworms. The product Sentinel does all the above but in addition has the distinct advantage of controlling fleas. Its flea controlling ability is achieved with the inclusion of an insect growth regulator that prevents flea eggs from hatching.
Two new products, Revolution and Advocate are also available. These are quite different to the others. While a monthly preparation, they are a ‘spot on the back of the neck’ formulation. Revolution has a multiple action. It prevents heartworm infections in dogs and cats and also controls adult fleas in puppies and kittens and in adult dogs and cats. It prevents flea eggs from hatching, and apparently kills fleas and their larvae in the environment. In addition, it also controls ear mites in dogs and cats, sarcoptic mange in dogs and, in cats, controls roundworm and hookworm.
Advocate is a new multi-purpose product that combines imidacloprid (Advantage) with selamectin so it kills fleas, as well as prevents heartworm, hookworm and roundworm in dogs and cats, plus whipworm in dogs, and controls lice and ear mites in cats, and mange and lice in dogs with monthly application.
Lastly, there is now a once-a-year heartworm preventative available from your veterinarian that gives your dog 12 months protection from heartworm disease in one dose. The new Once-A-Year injection is administered by your vet and can be used in all dogs from 3 months of age. For convenience it can be given with your dog's annual vaccination.
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Heartworm disease in cats
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It is only in recent years that heartworm disease in cats has been recognised as being a significant problem. A new test is now available through your veterinary surgeon, which makes the diagnosis of feline heartworm disease a lot easier. However, the disease is difficult, if not impossible, to treat in cats, even if it is diagnosed. The majority of cats are not on heartworm preventive medication at this stage, but they should be. It’s a killer.
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When should I start Heartworm prevention?
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It is very important that you know with certainty that your dog is free from heartworm disease before starting on any heartworm preventive medication. 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 to, as a nasty side-effect can occur if a dog is already infected with heartworm when the preventative is given. While cats can be placed onto the monthly preventive medication without a test, it is still advisable to have the test performed.
As a routine, heartworm preventive medication is best started when a pup or kitten is eight weeks of age. At this age, the pet does not require a blood test and if it is kept on the medication for life, the “Steven Spielberg Horror” will never haunt you or your pet!
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Dr Cam Day BVSc BSc MACVSc is a veterinary surgeon, an animal behaviour consultant and media presenter. In 1995 he qualified as a Member of the Australian College of Veterinary Scientists in the discipline of Animal Behaviour and is one of only 15 veterinarians with this qualification in Australia. He works full time in animal behaviour management in Queensland, consulting with dog, cat and bird owners on a daily basis as well as appearing on air as Brisbane’s radio Pet Vet, and writing for various magazines.
Image kindly supplied by Bayer Australia Limited.
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