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Heartworm in dogs

When should I start heartworm prevention?

Heartworm prevention should begin at six weeks of age and must continue for the whole of the dogs life. Older dogs that are not on heartworm prevention must have a test before going onto heartworm prevention medication.

Canine Heartworm is a small long spaghetti-like worm that lives in the right chamber of the heart and in the major arteries to the lungs. In severe infections it can back up into the veins of the liver. It is transmitted by mosquitoes. It was brought over from America approximately 40 years ago and started in the Northern States. It has been in Melbourne about 15 years. At the moment there is a 5% chance of an unprotected dog becoming infected. The incidence is higher in the wetter Eastern suburbs (20%) while around the bay it is lower (2%). The incidence in the Northern suburbs averages out at a 5%. Indoor dogs have less risk than outdoor dogs.

The heartworm lifecycle

  1. Mosquito takes in microfilariae while sucking blood from an infected dog.

  2. Mosquito feeds on an uninfected dog and deposits the larvae back into the pet via its saliva.

  3. The larvae (called an L3 stage) moves through the dog's tissue and blood developing and growing. It goes through a series of moults until it becomes an L5 stage. It then travels to the heart where it goes through its final moult. These stages take approximately four months.

  4. The immature final stage develops into a sexually mature adult within another two months. Mature heartworm live several years.

Clinical signs of heartworm

Unfortunately there are no early signs. As the disease slowly progresses, the dog may show symptoms such as coughing, decreased appetite, tiredness, loss of endurance during exercise, weight-loss, a pot belly and, in advanced stages, liver and heart failure. The worms interfere with the ability of the heart to pump blood. Clots can form and shower the lungs. The body reacts to the worms causing further damage especially to the lungs as well as to the vessels. Heartworm can be half a metre long.
Dogs should be wormed against Roundworm, Hookworm, Whipworm and Tapeworm.

Prevention of heartworm

Please note, dogs over six months of age must be tested and found free of heartworm prior to commencing heartworm medications.

There are four groups of drugs available to prevent heartworm:

  1. Diethylcarbamazine citrate or DEC is found in all the daily tablets. It's cheaper than the monthly tablets but must be given every single day. Once the dog is over six months if two tablets are missed in a row the dog must be re-tested before another tablet can be given. DEC given to an infected dog can be fatal.

  2. There are three different types of monthly tablets. Heartgard, Interceptor (Sentinal is Interceptor with flea prevention added) and Proheart. Monthly tablets should be given on the same day each month. If a tablet is missed then the next month should be given. Your dog should be re-tested after six months.

  3. The third medication is Revolution or Advocate. These are spot-on medications which also control fleas, ear mites and sarcoptic mange. These medications are easy to use by applying to the skin once monthly. Advocate also controls most intestinal worms.

  4. Proheart is a once yearly injection by your vet to prevent heartworm. This is suitable for dogs over six months of age. This injection is ideal for owners who want the convenience of not having to remember tablets. This injection can be given at the same time as your dog's yearly vaccinations.

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