Heartworm Disease (Dirofilaria immitus)

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What is it?

Heartworm disease occurs when a parasitic worm grows and lives in the bloodstream of the heart and lungs of dogs and cats. Pets can become infected after being bitten by mosquitos carrying the immature heartworm.

What causes it?

Heartworm disease can be an insidious disease because the onset of symptoms may be slow – months or sometimes years. Heartworm is spread between pets by mosquitoes. If a mosquito bites a dog with heartworm (or also a wild dog like a dingo), by feeding on the animals blood the mosquito will then carry the immature heartworm (“microfilariae”). Once these host mosquitoes then bite another dog or cat, they infect the pet with the heartworm larvae. These larvae will migrate to the pets heart and lungs, and sometimes other organs, where the adult worms grow and reproduce.

The presence of these heartworms in the pet’s bloodstream, causes blood clots, inflammation of lung tissue and blood leakage through vessel walls. The severity of the disease will vary between individuals depending on how their immune system reacts, and how many worms are present in the bloodstream. The smaller body size of cats means that if they are affected by heartworm disease the prognosis can be poorer, as the risk of blood clots and immune reactions can be greater than in dogs.

What animals are at risk?

Both dogs and cats can suffer from heartworm disease. The prevalence of heartworm disease is higher in areas where more mosquitoes are present - as infected mosquitoes spread the disease by biting pets. Pets can be at risk wherever mosquitoes are present, both outdoors and indoors. Mosquitos can occur in most geographical areas of Australia, with an increased prevalence in tropical areas and in areas associated with still or stagnant water, such as lakes, swamps and waterways.

Heartworm protection is readily available through the use of regular heartworm prevention treatments for pets. Pets who are not being given regular heartworm prevention medications, or who lapse in their coverage, are most at risk of developing heartworm disease.

What are the symptoms?

Depending on the size and age of the pet, it can often take several months or years for clinical signs of heartworm disease to become obviously apparent.

Symptoms of heartworm disease include:

  • Weakness and disinterest in exercise
  • Exercise intolerance
  • Dry and persistent cough
  • Losing condition and a poor coat
  • Symptoms in affected cats can be less obvious, meaning the infection may be far more advanced before it is detected.

    What should I do?

    If you notice your dog become less exercise tolerant or it develops a dry cough, seeking veterinary assessment is worthwhile.

    How will the vet diagnose it?

    The presence of heartworm disease can be easily confirmed in dogs with a blood test. The disease in cats can be more difficult to diagnose, however your vet can provide the best advice on what options may be possible.

    How can it be treated?

    If your pet is diagnosed with heartworm disease, the treatment will vary depending on the severity and extent of clinical symptoms. Treatment of heartworm disease involves use of a drug which kills the adult worms, often together with other medication to treat the larval stages of the heartworm which may be present in the dog’s circulation. Treatment is not without risk, as blood clots and blood-vessel blockages can develop as the parasites die, which can be fatal to the pet.

    Treatment in cats can also be difficult depending on the extent of the disease. A lot depends on the individual circumstances of the pet and their condition, so your veterinarian will be best placed to advise you on the best treatment and management options. This highlights why heartworm prevention is so important.

    Can it be prevented?

    Heartworm disease is easily preventable in dogs and cats through the use of a regular heartworm prevention medication. Depending on individual circumstances, your vet can provide further advice around what might be the best forms of prevention in your area. Preventative medication options include monthly tablets, chewable tablets, spot-on liquids and a yearly injection.

    Depending on the age of your dog, your vet may recommend a heartworm blood test prior to starting heartworm prevention, to ensure that there is no evidence of heartworm infection – otherwise side effects may occur, including difficulty breathing, collapse, and even anaphylaxis.