What you should know about heartworm


Animal Welfare League Queensland recommends preventive treatment to guard against disease.

Cats and dogs are both at risk of getting heartworm.

It is extremely important that heartworm preventives be administered strictly on schedule.

Heartworm disease, which can occur in both cats and dogs, is caused by a parasitic worm that lives in the heart, lungs and associated blood vessels, causing severe lung disease, heart failure and damage to other organs in cats and dogs.

Dogs are a host of heartworms, this means that they can grow and reproduce in the animal, while cats are a dead-end host, meaning the worms cannot reproduce in the animal.

Most cats are infected with only a few worms and they may not be fully mature, though the disease can be just as deadly for cats as it is for dogs.

How is heartworm spread to cats and dogs?

Heartworm disease is spread by mosquitoes – heartworm larvae are found in the bloodstream of infected dogs.

Mosquitoes pick up the larvae of the heartworms when feeding and then transfer them to other animals. The larvae then grow and mature into adult worms.

What types of heartworm prevention are available?

It is important to consult your veterinarian before starting any preventative treatments.

Several types of prevention are available for both dogs and cats, including monthly oral preventive, this works by killing the immature worms.

It is extremely important that heartworm preventives be administered strictly on schedule as unfortunately in as little as 51 days heartworm larvae can mature to the point where prevention is no longer effective. Those worms then mature into adults that cause heartworm disease.

What are the symptoms of heartworm disease in pets?

The most common signs in dogs are coughing, tiring easily, collapse or fainting episodes, decreased appetite and weight loss.

Many dogs, however, show no signs at all early on.

Later in the infection process, heart failure can occur, and this is often displayed as a distended abdomen full of fluid.

If a dog has large numbers of worms late in the disease, it can cause complete blockage of main arteries which is fatal unless emergency surgery is performed to physically remove the worms.

Heartworm disease in cats is less obvious, with one common sign being a cough.

What’s involved in testing for heartworm disease?

Early detection is key to treating heartworm disease successfully. Because many dogs show no signs during early infection, a blood test is required to indicate whether an animal is infected.

All dogs should be tested annually for heartworm infection, and this can usually be done during a routine visit.

Because most cats are infected with only a few worms and they may not be fully mature, heartworm infection in cats is harder to detect than in dogs.

The preferred method for testing is an antigen and an antibody test and your veterinarian may also use X-rays or ultrasound to look for heartworm infection.

How is heartworm disease treated and with what medications?

If your dog tests positive for heartworm disease, there are several steps to treatment:

  • Your dog’s normal physical activities must be restricted as soon as the diagnosis is confirmed because physical exertion increases the rate at which the heartworms cause damage to the heart and lungs.
  • Before heartworm treatment can begin, your dog’s condition may need to be stabilised with appropriate therapy. This is more common if your dog has pre-existing conditions.
  • Once your veterinarian has determined your dog is stable and ready for heartworm treatment, they will recommend a treatment plan involving several steps.
  •  After treatment is completed, your veterinarian will perform a heartworm test to confirm that all heartworms have been eliminated.

There is no treatment to eliminate heartworms in cats, so prevention is extremely important for felines.

The only option for cats is to provide treatment for the symptoms they are showing.

What’s the prognosis for heartworm disease?

With treatment, dogs who have mild or no clinical signs of the disease have a very good prognosis. Even dogs with severe disease or heart failure do well in most cases.

The main factor in how dogs fare during treatment is whether their activity is adequately restricted.

If heartworm disease is untreated, the dog will continue to experience damage to his heart and lungs, and ultimately it will be fatal.

- Animal Welfare League Queensland

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The information in this article has been prepared for general information purposes only and is not intended as legal advice or specific advice to any particular person. Any advice contained in the document is general advice, not intended as legal advice or professional advice and does not take into account any person’s particular circumstances. Before acting on anything based on this advice you should consider its appropriateness to you, having regard to your objectives and needs.