As you enter the vet to get your pet up to date with their annual vaccination (which sometimes slips our mind till we get a text message reminder from our vet), you might have been asked whether you would like your pet to be protected against heartworm as well. As this question is brought up year after year, a thought might creep up and you might even think to yourself, so what actually is heartworm and is it such a serious issue for it to crop up every year?

Matters of the heart

As its name suggests, heartworms are worms that affect the heart. Heartworm is caused by a parasitic worm, Dirofilaria immitis and is transmitted by mosquito bites. When a mosquito feeds on an infected dog, the mosquito itself becomes infected. It spreads the worms to another dog it bites. It only takes one bite from an infected mosquito and the worms can cause lasting damage to the heart, lungs and blood vessels of its victim.

Majority of dogs affected by heartworm show no obvious symptoms until the disease is in its advanced stages. This could take a long time as it takes up to 6 months for the larvae to mature into adults and 2 or more years for symptoms to show. Heartworms can grow up to 12 to 30 centimetres long. Just the thought of having fully grown worms of that length in your heart is a horrifying image.

Severe heartworm infections result in dogs having a cough (usually exacerbated by exercise) and are exercise intolerant. The disease is fatal and severely infected dogs can die quite unexpectedly due to blockage of the blood vessels.

Let’s talk prevalence (how common it is)

Like much of the rest of the world, Australia is not spared from the existence of heartworms. Recent data is lacking but the prevalence of canine heartworm in Australia is assumed to be generally low. Prevalence is much higher in New South Wales, Victoria and particularly high in Queensland and Northern Territory. This is due to the conditions and climate in those states that favour breeding of mosquitoes. Other weather phenomenon such as floods and humidity also increase the rate of heartworm infections each year. In 2018, a spike in heartworm cases were reported in Queensland due to floods that occur in 2011 – which increased mosquito numbers.

Can my cat get it?

Yes, cats are also susceptible and can get infected if bitten by an infected mosquito. While heartworm infection in cats may be uncommon (compared to dogs), the consequences are rapid and serious. The clinical signs in cats are not definitive and often involve coughing and rapid breathing as the worms target the lungs. There are no approved drug treatments currently for cats, however as cats are not ideal hosts for heartworm some cats may spontaneously resolve on their own.

So….is a prevention necessary?

Although heartworm cases are rare and prevention against it is not a requirement, it is still highly recommended. Several reasons:

  • If your pet is not protected and was infected, repercussions are high as treatments are lengthy and expensive
  •  Heartworm is still present in Australia
  • If you are a person of worrywart nature, protecting your pet will provide you with a peace of mind. After all, it does take a prolonged time for your pet to develop symptoms. So there is no definite way to determine whether they are possibly ill
  • Development of herd immunity. You might have heard the concept of herd immunity arise during this time of COVID-19. While it is not applicable to this COVID-19 situation yet, it is for heartworm. If majority of the dogs in an area are protected, the spread of heartworm can be eventually stopped. This is because there are only a few susceptible dogs left to infect.

The most common prevention is through an annual injection with your vet. There are heartworm prevention products available in the form of tablets and topicals as well. Often, they are combined with other preventions such as flea and other worms. It is worth discussing with your vet for advice on the product to use, as the products differ in terms of application method and frequency. For example, some owners might find it hard to tablet a dog each month hence would opt for an annual injection instead. It should be noted that there are currently no heartworm injections available for cats – there are topical and oral products available for cats.

Answering the main question…

No, heartworm is not a big deal BUT it can be. Although the number of cases has been relatively low, treating heartworm can be tricky and burdensome for both you and your pet. The risk of your pet contracting heartworm does differ in relation to different states in Australia. If you are deliberating about whether you want to put your pet on heartworm prevention, you can discuss this further with your vet. At this stage, prevention is the best cure.