6 Facts You Need to Know About Heartworm


This article is written by Pet Circle veterinarian, Dr Carla Paszkowski

Heartworm: a parasite most people have heard of, but few truly understand. Sure, we all generally know it's something we should protect our pets against. ('That's the one my vet was rattling on about, that they give the needle for, right?') But do you actually know how heartworm is spread, the signs your pet might be infected, or how prevalent in Australia it actually is?

To help give you a quick run-down, I've collated 6 important facts that I think every pet parent should know.

1. Heartworm is spread by mosquitoes

That's right! Just like malaria, heartworm is spread by mosquitoes. When an infected mosquito bites your pet, it injects baby heartworm larvae (known as microfilaria) into the blood. In dogs, these microfilaria make their way to the heart, where they mature and grow. Interestingly, it's a little different in cats - their highly active feline immune system often attacks the microfilaria before they get a chance to mature into an adult worm (however this isn't always the case - see our section on cat heartworm below!).

This insect-borne method of spreading is the reason why heartworm is more prevalent in tropical and warmer environments: more mosquitoes equals more heartworm.

Unfortunately, this also means that indoor-only animals are still at risk of catching heartworm. While keeping your pet indoors can reduce the risk of other parasites, such as intestinal worms, ticks, and fleas, unfortunately it won't help keep them protected from heartworm, as pesky mosquitoes can easily find their way inside!

2. It's a lot more common than most people believe

In general, many people (even some vets) believe that thanks to advances in heartworm prevention in the past decade, heartworm is now fairly rare in non-tropical areas of Australia. However, a study completed in 2016 found that there's actually been a recent spike in cases in Queensland and New South Wales. This demonstrates how heartworm distribution is not easy to 'stamp out' - it is very variable and can fluctuate based on weather anomalies, mosquito populations, and pet owner compliance.

Another study from 2016 found that up to 72.7% of wild dingo populations in low density housing areas were positive for heartworm. Unfortunately this can pose a major risk to domestic dogs. Even if dingoes or wild dogs don't come into direct contact with your dog, they can still act as a source of infection due to that mosquito transmission we talked about earlier.

Interestingly, it can work the other way too: domestic dogs with proper heartworm prevention can actually help decrease heartworm in wild dogs in the same area. As the mosquito population feeds off protected dogs, they will become free from microfilaria, and therefore infect less wild dogs (so next time you give Bella her monthly prevention, you're not just treating her - you could be helping wild dogs in your area too!).

3. Cats are also at risk, but the disease is different to dogs

For a long time, it was believed that heartworm infection with cats is so rare and asymptomatic, that cat owners don't really need to worry about heartworm prevention.

However, advances in diagnostic tests and research suggest that there may be more heartworm-positive cats than once realised. This is partly due to heartworm disease being almost identical to feline asthma (and therefore often misdiagnosed), and partly due to how difficult it is to test in cats.

So, how does it happen in cats? While cats aren't the natural host of heartworm, they can still be infected by a mosquito which has recently fed off an infected dog. Interestingly, it doesn't work both ways - infected cats can't then act as a source of infection back to mosquitoes. (So feral cats can't be blamed for spreading heartworm to dogs!).

Once an immature worm larvae is in the cat's blood, the worm will try to find the heart. But because the worm is designed to navigate inside dog vessels, it can get 'lost' and end up in other places - such as the cat's lungs. The highly reactive feline immune system then usually destroys these immature worms, so they never reach maturity. But unfortunately, this immune reaction can cause major disease, particularly when it happens in the lungs.

Plus, because cats have smaller vessels than dogs, any dead worm or consequential clot in a cat's blood can cause disastrous results much more quickly.

So, in dogs, heartworm disease is caused by mature heartworms becoming so large and numerous that they clog up the heart and vessels. But in cats, the baby heartworms cause all the disease - or more specifically, the body's intense immune reaction does.

Therefore, typically heartworm disease is a heart disease in dogs, and a lung disease in cats. So much so, that it can appear identical (even on x-rays) to feline asthma, and often it goes misdiagnosed.

The other reason it is hard to diagnose heartworm in cats has to do with the heartworm test itself. In dogs, diagnosis is easy - a two-minute blood test at your local vet easily detects proteins called antigens, which are unique to mature heartworm infection. However, because baby heartworms are usually eliminated in cats (thanks to that reactive immune system!) and therefore no mature heartworms are left, the easy 'antigen' test that can be done in dogs usually won't work. To definitively diagnose an infection in cats, a combination of tests need to be done (including an expensive blood test that requires sending off to an external lab.)

What cat products prevent heartworm? Currently there are only 3 products available that prevent heartworm in cats: Advocate, Revolution, and Milbemax.

4. Treatment is often deadly

While prevention of heartworm is easy and inexpensive (see our options below!), the treatment of a dog with heartworm is difficult, extremely expensive, and often results in disastrous side effects, including death.

Treatment of a heartworm positive dog usually requires a course of injections of a drug called an 'adulticide' (which literally means 'adult worm killer'), and close monitoring and management of any resulting complications. Complications can occur after the worm has been killed, due to the dead worm's body either causing a blockage, problems breathing, or an anaphylactic reaction. Part of the treatment regime actually involves antihistamines and/or steroids to reduce the risk of this anaphylactic shock.

Some dogs even require a surgery to physically remove heavy numbers of worms from the chambers of their heart (gross!).

Yuck! It's just awful to read about, and even more awful to experience first-hand.

So what's the take home message? Keep up with your prevention!

Which brings me two our next point...

5. There are over 12 preventative products for dogs, and 3 for cats

With so many prevention options, there really is no excuse for failing to keeping up with your pet's heartworm prevention. In the 'olden' days, dog owners had to give a daily heartworm tablet - now, thankfully, there are much easier monthly or yearly options available.

Options for heartworm prevention include:

  • An annual heartworm injection at your vet (dogs only). This is great for peace of mind, and works well for people who prefer to do a quarterly worming and flea / tick regime, such as the Bravecto / Drontal combination. (Remember; you still have to give an intestinal wormer if you've had the heartworm injection).
  • A monthly heartworm + worming combination treatment. This is a great option for people who prefer not to get a yearly heartworm injection, and who aren't worried too much about fleas (or have alternative flea prevention on board). Monthly worm + heartworm combination products for dogs include Interceptor, Milbemax, and Heartguard Plus. For cats, the only monthly worm + heartworm product is Milbemax.
  • A monthly flea + heartworm + worming combination treatment. This is a great option for people who prefer not to get a yearly heartworm injection, and who want to combine everything(ish) together. For dogs, options include Comfortis Plus, Advocate, Revolution, or Sentinel. Nexgard Spectra and The Big 5 include extra tick protection too! For cats, your options are Advocate or Revolution.

Note: it is a common myth that there are 3 or 6 monthly tablets for heartworm. This doesn't exist due to the lifecycle of the heartworm; a monthly dose is required to 'clear up' any microfilaria that might have been received in the past month, before they start to mature.

6. If you forget to give heartworm prevention for a few months, you can seriously hurt your dog if you give it again.

If you forget to give heartworm for a few months, it's important not to simply dose your dog once you remember. This can be very dangerous.

Why is this? Remember when we talked about the difference between immature larvae and adult worms? And how killing an adult worm can cause serious complications such as clogged arteries or anaphylactic shock?

Basically, most forms of heartworm prevention drugs target and kill the immature larvae, but don't have an effect on hardier adult worms. However, there are some forms of heartworm prevention, such as milbemycin, that can kill adult heartworms. If microfilaria are left for a few months, they will grow and mature. If you then give an adult-killing preventative once an adult worm has 'set up shop' in your dog's heart, there is the potential for a blockage or a serious reaction if the worm dies.

Therefore, most veterinarians recommend a heartworm blood test if you have missed your treatment by 3 months or more. Always talk to your vet if you aren't sure.

Which Product Do You Need?

For a full guide to parasite prevention products on the Australian market and which one YOUR pet needs, see the following guides: Parasite Prevention Guide For DOGS and Parasite Prevention Guide for CATS.

A Comparison Table of all Australian Parasite Preventatives and their Coverage

See our product comparison below for an easy guide to flea, tick and worming treatments and their coverage.

Further Reading

Want to read more? Check out our other articles:

Which Flea or Worming Product Do I Need?

What is the BEST Heartworm Treatment for Dogs?

How to Get Rid of Fleas in Cats

Complete Guide to Cat Parasite Products in Australia

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