Heartworm Facts


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

Heartworm: a parasite most people have heard of, but few truly understand. Do you actually know how heartworm is spread, the signs your pet might be infected and how to prevent it?

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

1. How do dogs get heartworm?
2. How common is heartworm?
3. Are cats at risk of heartworm?
4. What are the symptoms of heartworm in dogs?
5. Treatment for heartworm
6. Prevention for heartworm
7. What if I miss a dose of heartworm prevention?

How do dogs get heartworm?

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, and it 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!

How common is heartworm in dogs?

Many people believe that thanks to advances in heartworm prevention in the past decade, heartworm is rarely found in Australia. However, vet clinics across South East Queensland have recently reported a spike in heartworm cases in dogs. A study completed in 2016 also found an increase in cases in Queensland and New South Wales. This demonstrates how heartworm distribution is not easy to completely 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.

Can cats get heartworm?

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. Recently, 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.

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. 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 attachs these immature worms, while the worm won't reach maturity, the immune reaction itself can cause significant damage to the surrounding tissue, especially the lungs.

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.

Symptoms of heartworm in dogs

Common presenting signs of heartworm infection in dogs include:
  • Coughing
  • Lethargy
  • Loss of appetite
  • Weakness
  • Fainting
  • Distended abdomen

Treatment for heartworm infection in dogs

While prevention of heartworm is easy and inexpensive, 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. The moral of the story? Prevention is always better than cure

Prevention of heartworm disease

With so many prevention options, it's easier than ever to protect your pet from heartworm. Prevention is simple and inexpensive.

Some dog parents choose to have their vet give an injection at vaccination time which covers for a full year against heartworm.

For dogs who do not have the annual heartworm injection, monthly all-in-one products that combine heartworm control with flea, tick and intestinal worming protection are a popular and easy way to ensure that all bases are covered. If you use a long acting flea or tick treatment, such as Bravecto, then monthly heartworm products are a good choice. Some of these products, such as HeartGard, Interceptor, Milbemax and Milpro also cover for intestinal worms.

The heartworm injection is not available for cats, so monthly combination products that combine heartworm coverage with fleas, intestinal worms and in some cases, ticks, are a great way to ensure you have all the bases covered for your cat.

Top Heartworm Prevention for Dogs

Top Heartworm Prevention for Cats

What if I miss a dose of heartworm prevention?

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

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.

Got a pet health question or need help finding the right product? Get in touch with our Pet Circle Vets.

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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