Last Updated: 30/07/2026
Heartworm in dogs: facts, symptoms and prevention
Learn how dogs get heartworm, the symptoms to watch for, and the safest ways to prevent it , vet-reviewed facts every dog owner should know.
Author: Dr Carla Paszkowski BVSc (Hons)
Reading Time: 4 minutes - short read
Heartworm is a serious but entirely preventable disease in dogs, spread by nothing more than a mosquito bite. Do you know how it's transmitted, the symptoms to watch for, and how to keep your dog protected?
To help give you a quick run-down, I've collated the facts every dog owner should know.
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.
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!
Heartworm is a completely different parasite to intestinal worms, so don't assume your dog's regular wormer has it covered- see our guide to worms in dogs for the difference.
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, since heartworm spreads by mosquito bite rather than direct contact between dogs.
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?
Cats can be affected by heartworm too, though the disease looks quite different. Cats aren't the natural host, so the immature worms rarely mature fully, but the immune reaction to them can still cause serious lung damage.
Symptoms of heartworm in dogs
Common presenting signs of heartworm infection in dogs include:
- Coughing
- Lethargy
- Loss of appetite
- Weakness
- Fainting
- Distended abdomen
How is heartworm treated 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
Heartworm is easy and inexpensive to prevent, and there are more options than ever to protect your dog.
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 Interceptor, Milbemax and Milpro also cover for intestinal worms.
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.
If you'd like to compare all heartworm prevention options side by side, our vets break it down in our guide to the best heartworm treatment for dogs.
And for full coverage of fleas, ticks and worms together, see our complete flea, tick and worming guide for dogs.
Top recommended heartworm preventatives
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. 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 via our Contact Us page.
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History
Our experts continually monitor the health and wellness space and we update our articles when new information becomes available.
Thu Jul 30 2026
Edited by Dr Belinda Stancombe BVSc (Hons)Dr Carla Paszkowski BVSc (Hons)
Veterinarian
Dr. Carla graduated from the University of Queensland in 2013 with a Bachelor of Veterinary Science and worked for a number of years in small animal clinics across South East Queensland. While Carla enjoys most facets of clinical veterinary work, she holds a special passion for feline medicine, pocket pets, and nutrition.
