Health guide
Heartworm in Australian Dogs: Prevention, Testing and Missed Doses
Mosquito season is starting. Here is how heartworm actually spreads, why 'I gave the tablets' isn't always enough, and how to choose between monthly and yearly prevention
9 min read
The quick answer
- 1Heartworm is spread only by the bite of an infected mosquito. It cannot pass directly between dogs, and a single bite is enough to infect one.
- 2Early infection shows no signs, so waiting for symptoms means waiting until the disease is well advanced. Prevention is far easier than treatment.
- 3Monthly products only work if every dose is given on time. If you forget, ask your vet about testing, because heartworm cannot be detected in blood until about six months after infection.
When to see a vet
Heartworm is the disease that sounds like a problem for other people's dogs. Your dog seems perfectly healthy, the tablets are in the pantry somewhere, and you are fairly sure you gave one recently. The trouble is that heartworm gives you no warning. By the time a dog coughs, tires on walks or struggles to breathe, adult worms have been living in the heart and lungs for months.
Spring is when the mosquito population starts to climb, which makes it a good week to check your dog's protection is actually current. This article covers how the disease works, what the prevention options are, and the part most owners get wrong: what to do when a dose is late or missed. It is general information, and your vet is the right person to choose a product and schedule for your dog and your region.
Part 1
How Dogs Get Heartworm
There is only one route: the bite of an infected mosquito. A mosquito feeds on an infected animal, picks up immature worms, and passes them to the next dog it bites. Dogs cannot catch it from each other, from the environment or from sharing a bowl, so no amount of caution about other dogs protects yours. The only protection is stopping the immature worms from developing, which is what prevention products do.
Heartworm is found across Australia. It is most common in warm, humid coastal regions, but mosquitoes are present in many areas for much of the year, and Sydney vets note they are around all year. Your local risk is a question for your vet, who knows what is circulating in your area.
Part 2
Why It Is So Easy to Miss
In the early stages there are simply no signs. As the worms mature and multiply, dogs may develop a cough, become reluctant to exercise and tire quickly. Later signs include pale gums, difficulty breathing and collapse, and severe cases can end in sudden death. Treating an established infection is a long, expensive and risky process compared with a dose of prevention.
Part 3
Monthly or Yearly?
There are two main approaches, and the difference between them is mostly about how much depends on you remembering.
How it works
- Monthly chew or tablet
- Given by you every month
- Yearly injection
- Slow-release injection given by your vet
What you have to remember
- Monthly chew or tablet
- Every dose, on time, all year
- Yearly injection
- One appointment a year
If you are a few weeks late
- Monthly chew or tablet
- Little or no buffer, and protection may lapse
- Yearly injection
- Designed to cover a delay of a couple of months
Often bundled with
- Monthly chew or tablet
- Flea, tick and worm cover in combination products
- Yearly injection
- Usually given alongside the annual vaccination
Good fit for
- Monthly chew or tablet
- Owners who are reliable with a calendar
- Yearly injection
- Owners who struggle with monthly routines, or dogs hard to medicate
| Monthly chew or tablet | Yearly injection | |
|---|---|---|
| How it works | Given by you every month | Slow-release injection given by your vet |
| What you have to remember | Every dose, on time, all year | One appointment a year |
| If you are a few weeks late | Little or no buffer, and protection may lapse | Designed to cover a delay of a couple of months |
| Often bundled with | Flea, tick and worm cover in combination products | Usually given alongside the annual vaccination |
| Good fit for | Owners who are reliable with a calendar | Owners who struggle with monthly routines, or dogs hard to medicate |
Part 4
Testing: When It Matters
A heartworm test is a quick blood test, and there is a wrinkle that catches people out: it can only detect infection about six months after the mosquito bite, because the worms have to mature before the test can see them. That timing shapes the rules.
Dog over 7 months and never on prevention
- What vets generally advise
- Test before starting prevention
Dog on prevention, annual check-up
- What vets generally advise
- Many vets recommend a yearly test to confirm it is working
You missed one or more monthly doses
- What vets generally advise
- Speak to your vet. Testing straight away can miss a new infection, so they may advise a test months later
Puppy under about 7 months
- What vets generally advise
- Vets commonly start prevention without a test, since infections take months to show
| Situation | What vets generally advise |
|---|---|
| Dog over 7 months and never on prevention | Test before starting prevention |
| Dog on prevention, annual check-up | Many vets recommend a yearly test to confirm it is working |
| You missed one or more monthly doses | Speak to your vet. Testing straight away can miss a new infection, so they may advise a test months later |
| Puppy under about 7 months | Vets commonly start prevention without a test, since infections take months to show |
Part 5
Making Monthly Prevention Stick
A System That Does Not Rely on Memory
- 1
Pick a fixed date, not 'sometime this week'
The first of the month, or the day you pay the mortgage. Tie it to something you never forget.
- 2
Set a repeating reminder with an alarm
A calendar notification you can swipe away is easy to ignore. Use an alarm that needs a tap to confirm.
- 3
Mark it off where you will see it
A tick on the fridge calendar, or a note in the same place each month, so a glance tells you whether it was done.
- 4
Buy the next supply before this one runs out
Running out is a classic cause of a late dose. Reorder when you open the last packet.
- 5
Ask about the yearly injection
If you have missed doses before, an annual injection given at the vaccination visit removes the monthly task altogether.
Part 6
Products That Help
Affiliate Disclosure: As an Amazon Associate I earn from qualifying purchases. Product links on this page go to Amazon AU. Prices shown are approximate, check Amazon for current pricing.
Prevention products should be chosen with your vet, based on your dog's weight, age and health, and where you live. The two below are practical examples, not a recommendation to bypass that conversation.
All-in-One Monthly
NexGard Spectra Chews, Small Dogs 3.6-7.5 kg (3 pack)
A monthly beef-flavoured chew that combines heartworm prevention with flea, paralysis tick, mite and intestinal worm control in one dose, which cuts the number of separate products to remember. The listing is for the small dog weight band, and there are other sizes for larger dogs, so match the pack to your dog's current weight and check with your vet.
- Heartworm prevention, monthly
- Also covers fleas, ticks, mites and intestinal worms
- Soft, beef-flavoured chew
- This pack is for dogs 3.6 to 7.5 kg
Makes Dosing Easy
Greenies Pill Pockets, Capsule Size (30 count)
If your dog's monthly product is a tablet rather than a chew, or they need any other medication, a pill pocket makes it easy. Drop the tablet in, pinch it closed and hand it over, which beats wrestling with a reluctant dog. A dog who takes their dose happily is a dog who actually gets it.
- Masks the smell and taste of medication
- Pinch-closed pocket design
- Chicken flavour, natural ingredients
- 30 treats per pack
Heartworm Protection Checklist
- Confirm my dog is on a heartworm prevention product right now
- Work out the date the last dose was given
- Set a repeating alarm for the next dose
- Buy the next supply before this one runs out
- Book a yearly heartworm test with the annual check-up
- Ask my vet whether the yearly injection suits us
- Ring my vet straight away if a dose is missed
- Update the product when my dog moves to a new weight band
Part 7
What About Repellents?
Reducing mosquito exposure, such as emptying standing water and keeping dogs inside around dusk when mosquitoes are most active, is sensible, but it does not replace prevention. One bite is enough, and you cannot fully control where your dog goes. Do not use insect repellents made for people on your dog, since some ingredients are harmful if licked. Ask your vet about anything applied to the skin.
Check the date of your dog's last dose today. If it is more than a month ago, or you are not sure, that is the call to make this week.
Common questions
How do dogs get heartworm in Australia?
Only through the bite of an infected mosquito. The mosquito picks up immature worms from an infected animal and passes them to the next dog it bites. Dogs cannot catch it directly from each other, so the only protection is stopping the worms developing with a preventative.
Is monthly or yearly heartworm prevention better?
Both work when used correctly. Monthly products depend on every dose being given on time, and there is little buffer if you are late. The yearly injection is given by a vet, so there is nothing to remember, and it is designed to keep protecting for a period even if the next injection is delayed. Your vet can advise which suits your dog.
What should I do if I miss a monthly heartworm dose?
Ring your vet and tell them how late you were. The gap may have left your dog unprotected, and because heartworm does not show on a blood test until about six months after infection, testing straight away can miss it. They will advise on restarting prevention and when to test.
What are the signs of heartworm in dogs?
There are no signs early on. As the disease progresses, dogs may cough, tire quickly and be reluctant to exercise, and later develop pale gums, breathing difficulty and collapse. By this stage the infection is advanced, which is why prevention matters more than watching for symptoms.
Does my puppy need a heartworm test before starting prevention?
Vets commonly start young puppies on prevention without a test, since infection takes months to show, but dogs over about seven months who have never been on prevention are generally tested first. Follow your vet's advice for your puppy's age and history.