Heartworm symptoms are easy to miss until the disease has already done serious damage. That is what makes heartworm in dogs, and heartworm in cats, so dangerous. By the time most pets show obvious signs, the worms have often been growing inside the heart and lungs for months.
This guide explains what heartworm actually is, why dogs and cats experience it so differently, and what you can do to protect your pet.
What Causes Heartworm in Dogs and Cats
The cause of heartworm in dogs and cats is a parasite called Dirofilaria immitis, spread entirely through mosquito bites. It cannot pass directly from one pet to another, and it cannot pass to humans either.
Here is how it works. A mosquito bites an infected animal and picks up microscopic larvae from its blood. Over the next one to two weeks, those larvae develop inside the mosquito. When the mosquito bites another dog or cat, it deposits the larvae into the new host. From there, the larvae migrate through the body and mature into adult worms living in the heart and the blood vessels of the lungs. In dogs, that maturing process takes around six months. In cats, it takes slightly longer.
Adult worms can grow up to 30 centimetres long and live for years inside an infected animal, which is part of why even a small number of worms can cause lasting damage.
Heartworm Symptoms in Dogs
Heartworm symptoms in dogs tend to build slowly, and severity usually depends on how many worms are present and how active the dog is.
In the early stages, a dog might only have an occasional cough, or no visible symptoms at all. As the disease progresses, owners often notice their dog tiring more easily during exercise, developing a persistent cough, or losing weight without an obvious cause. In more advanced cases, dogs can struggle to breathe, look generally unwell, and show signs of heart failure.
In the most severe cases, called caval syndrome, worms physically block blood flow through the heart. This causes sudden laboured breathing, pale gums, and dark or coffee coloured urine, and it is a medical emergency.
Because symptoms often appear only after the disease has progressed, regular check ups and testing matter more than waiting for visible signs.
Cats and Heartworm: A Different Disease Entirely
Cats and heartworm do not behave the way most owners expect, and this is one of the most important things to understand. Cat symptoms of heartworm are not simply a milder version of what happens in dogs. It is a different disease process.
In cats, far fewer larvae survive to become adult worms, so an infected cat might only ever have one to three worms, sometimes just one. Those worms also live for a shorter time than they do in dogs. But fewer worms does not mean less danger. As the worms die naturally inside a cat’s lungs, even at the larval stage, they can trigger a serious immune reaction known as Heartworm Associated Respiratory Disease, or HARD. This condition causes coughing, wheezing, rapid or laboured breathing, and intermittent vomiting that is not related to eating.
HARD is frequently mistaken for feline asthma, because the symptoms look almost identical. One useful clue is age. Feline asthma usually starts when a cat is young, so if breathing problems appear for the first time in an older cat, heartworm is worth ruling out.
Perhaps the most alarming feature of feline heartworm is that sudden collapse or sudden death can be the very first sign, with no earlier symptoms at all. Indoor cats are not protected either. Studies suggest indoor only cats make up a meaningful share of confirmed feline heartworm cases, because mosquitoes can and do get inside.
Diagnosing heartworm in cats is also harder than in dogs. Standard antigen tests can come back negative even when a cat is infected, simply because the worm burden is so low. Vets typically combine blood tests with x-rays or ultrasound to get a clearer picture.
Heartworm Treatment for Dogs
Heartworm treatment in dogs is a structured process, not a quick fix. It usually starts with staging the infection through blood tests and x-rays, followed by a period of strict rest, since physical activity increases the risk of complications as worms begin to die.
Dogs are typically given a course of antibiotics to weaken the worms, along with anti-inflammatory medication, before receiving a series of injections that kill the adult worms over several weeks. Exercise restriction continues for weeks afterwards while the body clears the dead worms safely.
It is a serious undertaking, both for the dog and for the owner managing weeks of restricted activity, but most dogs recover well when the protocol is followed closely. It is also far more costly and stressful than prevention, which is why prevention is always the better path where possible.
Heartworm Treatment for Cats
This is where heartworm treatment takes a difficult turn. There is currently no approved medication that safely kills adult heartworms in cats. The drug used in dogs is toxic to cats, so treatment instead focuses on managing symptoms, such as using anti-inflammatory medication and supportive care during breathing difficulties.
Some cats do clear the infection on their own as the worms reach the end of their natural lifespan, but there is no guarantee, and the risk of sudden collapse remains throughout. This single fact, that heartworm in cats cannot be directly treated the way it can in dogs, is the strongest reason to focus entirely on prevention for cats.
Heartworm Prevention That Actually Works
Heartworm prevention is highly effective and far simpler than treatment, for both dogs and cats.
For dogs, options include monthly chews or spot on treatments, as well as a once yearly injection given by a vet. The yearly injection has a real practical advantage. It removes the risk of a missed dose, which is one of the most common reasons dogs end up infected in the first place. Whichever option you choose, dogs need a negative heartworm test before starting or restarting prevention, since giving these products to an already infected dog can trigger a dangerous reaction.
For cats, monthly spot on treatments are the standard approach, and they are recommended even for cats that never go outside. Given that there is no reliable treatment for an infected cat, consistent prevention is genuinely the only protection available.
Is Heartworm a Real Risk in Brisbane?
It is a fair question, and the honest answer is yes, though the risk shifts with the seasons. Heartworm depends on mosquitoes, and mosquitoes need warmth to allow the parasite to develop. Research into Australian conditions has found that transmission becomes far less likely once temperatures drop below a certain threshold, which means risk in southeast Queensland rises through the warmer months and eases off in the coolest part of winter.
That seasonal pattern does not mean prevention can be paused. Mosquito activity varies year to year, missed doses are far more dangerous than a slightly early or late mosquito season, and the consequences of an actual infection, particularly in cats, are severe enough that consistent year round protection remains the recommended approach across Australia.
Frequently Asked Questions
What are the first signs of heartworm in dogs?
The earliest sign is often a mild, occasional cough with no other obvious symptoms, which is part of why heartworm is so easy to miss in its early stages.
Can heartworm be cured?
In dogs, yes, through a structured treatment protocol involving medication and a series of injections. In cats, there is no approved cure, only supportive management of symptoms, which is why prevention matters so much more for cats.
Can indoor cats get heartworm?
Yes. Mosquitoes can enter homes, and indoor cats make up a meaningful proportion of diagnosed feline heartworm cases, so being indoors does not remove the risk.
Is heartworm contagious between pets?
No. Heartworm only spreads through mosquito bites. A dog or cat cannot catch heartworm directly from another infected animal.
How long does it take for heartworm symptoms to appear?
It can take many months, sometimes over a year, for a heartworm infection to cause noticeable symptoms, which is why regular testing is more reliable than waiting for visible signs.
Do dogs need a test before starting heartworm prevention?
Yes. Vets recommend testing dogs before starting or restarting prevention, because giving these products to a dog that already has an infection can cause a serious reaction.
Concerned About Heartworm? Talk to Plus One Vet
If you have noticed any unusual coughing, breathing changes, or tiredness in your dog or cat, or you simply want to make sure your pet’s prevention is on track, the team at Plus One Vet is here to help. With clinics in Sunnybank and Nundah, we provide thorough, caring veterinary support for Brisbane pets and their families.
Contact us today to discuss heartworm testing or prevention options suited to your pet.
