Of all the parasites vets talk about, heartworm is the one that tends to get the most emphatic "please don't skip this" speech — and for good reason. Unlike fleas or intestinal worms, heartworm disease is genuinely dangerous, expensive and difficult to treat once a pet is infected, and in some cases can be fatal. The good news is that it's also one of the most reliably preventable diseases in veterinary medicine, as long as prevention is kept consistent.
What Heartworm Disease Actually Is
Heartworms are a type of parasitic worm spread exclusively through mosquito bites — a mosquito picks up immature larvae from an infected animal and passes them along when it bites another dog or cat. Once inside a new host, those larvae migrate through the body over several months and mature into adult worms that live in the heart and the blood vessels of the lungs. Left untreated, adult worms can grow to be several inches long, and a significant infestation puts real strain on the heart and lungs.
Dogs are the natural host and tend to carry higher worm burdens, but cats can be infected too — and heartworm disease in cats is often harder to detect and has no approved treatment for adult worms, which makes prevention even more critical for them.
Why "Monthly, No Exceptions" Is the Rule
Heartworm preventives don't work the way you might assume — they aren't a repellent that keeps mosquitoes away, and they don't prevent infection from happening at the moment of a bite. Instead, they work retroactively, clearing out the larval stages that may have been deposited in the weeks before the dose was given. That's exactly why a missed month creates a real gap: if larvae have had enough time to develop past the stage the preventive can clear, the next dose won't undo that.
Year-round, even where winters are cold
It's tempting to think prevention can pause once mosquito season ends, but many vets recommend year-round dosing regardless of climate. Mosquitoes can survive indoors, warm spells can trigger unexpected activity even in cooler months, and staying on a consistent monthly schedule is simply easier to maintain than starting and stopping.
Testing before (and during) prevention
Before starting a preventive, and periodically afterward, vets test a blood sample for heartworm antigen. This matters because giving certain preventives to an already-infected pet can trigger a dangerous reaction, and because early detection makes treatment (for dogs) far safer and more successful. Annual testing is a standard part of most preventive care plans, alongside the sort of routine screening covered in our annual fecal test guide.
Signs of Heartworm Disease
In the early stages, many infected pets show no symptoms at all, which is part of why regular testing matters more than waiting for signs to appear. As the disease progresses, owners may notice a mild, persistent cough, tiring more easily on walks or play, reduced appetite, and weight loss. In cats, symptoms can look surprisingly like asthma — coughing, difficulty breathing, or vomiting unrelated to eating. Any of these warrant a prompt vet visit rather than a wait-and-see approach.
In more advanced cases, dogs can develop a swollen belly from fluid buildup, or a sudden collapse related to blocked blood flow through the heart — a genuine emergency. This is a lot to describe, and it can sound frightening, but it's worth keeping in perspective: these advanced presentations are largely what consistent, year-round prevention exists to prevent from ever happening in the first place. The whole point of monthly dosing is that pets on a reliable schedule essentially never reach this stage.
If a Pet Does Test Positive
A positive heartworm test in a dog is serious, but it isn't hopeless — treatment exists and has a good success rate when a pet is otherwise reasonably healthy, though it involves a period of strict rest since physical exertion can dislodge dying worms and cause complications. This is one more reason vets are so consistent about testing before and during a preventive routine: catching an infection early, before serious lung or heart damage occurs, makes the whole treatment process safer and more likely to succeed. For cats, since there's no approved drug to kill adult heartworms, management typically focuses on supporting the cat through the illness, which underscores why prevention carries even more weight for them.
The Bottom Line
Heartworm disease is far easier and cheaper to prevent than to treat, and prevention only works if it's consistent — a monthly dose given reliably, year-round, backed by periodic testing. If you've ever wondered why your vet asks about heartworm prevention at every single visit, this is why: it's one of the few parasite risks where a lapse can have serious, lasting consequences.