Heartworm disease is preventable with a $5-to-$15-a-month pill or injection, and treating an established infection can cost $1,000 or more and puts real strain on a dog’s body. That gap is the whole story: prevention is cheap and easy, treatment is expensive and hard on your dog, and by the time symptoms show up, the worms have usually been there for months.
Heartworms are exactly what they sound like — parasitic worms, up to a foot long, that live in a dog’s heart, lungs, and the blood vessels connecting them. They spread only one way: through the bite of an infected mosquito. According to the American Heartworm Society, heartworm disease has been reported in all 50 US states, and incidence has been rising in areas that previously saw few cases, likely tied to shifting mosquito ranges and increased pet travel.
How a Dog Actually Gets Heartworm
A mosquito bites an infected animal, picks up microscopic larvae, and then bites your dog. Those larvae migrate through tissue and mature over about six months, eventually settling in the heart and pulmonary arteries as adult worms that can live five to seven years. A single infected dog can host dozens of worms, and each mosquito bite is a fresh chance at another one.
This means indoor dogs aren’t automatically safe — mosquitoes get inside — and it means a single missed month of prevention during mosquito season is enough for an infection to take hold. It also means heartworm isn’t contagious between dogs directly; a mosquito has to be the middleman.
Symptoms: Why Early Heartworm Is Nearly Silent
In the early stages, most dogs show no symptoms at all, which is exactly why the American Heartworm Society recommends annual testing even for dogs on year-round prevention — missed doses happen, and prevention isn’t 100% foolproof.
As the worm burden increases, watch for:
- A mild, persistent cough, often the first noticeable sign
- Fatigue after moderate activity — a dog who used to run around now gets tired on a normal walk
- Reduced appetite and weight loss
- A swollen belly, caused by fluid buildup from heart failure in advanced cases
- Labored breathing
In severe, untreated cases, a sudden blockage of blood flow in the heart — called caval syndrome — can cause collapse and is a life-threatening emergency. This is uncommon but underscores why catching heartworm early, before symptoms appear, matters more than treating it once a dog is visibly sick.
Diagnosis: A Simple Blood Test
Vets diagnose heartworm with a blood test that detects proteins released by adult female worms, usually available as an in-clinic result within about 10 minutes. Because it takes roughly six months after infection for worms to mature enough to be detected, a dog can test negative shortly after infection and still be carrying larvae — which is part of why annual re-testing is standard even on dogs taking monthly prevention.
If a test comes back positive, your vet will typically run additional bloodwork and possibly X-rays or an echocardiogram to gauge how advanced the infection is and how much strain it’s putting on the heart and lungs before deciding on a treatment plan.
Treatment: Effective, But Not Simple
The American Heartworm Society’s approved treatment protocol uses a series of melarsoninic injections (marketed as Immiticide or Diroban) to kill adult worms, spaced out over several weeks and paired with strict exercise restriction — this part matters as much as the medication. As worms die, they break apart in the bloodstream, and a dog that’s running or playing during treatment is at real risk of a fragment causing a fatal blockage in the lungs.
Treatment typically runs several thousand dollars depending on the dog’s size and infection stage, takes a few months from first injection to clearance, and requires confinement or severely limited activity throughout. It’s effective — most dogs recover fully — but it’s a hard few months for both dog and owner, and it’s the reason vets are emphatic that prevention is the better path by a wide margin.
Prevention: The Actual Fix
Monthly heartworm preventives (oral, topical, or a twice-yearly injectable option) are highly effective when given on schedule and are available only by prescription because your dog needs a negative heartworm test first — giving prevention to an already-infected dog can cause a dangerous reaction.
A few things worth knowing about prevention:
- Year-round dosing is now the standard recommendation in most of the US, even in colder climates, because mosquito seasons are lengthening and a missed winter dose is an easy way to lose a full year of protection.
- Consistency matters more than brand. A monthly chewable given reliably beats a “better” product given inconsistently.
- Combination products often bundle heartworm prevention with flea, tick, or intestinal parasite control — worth asking your vet whether one product covers everything your dog needs rather than juggling several.
When to See a Vet
Book a vet visit if your dog develops a new, unexplained cough, seems to tire faster than usual on walks, loses weight without a diet change, or develops a visibly swollen abdomen. None of these symptoms are exclusive to heartworm — but they’re exactly the kind of vague, easy-to-dismiss signs heartworm produces, so they’re worth ruling out with a simple blood test rather than waiting to see if they resolve on their own. If your dog hasn’t had an annual heartworm test, that’s also worth scheduling regardless of symptoms — see our guide on essential vaccines and preventive care for dogs for how it fits into a broader wellness schedule.
The Practical Takeaway
Heartworm prevention is one of the cheapest, highest-value things you can do for a dog — far cheaper and far kinder than treatment. If your dog isn’t currently on a vet-prescribed preventive, that’s worth fixing this week, not after the next vet visit. And if your dog is already showing a new cough or unexplained fatigue, don’t wait for it to look more serious — a heartworm test is fast, inexpensive, and rules out one of the more preventable causes. For related respiratory symptoms that aren’t heartworm, our article on why your dog is panting so much covers the more common causes and when they warrant a vet visit.