I still remember the vet visit that scared me straight about heartworms. My second dog, a goofy Lab mix named Duke, had missed about four months of prevention during a chaotic move across states. When the vet ran a routine heartworm test at his annual checkup, I held my breath for what felt like forever. He was negative — thank God — but the lecture I got that day stuck with me permanently.
Heartworm disease kills dogs. That’s not me being dramatic. It destroys their hearts and lungs from the inside out, and by the time most owners notice something’s wrong, serious damage has already been done. The frustrating part? It’s almost entirely preventable. A monthly pill or a yearly injection, and your dog never has to deal with foot-long worms living in their pulmonary arteries. Yet the American Heartworm Society estimates that over a million dogs in the U.S. are infected right now.
So let’s talk about all of it — how dogs get heartworms, what to watch for, and what your options are if the worst happens.
How Dogs Actually Get Heartworms
Here’s something that surprises a lot of people: dogs can’t catch heartworms from other dogs. Not directly, anyway. The entire transmission cycle depends on mosquitoes.
A mosquito bites an infected dog (or fox, coyote, or wolf) and picks up microscopic heartworm larvae called microfilariae. Those larvae develop inside the mosquito over about two weeks, and when that mosquito bites your dog, the larvae enter through the bite wound. From there, they migrate through your dog’s body over the next six months, eventually settling in the heart, lungs, and surrounding blood vessels.
Once they’re adults, these worms can reach 12 inches long. A single dog can harbor 30 to 250 worms, and they can live 5 to 7 years inside your pet. Let that sink in for a second.
The scary thing about geography? Heartworm disease has been reported in all 50 states. If you’re thinking “well, I live somewhere cold, so we’re fine” — I used to think that too. Mosquitoes are adaptable. They breed in standing water, and all it takes is one warm spell. Vets across the northern U.S. and Canada now recommend year-round prevention, and honestly, I agree with them.
Signs and Symptoms to Watch For
This is the part that really gets me. Heartworm disease is sneaky. In the early stages, your dog might show absolutely zero symptoms. By the time you’re noticing something, the worms have likely been setting up shop for months.
Early Stages (Class 1-2)
- A mild, persistent cough — especially after exercise
- Your dog tiring out faster than usual on walks
- Decreased appetite
- Slight weight loss
I’ve talked to owners who dismissed the cough as allergies or kennel cough. Easy mistake to make. But if your dog is coughing and seems less enthusiastic about their usual walk route, get them tested.
Advanced Stages (Class 3-4)
This is where things get dangerous:
- Pronounced fatigue — even short walks become too much
- Difficulty breathing
- A swollen belly from fluid accumulation (ascites)
- Dark or bloody urine
- Pale gums
Class 4 is called caval syndrome, and it’s an emergency. The worm burden is so heavy that blood flow to the heart is physically blocked. Without immediate surgical removal of the worms, it’s almost always fatal. Dogs at this stage can collapse with no warning.
A friend of mine adopted a rescue from Alabama that turned out to be heartworm positive. She’d had him three weeks and he seemed totally fine — playful, eating well, normal energy. The only reason they caught it was the standard intake test at her vet. That dog went through six months of treatment and recovered fully, but only because they found it early.
Testing and Diagnosis
Heartworm testing is simple. It’s a blood test that detects proteins (antigens) produced by adult female heartworms. Most vets run it as part of your dog’s annual wellness exam, and results come back in about 10 minutes with an in-clinic test.
A few things worth knowing:
It takes about 5-7 months after infection for a test to come positive. The test detects adult worms, and larvae need time to mature. This is why your vet might want to retest a dog with a gap in prevention — the infection could’ve happened months ago but not show up yet.
| Test Type | What It Detects | Turnaround | Cost |
|---|---|---|---|
| Antigen test (standard) | Adult female worm proteins | 10 minutes in-clinic | $35-75 |
| Microfilaria test | Larval worms in blood | Same day | $15-40 |
| Chest X-ray | Heart/lung damage | Same day | $150-300 |
| Echocardiogram | Worm visualization, heart function | Same day | $300-600 |
If the antigen test comes back positive, your vet will likely confirm with a second test and then run additional diagnostics — X-rays and possibly an echocardiogram — to assess how much damage has been done. The treatment plan depends entirely on the disease stage.
Prevention Options That Actually Work
This is the hill I’ll die on: heartworm prevention is non-negotiable. The monthly cost is somewhere between $6-18 depending on your dog’s size and the product. Treatment for an actual infection? $1,000-3,500. And that’s not counting the emotional toll of watching your dog go through it.
Monthly Preventatives
These are the most common, and they all work well:
- Heartgard Plus (ivermectin/pyrantel) — Probably the most recognized name. Soft chew, dogs love it. Also covers roundworms and hookworms.
- Interceptor Plus (milbemycin/praziquantel) — Covers heartworms plus tapeworms, which Heartgard doesn’t. Good choice if your dog eats random stuff outside.
- Simparica Trio (sarolaner/moxidectin/pyrantel) — The all-in-one. Heartworm, flea, tick, and intestinal parasite coverage in a single monthly chew. This is what I use for my dogs now.
Injectable Option
- ProHeart 12 (moxidectin) — A single injection at your vet’s office that lasts a full year. I love this option for people who are forgetful about monthly doses. No judgment — life gets busy. ProHeart 12 takes the guesswork out of it.
Topicals
- Advantage Multi (imidacloprid/moxidectin) — Monthly topical that covers heartworms and fleas. Good if your dog won’t take chewables, though topicals can be messy and you have to keep kids away from the application site until it dries.
One critical thing: these preventatives kill heartworm larvae that your dog picked up in the previous 30 days (or 12 months for ProHeart). They don’t repel mosquitoes or prevent bites. They work retroactively. That’s why consistency matters — a missed month creates a window where larvae can mature past the point these drugs can kill them.
Treatment for Heartworm-Positive Dogs
If your dog tests positive, take a breath. It’s serious, but it’s treatable in most cases. The process just isn’t fun for anyone involved.
The Standard Protocol (Melarsomine-Based)
The American Heartworm Society recommends a multi-step approach:
Month 1: Start heartworm preventative (yes, even though your dog’s already infected — this kills new larvae and prevents further transmission). Your vet will also prescribe doxycycline, an antibiotic that weakens the worms by killing a bacteria they depend on called Wolbachia.
Month 2: Continue doxycycline and preventative. Strict exercise restriction begins now. And I mean strict — leash walks only, no running, no roughhousing, no dog parks.
Month 3: First injection of melarsomine (Immiticide), given deep into the back muscles. This is the drug that actually kills adult worms.
Month 4: Two more melarsomine injections, 24 hours apart. This is the hardest part. Your dog will be sore at the injection sites, and exercise restriction becomes even more important — as the worms die, fragments can block blood vessels in the lungs. A dog that’s running and jumping has increased blood flow, which increases the risk of a pulmonary embolism.
Months 5-6: Continued rest and monitoring. Your vet will retest about 6 months after treatment to confirm it worked.
The Exercise Restriction Reality
I won’t sugarcoat this — keeping an otherwise energetic dog calm for 4-6 months is brutal. Frozen Kongs, puzzle feeders, and short leash walks become your whole life. My friend with the rescue dog said it was harder on her than the dog. He wanted to play so badly and couldn’t understand why she kept stopping him.
But this part isn’t optional. The number one complication from heartworm treatment is pulmonary thromboembolism — dead worm fragments lodging in the lung vessels. Exercise dramatically increases this risk.
What About the “Slow Kill” Method?
You might hear about just keeping a dog on monthly preventative without the melarsomine injections, letting the worms die naturally over a couple years. Most vets — and the American Heartworm Society — strongly advise against this. The worms keep damaging the heart and lungs the entire time they’re alive. Every extra month those worms are living in your dog’s body is another month of organ damage.
Cost Breakdown
Money matters. I’m not going to pretend it doesn’t. Here’s what you’re looking at:
| Prevention (Annual) | Treatment | |
|---|---|---|
| Small dog (under 25 lbs) | $70-120 | $1,000-1,500 |
| Medium dog (25-50 lbs) | $90-150 | $1,200-2,000 |
| Large dog (50-100 lbs) | $120-200 | $1,500-3,000 |
| Giant breed (100+ lbs) | $150-250 | $2,000-3,500+ |
Treatment costs include diagnostics, injections, medications, follow-up testing, and the inevitable extra vet visits when your dog is sore or you’re worried about complications. Some pet insurance plans cover heartworm treatment, but many exclude it if prevention wasn’t being used — so read your policy carefully.
The math is simple. A lifetime of prevention for a medium-sized dog costs roughly what a single round of treatment does. Prevention wins every time.
Special Considerations by Breed
Not all dogs handle heartworm prevention and treatment the same way.
Collies and herding breeds (Shelties, Australian Shepherds, Border Collies) can carry the MDR1 gene mutation, which makes them sensitive to certain drugs including ivermectin at high doses. The doses used in heartworm preventatives like Heartgard are safe — that’s been well established — but it’s worth mentioning to your vet. Milbemycin-based products like Interceptor are an alternative if you’re more comfortable with that.
Brachycephalic breeds (Bulldogs, Pugs, French Bulldogs) already have compromised respiratory systems. Heartworm disease hits them harder because their lungs are working overtime as it is. Prevention is even more important for these guys.
Senior dogs need testing before starting prevention, even if they’ve been on it their whole lives. A lapse of even a few months warrants a retest.
Puppies can start heartworm prevention as early as 6-8 weeks old. Don’t wait for their first annual exam — talk to your vet at the first puppy visit.
Frequently Asked Questions
Can indoor dogs get heartworms?
Yes. Mosquitoes get inside — through open doors, torn screens, garages, you name it. I’ve slapped mosquitoes in my living room in January. Indoor-only dogs absolutely need heartworm prevention. The American Heartworm Society doesn’t make any distinction between indoor and outdoor dogs in their recommendations.
Is heartworm treatment painful for dogs?
The melarsomine injections are given deep into the lumbar muscles and can cause significant soreness for several days. Most vets prescribe anti-inflammatory medication to manage pain. Dogs are usually uncomfortable but not in acute distress. The injection sites can be swollen and tender — some dogs don’t want to be touched on their backs for a week or so.
Can humans get heartworms from dogs?
Technically, heartworm larvae can be transmitted to humans through mosquito bites. But here’s the good news — they can’t complete their life cycle in our bodies. The larvae die off and occasionally create small nodules in the lungs that show up on X-rays (sometimes mistaken for tumors). It’s extremely rare and not considered a significant health threat.
How long can a dog live with untreated heartworms?
Without treatment, heartworm disease is progressive and ultimately fatal. Dogs with mild infections might survive 2-4 years, but their quality of life deteriorates steadily. The worms cause irreversible damage to the heart, lungs, and blood vessels. There’s really no good reason to leave heartworms untreated when effective treatment exists.
Do I really need to give heartworm prevention year-round?
The American Heartworm Society recommends 12-month prevention regardless of where you live. Even in northern climates, indoor heating can keep mosquitoes alive longer than you’d think. A single missed month during an unexpected warm spell is all it takes. Plus, year-round dosing protects against gaps if you accidentally miss a month — the preventative kills larvae from up to 30 days prior, so consistent dosing provides a safety net.
The Bottom Line
Heartworm disease is one of those things in dog ownership that’s almost entirely within your control. A monthly chew or a yearly shot, and your dog never has to deal with worms living in their heart. Skip it, and you’re gambling with an expensive, stressful, and potentially fatal disease.
I’ve been giving heartworm prevention to every dog I’ve owned since Duke’s close call, and I’ll keep doing it until my last dog. It’s the easiest, most impactful thing you can do for your dog’s long-term health. Talk to your vet, pick a product that works for your dog and your lifestyle, set a monthly reminder if you need to — and just do it.
Featured Image Source: Pexels
