Written by Dr. Asmat Ullah Khan, DVM | Last Reviewed: March 2026
This article is for educational purposes only and is not a substitute for professional veterinary advice. Always consult your veterinarian for guidance specific to your dog’s health needs.
Heartworm disease in dogs is a serious, potentially fatal condition caused by parasitic worms living inside your dog’s heart and lungs, but the good news is it’s almost entirely preventable. Understanding how it spreads, what it does, and how to stop it before it starts is the most important thing you can do for your dog’s long-term health.
What Is Heartworm Disease in Dogs?
Heartworm disease is caused by Dirofilaria immitis, a parasitic worm that takes up residence in the pulmonary arteries — the vessels carrying blood from the heart to the lungs — and occasionally within the heart itself. According to the American Heartworm Society, these worms can grow up to a foot in length and survive for five to seven years inside an untreated dog.
Adult heartworms cause direct, progressive inflammation of the blood vessels in the lungs. Over time, this leads to pulmonary hypertension (elevated blood pressure in the lung’s vessels), respiratory distress, weight loss, weakness, and in advanced cases, right-sided congestive heart failure. As outlined in detail by Cornell University’s Riney Canine Health Center, the damage heartworms inflict on the heart and lungs can be permanent, even after successful treatment.
How Big Can the Problem Actually Get?
A single dog can harbor hundreds of adult worms. Even a moderate worm burden causes significant cardiovascular stress. The insidious part? Many dogs show no obvious symptoms in the early stages, they look and act completely normal while the damage quietly accumulates inside.
How Dogs Get Heartworms: The Mosquito Connection
Mosquitoes are the sole vector of heartworm disease — meaning without a mosquito bite, your dog cannot contract heartworms. When a mosquito feeds on an infected animal (dogs, coyotes, foxes, wolves), it ingests tiny immature heartworms called microfilariae. These develop inside the mosquito into infective third-stage larvae (L3) , but only if environmental temperatures are sustained above 14°C (57°F), a threshold measured in heartworm development units (HDUs).
The Heartworm Lifecycle, Step by Step
Once an infected mosquito bites your dog, the L3 larvae enter through the puncture wound and migrate under the skin. Over several weeks, they progress through additional larval stages before entering the bloodstream. They then travel to the pulmonary arteries, where they mature into adults — a process the Merck Veterinary Manual notes takes approximately six to seven months. Adult worms then mate, and females release new microfilariae into the bloodstream, restarting the cycle.
Heartworm Disease Is More Common Than You’d Think
Globally, heartworm disease is most prevalent in warm, humid, tropical, and subtropical regions where mosquito populations are large and persistent. But it exists on every continent except Antarctica, and its range is expanding. According to research on Dirofilaria immitis exposure risk in Canada published in PMC, climate change is shifting mosquito ranges northward and extending active transmission seasons in temperate regions — including Canada.
In Canada, heartworm has historically been concentrated around the Great Lakes region of Ontario, southern Quebec, and southern Manitoba — but cases in Atlantic Canada have been documented, and that geographic footprint continues to grow. Wild canid populations — coyotes especially — act as natural reservoirs, keeping the parasite circulating in the environment even in areas where domestic dog prevalence is low.
The Biggest Myth: “It’s Too Cold Here for Heartworms”
This is probably the most common — and most dangerous — misconception among Canadian dog owners, and honestly, it’s understandable. Heartworm larval development inside a mosquito does require temperatures above 14°C (57°F) sustained for long enough to accumulate 130 HDUs. But here’s the catch: Canadian summers absolutely hit that threshold. A mean daily temperature of just 17°C delivers 3 HDUs per day, and it doesn’t take long to reach 130 in July or August in most of the country.
A 2024 study published in the Canadian Veterinary Journal found that the traditional June 1st start date for heartworm prevention — based on Windsor, Ontario weather data from 1957–1986 — may need to be revisited entirely, given how significantly seasonal temperature patterns have shifted. That early warm spell in April or May? It can catch owners completely off guard if prevention hasn’t been started yet.
The myth-busting takeaway: We do get a break in the dead of winter — but our summers are more than warm enough, and one infected mosquito is all it takes.
What About Indoor Dogs?
This one comes up almost as often. Indoor living does lower exposure risk — but mosquitoes are remarkably good at getting inside. They slip through window screens, open doors, garage entrances, and even bathroom exhaust vents. And most “strictly indoor” dogs still get walks, trips to the park, or backyard time. As PetMD notes, even a low-activity indoor dog with a lapse in prevention is still a candidate for infection and should be tested and re-evaluated.
The myth-busting takeaway: Mosquitoes don’t knock before they come in — and your dog doesn’t have to live outside to be at risk.
How to Prevent Heartworm Disease in Dogs
Prescription Preventatives — Oral, Topical, and Injectable
Heartworm preventatives are available by veterinary prescription in three main forms. The American Heartworm Society and the AVMA both recommend FDA-approved heartworm preventatives year-round for all dogs — regardless of climate or geography. A 2024 randomized field trial by Dantas-Torres et al. published in PubMed found that a topical moxidectin/imidacloprid/praziquantel combination achieved 100% prevention of microfilaremia in treated dogs in an endemic region — reinforcing just how effective consistent prevention is when used correctly.
📊 Heartworm Prevention Options for Dogs at a Glance
| Prevention Type | Common Forms | How Often | Also Covers | Vet Prescription Required? |
|---|---|---|---|---|
| Oral tablet/chew | Flavored chewables | Monthly | Some cover intestinal parasites, fleas | Yes |
| Topical (spot-on) | Liquid applied to skin | Monthly | Fleas, ticks, mites (product-dependent) | Yes |
| Injectable | ProHeart 6 or 12 | Every 6 or 12 months | Heartworm only | Yes |
| Combo oral | Broad-spectrum chews | Monthly | Fleas, ticks, intestinal worms | Yes |
Always rule out an active heartworm infection before starting or switching preventatives. Starting prevention in a heartworm-positive dog without proper evaluation can cause serious complications.
Environmental Tips to Reduce Mosquito Exposure
Preventatives do the heavy lifting, but reducing mosquito exposure is a smart layer of added protection. Here’s what actually helps:
- Avoid lakes, streams, and floodplain areas during warm, humid weather when large mosquito hatches occur
- Check your yard for standing water in containers, flowerpots, gutters, and bird baths — these are prime mosquito breeding grounds
- Consider porch nets, screened-in areas, or mosquito barrier treatments for your property
- Indoor dogs still benefit from keeping windows screened and doors closed during peak mosquito hours (dawn and dusk)
- Contact your local public health department if you have concerns about mosquito populations on your property
According to a review by Noack et al. (2021), ectoparasiticides and repellents can serve as valuable adjuncts to chemoprophylaxis — especially in high-exposure environments — though they should never replace prescribed preventatives.
Detecting Heartworm Infection Early
When and How Often Should Your Dog Be Tested?
Annual heartworm antigen blood testing is recommended for all dogs over seven months of age — even dogs on consistent prevention. The antigen test detects proteins released by adult female heartworms and is highly accurate. A 2024 retrospective cohort study by Mwacalimba et al. found that better adherence to prevention protocols was directly linked to reduced positive test rates — and importantly, that dogs who had been treated for heartworm disease faced increased long-term cardiac risks compared to dogs that had never tested positive, underscoring why prevention matters so much more than treatment.
If you’ve missed doses, switched products, or recently adopted a dog with an unknown history, more frequent testing may be recommended. If your dog tests positive, your veterinarian will stage the infection and discuss treatment — which typically involves strict exercise restriction, pre-treatment stabilization, and a multi-step injectable melarsomine protocol. It’s costly, time-consuming, and hard on your dog. Prevention is always the better path.
🩺 Dr. Khan’s Clinical Corner
“A few months into my time at the hospital, a sweet mixed-breed came in for a routine wellness visit , completely asymptomatic, tail wagging the whole exam. The owner had been diligent with prevention, or so they thought , turns out they had stopped giving it in late September, figuring the mosquito season was ‘done.’ The heartworm antigen test came back positive. It was a good reminder that prevention doesn’t just stop infection going forward — it works retroactively, clearing larvae from the previous 30 days, so that late-fall gap is anything but safe.”
— Dr. Asmat Ullah Khan, DVM
🔬 Research Radar
The science on heartworm prevention is clear and consistent. A landmark experimental challenge study by Bowman et al. (2016) demonstrated that after four consecutive monthly doses of topical moxidectin, 100% of dogs were protected even when experimentally infected 28 days after the last treatment — validating the retroactive “reach-back” mechanism that makes monthly prevention so powerful. Complementing this, a practice-based study by Ku et al. (2017) identified poor owner compliance — not treatment failure — as the predominant cause of heartworm infection in clinical settings, reinforcing that the medication works when it’s actually given consistently. Together, these studies tell a simple story: the drug works. The challenge is the human side of the equation.
🚨 Emergency Red Flags
Take your dog to a veterinarian immediately if you notice any of the following:
- Persistent cough that doesn’t resolve, especially after mild activity
- Exercise intolerance — your dog tires quickly, stops mid-walk, or refuses activity they previously enjoyed
- Labored or rapid breathing at rest
- Distended abdomen (fluid accumulation from heart failure)
- Fainting or collapse, particularly after exertion
- Sudden weight loss with no change in diet
- Pale or bluish gums — a sign of poor oxygenation
- Sudden leg weakness or paralysis (rare but possible with caval syndrome, a life-threatening complication)
These signs may indicate advanced heartworm disease, right-sided heart failure, or caval syndrome — all of which require urgent veterinary care.
🏠 Home Care & Prevention Tips
These practical steps work best alongside veterinary-prescribed prevention — not instead of it:
- Set a monthly phone reminder on the same date every month for prevention doses — consistency is everything
- Tie it to a routine — give prevention on the same day you pay a monthly bill, or on the first of every month
- Store preventatives properly — keep them at room temperature, away from humidity, in their original packaging
- Never skip the fall doses — as Dr. Khan’s case illustrates, September and October can still carry real transmission risk
- If you adopt a new dog or rescue, always get a heartworm test before starting prevention — starting prevention in a heartworm-positive dog without evaluation can trigger a dangerous reaction
- Keep records — note the date of every dose in your pet’s health file or a simple notebook
- Ask about combo products — if your dog is also at risk for fleas, ticks, or intestinal parasites, a combination product may simplify your prevention routine (confirm with your vet)
❓ FAQ
Q: Can my dog get heartworms even if they’re on prevention?
No preventative is 100% effective if doses are missed or given late. When used consistently and correctly, FDA-approved heartworm preventatives are highly effective — close to 100%. The most common reason dogs on prevention test positive is a gap in dosing, not product failure.
Q: How long does heartworm treatment take?
Treatment is a multi-month process involving pre-treatment stabilization, antibiotic therapy (doxycycline), and a series of melarsomine injections. Dogs must be strictly exercise-restricted throughout — sometimes for months. It’s expensive, physically demanding for your dog, and carries real risks. Prevention is dramatically safer and cheaper.
Q: My dog has been on prevention for years — do they still need annual testing?
Yes. The American Heartworm Society recommends annual testing for all dogs regardless of prevention history. Gaps in dosing can happen without owners realizing, and early detection leads to far better outcomes than catching it at an advanced stage.
Q: At what age should I start heartworm prevention?
Most veterinarians recommend starting heartworm prevention as early as six to eight weeks of age, depending on the product. Puppies under seven months can be started on prevention without prior testing — after that, a negative test should be confirmed first.
Conclusion
Heartworm disease in dogs is serious, progressive, and largely silent until it has already done significant damage — but it is also one of the most preventable diseases in veterinary medicine. Year-round, veterinarian-prescribed prevention, combined with annual testing and a few smart environmental habits, gives your dog the best possible protection. Don’t wait for symptoms that may never come until it’s too late — book a wellness visit with your veterinarian, get your dog tested, and start or continue prevention today. Your dog’s heart depends on it.
