Heartworm Disease: Prevention Is the Only Cure

The thought of worms living in your dog's heart is disturbing, and rightly so — heartworm disease is a serious, potentially fatal condition. But here's the most important thing you'll read about it: prevention is simple, affordable, and nearly 100% effective. This is one disease you have real power to prevent.

What Is Heartworm Disease?

Heartworm disease is caused by Dirofilaria immitis, a parasitic worm that lives in the heart, lungs, and associated blood vessels of infected animals. Adult heartworms can grow up to 12 inches long, and a single dog can harbor up to 250 worms.

The worms cause lasting damage to the heart, lungs, and arteries. Even after treatment, some damage is permanent. This is why prevention — not treatment — is the standard of care.

How Dogs Get Heartworm

The lifecycle involves mosquitoes as an essential intermediate host:

1. A mosquito bites. Prevention is especially important for breeds prone to heart conditions — check your breed's health profile on our breed pages an infected animal (dog, wolf, coyote, fox) and picks up microscopic baby heartworms (microfilariae)
2. Inside the mosquito, the microfilariae develop into infective larvae over 10-14 days
3. The mosquito bites your dog, depositing larvae into the skin
4. Larvae migrate through tissue, entering the bloodstream and traveling to the heart and lungs
5. Over 6-7 months, larvae mature into adult worms and begin reproducing
6. Adult worms live 5-7 years, continuously producing microfilariae

It takes just one mosquito bite. Indoor dogs are at risk because mosquitoes get inside. Every state in the US has documented heartworm cases. The American Heartworm Society reports that over 1 million dogs in the US are estimated to be heartworm positive.

Symptoms of Heartworm Disease

Heartworm disease progresses through four classes of severity:

Class 1 (Early):

• No symptoms, or occasional mild cough
• Heartworms detectable only by blood test
• Many dogs remain at this stage for months with no visible signs

Class 2 (Moderate):

• Persistent cough
• Exercise intolerance — tiring more quickly on walks
• Mild weight loss

Class 3 (Severe):

• Difficulty breathing
• Significant exercise intolerance
• Persistent cough, sometimes with blood
• Enlarged abdomen from fluid accumulation (right-sided heart failure)
• Fainting during activity

Class 4 (Caval Syndrome — Emergency):

• Cardiovascular collapse
• Dark brown or red urine
• Labored breathing
• Pale gums
• Without emergency surgery to remove worms, Class 4 is typically fatal

The insidious part is that heartworm disease can progress significantly before symptoms appear. By the time most owners notice something wrong, the disease has been present for months.

Diagnosis

Annual testing is recommended for all dogs, even those on preventive medication:

• Antigen test — detects proteins produced by adult female heartworms. This is the standard screening test. A positive result is confirmed by a second test.
• Microfilariae test — detects baby heartworms in the bloodstream. Used alongside antigen testing.
• Chest X-rays — assess heart size and lung damage in positive dogs
• Echocardiography — ultrasound to visualize worms in the heart and assess cardiac function

Why test if your dog is on prevention? No prevention is 100% effective if a dose is missed, given late, or vomited. Annual testing catches breakthrough infections early, when treatment is safest.

Labrador Retrievers and other active outdoor breeds may have higher exposure risk due to time spent outside during mosquito season.

Treatment: Expensive, Risky, and Necessary

Treating established heartworm disease is complex, expensive, and carries real risks. This is not a "take some pills and you're fine" situation.

The standard treatment protocol (American Heartworm Society):

1. Confirm diagnosis and stage the disease (X-rays, blood work, sometimes echocardiography)
2. Stabilize any concurrent conditions (heart failure, respiratory distress)
3. Start monthly heartworm prevention — kills new incoming larvae and prevents further infection
4. Doxycycline antibiotic course — 4 weeks of twice-daily doxycycline, which weakens heartworms by killing Wolbachia bacteria they depend on
5. Melarsomine injections (Immiticide) — the adulticidal drug. Given as a series of 3 deep intramuscular injections over 2 months. This is the only FDA-approved drug that kills adult heartworms.
6. Strict exercise restriction — for the entire treatment period and several months after. This is the hardest part for dog owners. As worms die, they break apart and travel to the lungs as emboli. Exercise increases heart rate and blood flow, raising the risk of a pulmonary embolism (dead worm fragments blocking lung blood vessels).

Treatment costs: $1,000-3,000 depending on disease severity, dog size, and geographic location. Severe cases requiring hospitalization can exceed $5,000.

Treatment risks: Dead worm fragments can cause life-threatening pulmonary embolism. Even with the slow-kill approach (melarsomine series), there's inherent risk.

Prevention: Simple and Effective

Monthly heartworm prevention kills the larval stages of heartworm before they can mature. It's safe, affordable, and nearly 100% effective when given consistently.

Oral preventives:

• Heartgard Plus (ivermectin/pyrantel) — monthly chewable, also covers some intestinal parasites
• Interceptor Plus (milbemycin/praziquantel) — monthly, covers heartworm plus four intestinal parasites
• Simparica Trio (sarolaner/moxidectin/pyrantel) — monthly, covers heartworm, fleas, ticks, and intestinal parasites
• Tri-Heart Plus — generic equivalent to Heartgard Plus

Topical preventives:

• Revolution (selamectin) — monthly topical, covers heartworm, fleas, some mites
• Advantage Multi (imidacloprid/moxidectin) — monthly topical

Injectable preventive:

• ProHeart 12 (moxidectin) — a single injection that prevents heartworm for 12 months. Given by your vet. Eliminates compliance concerns.

Cost comparison:

• Monthly prevention: $5-15/month ($60-180/year)
• Treatment: $1,000-3,000+
• Prevention is 6-50x cheaper than treatment

Year-Round Protection: Why It Matters

The American Heartworm Society recommends year-round, 12-month prevention regardless of where you live. Here's why:

• Mosquitoes can survive indoors during winter
• Climate variability means unseasonably warm days occur even in northern states
• Missing even one month creates a gap in protection
• Monthly preventives also cover intestinal parasites that are active year-round
• Consistency eliminates the risk of forgetting to restart in spring

For a complete guide to canine vaccinations and preventive care, see our dog vaccination schedule.

Heartworm in Cats

Cats can also get heartworm disease, though it presents differently:

• Cats are not natural hosts — most infections involve only 1-3 worms
• Even a single worm can cause severe illness (HARD — Heartworm Associated Respiratory Disease)
• There is NO approved treatment for heartworm in cats
• Prevention is the only option — monthly products like Revolution are recommended for at-risk cats

The Bottom Line

Heartworm prevention is one of the most straightforward decisions in pet care. The math is clear: a few dollars per month versus thousands in treatment costs and significant health risks. Talk to your vet about which preventive is right for your dog, and commit to year-round protection.

When in doubt, test annually and give prevention monthly. Your dog's heart will thank you.

FAQ

Can heartworm disease be cured?

Yes, heartworm disease can be treated, though "cured" is conditional. Treatment kills the worms, but some heart and lung damage may be permanent. Earlier treatment results in better outcomes and less lasting damage.

Can indoor dogs get heartworm?

Yes. Mosquitoes enter homes through doors, windows, and screens. The American Heartworm Society recommends prevention for all dogs, regardless of lifestyle. Indoor-only dogs are at lower risk but not zero risk.

What happens if I miss a month of heartworm prevention?

Give the missed dose as soon as you remember and contact your vet. If the gap exceeds 2 months, your vet may recommend retesting 6 months later (it takes that long for infections to become detectable). Resume monthly prevention immediately.

Is heartworm prevention necessary in cold climates?

Yes. Year-round prevention is recommended everywhere. Winter mosquito activity is unpredictable, and monthly preventives also protect against intestinal parasites. The cost of year-round prevention is minimal compared to the risk of a gap in coverage.

Can humans get heartworm?

Technically, but it's extremely rare and not clinically significant. Humans are not natural hosts — if larvae enter human tissue, they die before reaching the heart. The rare finding is an incidental lung nodule on imaging.