How Do Dogs Get Heartworm? Plus 10 Symptoms You Need to Know

Heartworm disease is a serious and potentially fatal condition that affects dogs. Caused by the parasitic worm Dirofilaria immitis, this disease attacks a dog’s heart, lungs, and blood vessels, leading to severe complications if left untreated, according to the Cornell Richard P. Riney Canine Health Center.

Many pet owners have heard of heartworm, but not everyone understands how dogs get infected, what symptoms to watch for, or how to prevent it. Since heartworms spread through mosquito bites, any dog—indoors or outdoors—is at risk. The good news is that heartworm disease is preventable with the right care.

How Do Dogs Get Heartworm?

Understanding how heartworm is transmitted is the first step in protecting your pet. Heartworm disease spreads through mosquito bites, not from dog to dog, so multi-dog households don’t need to worry about direct transmission.

When a mosquito bites an infected animal, it picks up microscopic heartworm larvae known as microfilariae. These larvae mature inside the mosquito, and then the mosquito passes them to another dog during a later bite.

Once inside the dog’s bloodstream, the larvae grow over several months into adult worms—sometimes up to a foot long. These adult worms take up residence in the heart, lungs, and arteries, causing irreversible damage if untreated.

While dogs in warm, humid climates face a higher risk, heartworm disease has been reported in all 50 states. According to the American Heartworm Society (AHS), more than one million dogs tested positive for heartworm in 2022. Even mostly indoor dogs remain vulnerable, as it only takes one mosquito bite.

10 Heartworm Symptoms in Dogs

Heartworm symptoms can be subtle at first, becoming more obvious as the disease progresses. Watch for the following warning signs:

• Persistent, soft, dry coughing
• Fatigue after mild exercise
• Weight loss or appetite changes
• Difficulty breathing
• Swollen abdomen from fluid buildup
• Fainting or collapsing
• Nosebleeds
• Pale gums
• Rapid or irregular heartbeat
• Sudden death in severe, untreated cases

Since early detection is vital, schedule routine vet checkups and book an appointment if you notice any concerning symptoms.

How Is Heartworm Diagnosed in Dogs?

If heartworm is suspected, veterinarians generally use a blood test to detect heartworm proteins released by adult female worms. Additional diagnostics may include:

• Microfilaria testing to detect baby heartworms in the bloodstream
• Chest X-rays to evaluate heart and lung damage
• Echocardiogram (heart ultrasound) to assess heart function

Even if your dog appears healthy, annual heartworm testing is recommended to ensure early detection.

Heartworm Treatment for Dogs

Treating heartworm can be challenging, expensive, and requires careful activity restriction. A typical treatment plan may include:

• Melarsomine injections to kill adult worms
• Antibiotics (like doxycycline) to weaken heartworms and reduce inflammation
• Steroids and supportive medications to manage side effects and protect the lungs

Strict exercise restriction is necessary during treatment. As adult heartworms die, they can obstruct blood flow. Keeping your dog inactive reduces the risk of life-threatening complications.

In severe cases, surgery may be required to remove adult heartworms. Even with successful treatment, permanent heart or lung damage may occur, which is why prevention is always the safer option.

Heartworm Prevention for Dogs

The best defense against heartworm disease is consistent, year-round prevention. Common options include:

Chewable Preventatives

These monthly chewables kill heartworm larvae and also protect against other parasites. Dogs as young as six weeks old can typically start on these medications. Before use, your veterinarian should test your dog for existing heartworm infections.

Topical Preventatives

Topical treatments provide broad-spectrum parasite protection and prevent heartworm disease when applied monthly. A prescription may be required.

Injectable Prevention

A veterinarian-administered injection can provide up to 12 months of continuous heartworm protection. This option is typically recommended for dogs one year of age and older.

Additional Protection

To reduce mosquito exposure:

• Keep dogs indoors during peak mosquito times (dawn and dusk)
• Use pet-safe mosquito repellents
• Maintain clean outdoor areas to reduce mosquito breeding

For year-round parasite protection options, explore Dog Medications, Dog Flea and Tick Protection, and Dog Health and Wellness.

When to See a Veterinarian

Schedule a veterinary visit if:

• Your dog tests positive for heartworm
• Your dog shows signs such as coughing, fatigue, or breathing difficulties
• Your dog has not had a heartworm test within the past year

Routine screenings are essential because no prevention method is completely foolproof.

Keeping Your Dog Heartworm-Free

Heartworm disease is serious and potentially life-threatening, but prevention makes all the difference. Because dogs get heartworm from mosquito bites, year-round preventative care is crucial—even for indoor dogs.

Talk with your veterinarian about the best heartworm preventative for your dog, stay consistent with dosing, and maintain regular testing. Keeping your pup protected from heartworm is one of the most important steps you can take to support a long, healthy life.

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