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 called “Dirofilaria immitis,” this disease attacks a dog’s heart, lungs, and blood vessels, leading to severe health complications if left untreated, according to Cornell Richard P. Riney Canine Health Center.

Many pet owners have heard of heartworm, but not everyone knows how dogs get infected, what symptoms to watch for, or how to prevent it. Because heartworms are spread through mosquito bites, any dog—whether they live indoors or outdoors—is at risk. Luckily, heartworm disease is preventable with the right care.

How do dogs get heartworm?

Understanding how dogs get heartworm is the first step in protecting them. First off, heartworm disease spreads through mosquito bites—not from dog to dog. So don’t be afraid if you are a multi-dog household and one of your pups gets it. 

When a mosquito bites an infected animal, it picks up microscopic heartworm larvae (microfilariae). 

These larvae mature inside the mosquito before being passed on to another dog through a bite. Once inside the dog’s bloodstream, the larvae grow over several months, eventually developing into adult worms that can reach up to a foot long. These worms take up residence in the dog’s heart, lungs, and arteries, causing serious damage. 

Dogs in warm, humid areas with high mosquito populations are most at risk. However, heartworm disease has been reported in all 50 states by Parasites and Vectors research in BioMed Central, Springer Nature, so don’t assume you’re out of the woods if you’re in a cold-weather location. According to the American Heartworm Society (AHS), there were over 1 million heartworm-positive dogs in 2022.

Dogs who stay mostly indoors aren’t completely safe either. It only takes one mosquito bite to become infected.

10 heartworm symptoms in dogs

So, how do you know your dog has fallen victim to a heartworm larvae-carrying mosquito? Heartworm disease is tricky because symptoms don’t always appear right away. Early infections may go unnoticed, but as the disease progresses, signs become more severe.

Here’s what to look out for: 

• Persistent coughing: A soft, dry cough is often one of the first warning signs as heartworms irritate the lungs.
• Fatigue after exercise: If your dog gets winded easily, even after short walks, it could be due to reduced heart and lung function.
• Weight loss & appetite changes: Dogs with heartworm may eat less and begin losing weight without explanation.
• Difficulty breathing: As heartworm disease advances, inflammation and fluid buildup in the lungs make breathing harder.
• Swollen abdomen: In severe cases, heartworms cause fluid accumulation in the belly, making it appear bloated.
• Fainting or collapsing: If heartworms block blood flow, dogs may experience sudden weakness, collapse, or fainting episodes.
• Nosebleeds: Some infected dogs may develop nosebleeds due to damaged blood vessels.
• Pale gums: Poor circulation from heartworm disease can cause gums to lose their healthy pink color.
• Rapid or irregular heartbeat: Heartworms strain the cardiovascular system, leading to abnormal heart rhythms.
• Sudden death: In extreme cases, untreated heartworm disease can lead to fatal heart failure.

Since early detection is critical, schedule regular vet checkups—especially if your dog shows any of these signs. Don’t skip your dog’s annual physical, but definitely make an appointment if you see anything abnormal. 

How is heartworm diagnosed in dogs?

If you do have suspicions, your veterinarian can use a blood test to detect heartworm disease. A heartworm antigen test will look for proteins released by adult female heartworms. If the test is positive, additional diagnostic tools may be used to confirm that heartworm is present. 

A vet might also use a microfilaria test, which checks for microscopic baby heartworms in the bloodstream.

A chest X-rays can also assess heart and lung damage, as can an echocardiogram, an ultrasound of the heart. An echocardiogram can provide a vet a detailed view of the dog’s heart function.

Just keep in mind that even if your dog doesn’t show symptoms, annual heartworm testing can be administered to be sure.

Photo by Karsten Winegeart on Unsplash

Heartworm treatment for dogs

So what should you do if your dog does have heartworm? We won’t sugarcoat it for you. Treating heartworm disease can be complex, expensive, and requires strict activity restrictions to prevent complications.

A standard treatment plan may include the following options:  

• Melarsomine injections: A powerful drug that kills adult heartworms that will need to be administered over multiple doses.
• Antibiotics (like Doxycycline): Helps weaken heartworms while reducing inflammation.
• Steroids & supportive medications: Manages side effects of heartworm treatments like the ones mentioned above, and reduces lung damage.

During heartworm treatment, there will need to be strict exercise restrictions for your dog. As heartworms die, they can block blood flow. Dogs must stay calm and inactive to prevent life-threatening complications.

It’s also important to note that in severe cases, surgical removal of heartworms may be the only option. Unfortunately, even after successful treatment, some dogs may suffer permanent heart and lung damage. This is why prevention is far better than treatment.

Heartworm prevention for dogs

The best way to protect your pup from heartworm disease is consistent, year-round prevention. 

Vets recommend the following preventative products: 

Chewable tablet

Heartgard Plus Heartworm, Hookworm, Roundworm Prevention Chewable Tablet for Dogs

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This vet-recommended pill kills heartworm larvae. Give it to your dog once per month to treat and control heartworm, roundworms, and hookworms. Choose the real-beef chew flavor that dogs as young as 6 weeks old will love. When given to dogs consistently, this chewable tablet provides year-round protection. 

What to consider: Before giving this product to your pet, your dog should be tested for existing heartworm infections. While side effects are rare, some digestive and neurologic reactions have been reported.

Topical treatments

Selarid Selamectin Flea, Tick, Mite, and Mange Topical for Dogs & Cats

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The monthly topical parasiticide Selarid is meant for dogs ages 6 weeks and older (plus, cats 8 weeks and older). It aims to prevent heartworm disease, but also kills adult fleas, prevents flea eggs from hatching, and treats ear mites, sarcoptic mange, and certain intestinal parasites. When applied directly to the skin, Selarid provides broad-spectrum protection against common parasites. 

What to consider: This treatment will require a prescription from a veterinarian. In addition, while it’s generally well-tolerated by both dogs and cats, side effects like hair loss at the application site, vomiting, or rare neurological reactions may occur. As always, pet owners should consult their vet before use.

Injectable prevention

ProHeart 12

FDA-approved ProHeart 12 is an injectable heartworm prevention product that provides year-long protection with one single dose. It works by slowly releasing the antiparasitic drug moxidectin over 12 months. 

What to consider: While this product has undergone extensive safety testing and adverse reactions have been rare and typically mild, some pet owners have reported vomiting, lethargy, and diarrhea from their dogs after use. You should also only consider ProHeart 12 for dogs one year old and older. As always, pet owners should consult their veterinarian to determine if this is the right choice for their dog.

These medications work by killing immature heartworms before they can grow into adults.

You should also keep mosquitoes away from your dog regardless. Try to keep your dogs indoors during the peak mosquito hours of dawn and dusk. You can also use pet-safe mosquito repellent, like Flys-Off Insect Repellent for Dogs & Cats and Absorbine UltraShield EX Insecticide & Repellent, when you bring your dog outside. 

When to see a veterinarian

While you should always take your dog in for regular vet visits, you should schedule an appointment if:

• Your dog tests positive for heartworm. And in this case, immediate treatment is essential.
• Your dog shows symptoms like coughing, fatigue, or breathing issues.
• Your dog hasn’t had a heartworm test in over a year, and you want the reassurance. 

Remember, no prevention method is 100% foolproof, so regular testing ensures early detection and quick treatment if needed.

Heartworm disease is dangerous and can be life-threatening, but it’s also avoidable and preventable with proper care. 

Since dogs get heartworm from mosquito bites, prevention (which is cheaper than treatment if your dog does get heartworm) is critical year-round—even for indoor dogs. Regular testing and vet checkups are essential. Talk to your veterinarian about the best heartworm preventative products for your dog, and stay proactive. Keeping your pup heartworm-free is one of the best ways to ensure a long, healthy life.