Understanding heartworms in dogs

June 7, 2024 - 4 min read

The information in this article has been reviewed by Dr. Rebecca MacMillan on June 7, 2024 . Although it may provide helpful guidance, it should not be substituted for professional veterinary advice.

A hand feeding a square brown tablet to a curly haired dog

Overview of heartworm disease

Heartworm disease is a serious and potentially fatal condition in dogs caused by parasitic worms living in the heart and lungs.

We’ll explain what heartworms are, the symptoms of heartworm disease, how it's diagnosed, the available treatments, and preventive measures, all to help you protect your furry best friend.

What are heartworms in dogs?

A small, fluffy dog is being examined by a veterinarian wearing a white coat and blue gloves. The vet is using a stethoscope to listen to the dog's chest. A clipboard with a pen is on the examination table in the foreground.

Heartworms (Dirofilaria immitis) are parasitic worms that can infect dogs, causing severe health issues.

Life cycle of heartworms

Heartworms have a complex life cycle involving mosquitoes as intermediate hosts.

When a mosquito bites an infected dog, it ingests microfilariae (baby heartworms). These microfilariae develop into larvae within the mosquito.

When the mosquito bites another dog, it passes on the larvae, which then mature over several months into adult heartworms in the dog's heart and lungs.

These adult worms mate, producing microfilariae that enter the bloodstream, completing the heartworm life cycle.

How heartworms are transmitted

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Heartworms are primarily transmitted when an infected mosquito bites a dog and deposits heartworm larvae into the dog's bloodstream.

Common environments where transmission is likely

Heartworm transmission is most common in warm, humid environments where mosquitoes thrive. Regions with standing water, such as ponds and swamps, are particularly high-risk areas.

Symptoms of heartworm disease in dogs

Heartworm disease can manifest in various ways, depending on the stage of infection. It can take several years before symptoms are seen, by which time the condition is usually well advanced.

Here are the symptoms to watch for at different stages of the disease:

Stage Symptoms
Initial signs First signs could be a mild cough and fatigue after moderate activity. Note that dogs can be asymptomatic for years
More advanced heartworm symptoms Persistent cough, difficulty breathing, unexplained weight loss, swollen belly due to fluid accumulation
Severe heartworm cases Heart failure, blockage of blood flow within the heart, sudden death

Remember, the sooner you recognize these symptoms, the sooner your vet can intervene, which can significantly improve your dog’s prognosis. Consult your vet ASAP if you notice any of these signs!

Diagnosis of heartworm disease in dogs

Heartworm disease is diagnosed through blood tests that detect the presence of heartworm proteins. Your veterinarian may also use X-rays, ultrasounds, and other tests to assess the severity of the infection.

Treatment options for heartworm disease

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Treating heartworm disease can be a complex process, and it all depends on the severity of the infection. Early detection and timely intervention are crucial for a successful outcome.

Unfortunately, by the time most dogs show symptoms of heartworm disease, significant damage will have already occurred.

Unfortunately, by the time most dogs show symptoms of heartworm disease, significant damage will have already occurred.

This includes scarring of the heart and lungs and narrowing of the blood vessels. That's why it's so important to make sure to prevent heartworm from occurring (more on that later) in the first place.

Here are some of the basic steps your vet may take to treat heartworm disease:

Pre-treatment evaluation

Before starting treatment, your vet will examine your dog's overall health and the severity of the infection.


Treatment usually involves a series of injections to kill the adult heartworms, along with medications to eliminate microfilariae and reduce inflammation.

Post-treatment care

After treatment, dogs need rest and limited activity to prevent complications. Follow your vet's instructions for follow-up appointments!

How to prevent heartworm disease

Happy looking corgi leaping through a field

Preventing heartworm disease is far easier and more cost-effective than treating it.

Here are some key preventive strategies to follow if you live in an area where heartworm infection is prevalent (or if your pet is traveling to a country where they are common).

Keep up with regular veterinary check-ups

Big white dog sitting on the veterinarian scales

Routine check-ups give your vet the opportunity to monitor your pup for signs of parasites like heartworms. Even if your dog's on preventive medications, they'll still need to have a heartworm test annually.

Use preventive heartworm medications

poodle with eyes obscured, being offered a monthly heartworm chew by a human hand

Don't underestimate the power of monthly heartworm preventives! These medications kill heartworm larvae before they mature into adults. Ask your vet what they recommend for your dog.

You'll need a prescription to order it, and typically vets will require that you bring your dog in for an exam so they can test them for heartworm first.

Environmental control

A middle aged women enjoys a morning run on a beautiful sunny day with her pet, the dogs appreciating the time outside. They jog on a forest trail, the setting sun casting an orange glow on the scene.

Reduce your dog's exposure to mosquitoes by getting rid of standing water in your yard and using physical bug repellents, lights, and citronella candles outside.

Look into dog-safe bug repellent

There are plenty of dog-safe mosquito and bug repellents on the market. But before you buy and apply, ask your vet what they recommend for your dog. Even "natural" ingredients can irritate your dog's skin.

Myths and misconceptions about heartworms

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Don't fall prey to the myths and misconceptions about heartworm disease! When you know the facts, you won't underestimate the risk that teeny little mosquitoes can pose to your pup.

Only outdoor dogs are at risk

Heartworm disease can affect both indoor and outdoor dogs, especially if you camp, hike, or live in a mosquito-prone area (or location with temperate weather that allows mosquitos to thrive much of the year). Mosquitoes can also easily find their way indoors, putting all dogs at risk.

Heartworm disease is not serious

Heartworm disease is severe and potentially fatal. Left untreated, it can cause lasting damage to the heart, lungs, and arteries.

Heartworm prevention is too expensive

Preventive measures are cost-effective compared to the high cost and risk of treating an established heartworm infection. A non-insurance wellness plan (like ours!) can help you pay for heartworm prevention meds.

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ManyPets Wellness Plan for Dogs

Save on preventative care for your pup

The non-insurance ManyPets Wellness Plan can help reimburse you for the cost of routine and preventative care, including routine vet visits and certain over-the-counter products.

Dog being cleaned


Heartworm disease is a serious threat to dogs, but with prevention, early detection, and appropriate treatment, it can be managed effectively. 

Regular veterinary care and preventive measures are key to keeping your dog healthy and the risk of developing heartworm disease low!

Of course, you can’t always anticipate every accident or illness that might befall your dog. That’s where pet insurance comes in.

It’s designed to help reimburse you for those unforeseen costs for vet treatments and emergencies.* Get your quote today:

Get a risk-free dog insurance quote

*Pre-existing conditions excluded. See your policy for details.

Leanna Zeibak
Content Manager

Leanna Zeibak is a Content Manager at ManyPets. In her spare time, she paints pet portraits and bakes far too many chocolate chip cookies.