Heartworm in Dogs: Everything You Need to Know

Keep these unwelcome guests away from your pup.

Team LemonadeTeam Lemonade
heartworm symptoms dogs

We hate to break it to you, but heartworm is a lot less cute than it sounds. 

Heartworm disease, also known as dirofilariasis, is a serious but super common and entirely preventable condition in dogs.

Every pet owner wants their pup to live a long and healthy life. Understanding what heartworm disease is, as well as the basics of heartworm prevention and treatment, are essential for any pet parent. 

  • Heartworm is spread through mosquito bites, which enable parasite larvae to travel through an animal’s bloodstream and mature into adult worms, where they eventually infect a dog’s heart.
  • Symptoms of heartworm include mild, persistent cough, difficulty breathing, low energy, decreased appetite, listlessness, and weight loss.
  • Treatment for heartworm in dogs takes several months to mitigate against dangerous side effects, and usually involves three separate injections.
  • A full round of treatments for heartworm can average around $2,500–$3,000.
  • Your best bet: Heartworm can be easily prevented by regularly giving your dog chewable tablets, topical treatments, or injections.

What is heartworm disease? 

Heartworm disease is a life-threatening illness in dogs and cats caused by a parasitic worm called Dirofilaria immitis. That sounds like a comic book villain we don’t want to meet.

Once your pup is infected (via a mosquito bite) the parasitic larvae mature and breed in the bloodstream and eventually make their way to the heart, pulmonary artery, and adjacent large blood vessels. This can potentially lead to heart failure, lung disease, and even death if left untreated.

What are the symptoms of heartworm in dogs? 

Symptoms of heartworm include:

  • Mild, persistent cough
  • Difficulty breathing
  • Low energy
  • Decreased appetite
  • Listlessness and weight loss
  • As the disease progresses, dogs may develop a swollen-looking abdomen due to excess fluid build-up due to heart failure

The severity of the disease depends on how many worms are living inside the dog, and how long the dog has been infected.

While dogs with few worms and lower activity levels may show very few or no signs of heartworm, active dogs infected with many worms are likely to show more severe symptoms. Dogs with other health problems are also more likely to show more severe symptoms, as heartworm can cause additional complications.

If a large number of heartworms exist in a dog’s heart, blood flow blockages can lead to cardiovascular collapse, a condition known as caval syndrome. If you notice your dog is suddenly having an extremely hard time breathing, or has dark urine and pale gums, take them to the vet immediately to be examined. It‘s possible your dog might need surgery at this stage in order to stabilize and cure them of the disease.

How do dogs get heartworm? 

Infected mosquitos are responsible for spreading heartworm from animal to animal. As if we didn’t already have enough reason to despise these tiny bloodsuckers…

When a mosquito bites an animal infected with heartworm, heartworm larvae that live in the bloodstream of the host, called microfilariae, are transmitted to that mosquito. The larvae become infectious over the next two weeks. When that same mosquito bites a new host—whether it be a dog, cat, or coyote—it transmits the infectious microfilariae into the animal‘s bloodstream through its bite. (FYI, humans are only affected in very rare instances.)

It can then take up to seven months for the parasites to mature into adult worms and breed inside the dog‘s body. Without treatment, they‘ll eventually infect the dog’s heart.

However, detection can be difficult at this stage because most dogs won‘t show any symptoms. Some dogs may even live for years with heartworms, without showing any symptoms at all. 

How is heartworm in dogs diagnosed?

Heartworm is diagnosed through a blood test, which can be routinely taken during your dog’s annual wellness exam.

On a blood smear, veterinary professionals can see the microfilaria (think: very young parasites) swimming in your pup’s blood. And yes, that’s just as creepy as it sounds. See for yourself, if you dare.

If your vet sees something in the blood sample, they might verify the diagnosis through an ultrasound, x-ray, and/or a CBC (complete blood bound)/blood chemistry test.

It’s super important to test your dog for heartworm annually, because if a dog has early-stage heartworm, they might test negative one year, and then test positive the next once the worms have matured. Even if your pup is on heartworm prevention medication, their existing heartworm infection could still progress, putting your dog’s health in danger if left untreated.

How is heartworm disease in dogs treated? 

Since it can take time for dog’s to show symptoms, by the time heartworm disease is detected, it’s important to begin treatment as quickly as possible.

Veterinarians treat heartworm disease in dogs with an injectable medication called Melarsomine, the only FDA approved medication to treat heartworms in dogs. This heartworm treatment kills adult heartworms, and has a 95% success rate of treating dogs who get the three-part injection.

According to the American Heartworm Society, the gold standard of treatment for heartworm involves a several month process in order to reduce the risk for serious complications and side effects:

  • Day 1–30. Pretreatment with antibiotics 
  • Day 31–59. No treatment 
  • Day 60. First dose of Melarsomine
  • Day 61–89. No treatment 
  • Day 90. Second injection of Melarsomine 
  • Day 91. Third and final injection of Melarsomine 

Don’t be surprised if your vet asks you to leave your dog at the office overnight on days of treatment to monitor any negative reactions to the medication.

In severe cases of heartworm disease in dogs, surgery may be required to manually remove the heartworms from your dog’s heart or arteries. However, dog owners need to evaluate if the risks are worth it, as this surgery is very dangerous. At this stage, owners can also consider palliative care to keep their dog comfortable.

How much does it cost to treat heartworm in dogs?

At Lemonade’s in-house vet’s Dr. Stephanie Liff’s New York City practice, a full round of treatments and hospitalizations and treatments for heartworm will set you back around $2,500–$3,000.

If you have your pup covered with pet insurance, like coverage offered by Lemonade, diagnostics, procedures, and medications related to a new case of heartworm would likely be covered. Getting your pup covered before the first sign of illness can help take the bite out of big-ticket vet bills down the line.


How can you prevent heartworm disease in dogs?

Heartworm disease is the last thing any pet owner wants for their dog, but luckily it can be easily prevented. It‘s imperative to bring your dog to the vet for routine checkups and provide them with heartworm prevention regularly. 

Preventative medications can be easily and effectively given to your dog every month. They’re administered either through a chewable tablet (Sentinel or Heartgard) or a topical liquid (Revolution) applied between the dog‘s shoulder blades. There’s also an injection available, Proheart, which your vet can give your pup every six or 12 months.

These methods are extremely effective at preventing heartworm, and are typically available for purchase from your vet’s office or from your local pet store (except for the injection, of course, which needs to be done by a doctor).

With Lemonade pet insurance, you can add the Preventative package to your base accident & illness policy to cover things you’re probably already paying for towards your pet’s ongoing care. That includes things like an annual heartworm test, as well as your pet’s annual wellness exam, three important vaccinations, and a blood and fecal test.

If you’re looking for even more coverage for your dog, Lemonade’s Preventative+ package includes coverage for heartworm prevention, as well as routine dental cleaning and flea prevention.

Cover your pet

Before we go… 

When it comes to protecting your dog from heartworm, defense is the best offense against this invasive disease.

By taking your pup in for regular check-ups and giving them regular heartworm prevention medication, your furry friend will be set up for success to enjoy a long, healthy, worm-free life.

And hey, now that you’ve made it to the end of this article—and subjected yourself to a lot more gnarly medical information than you may have liked—go relax and snuggle with your pup.


Please note: Lemonade articles and other editorial content are meant for educational purposes only, and should not be relied upon instead of professional legal, insurance or financial advice. The content of these educational articles does not alter the terms, conditions, exclusions, or limitations of policies issued by Lemonade, which differ according to your state of residence. While we regularly review previously published content to ensure it is accurate and up-to-date, there may be instances in which legal conditions or policy details have changed since publication. Any hypothetical examples used in Lemonade editorial content are purely expositional. Hypothetical examples do not alter or bind Lemonade to any application of your insurance policy to the particular facts and circumstances of any actual claim.