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Heartworm disease in dogs is a very serious and potentially life-threatening disease that can cause heart failure, severe lung disease, and other serious organ damage. It’s caused by a parasitic worm called Dirofilaria immitis and is spread through the bite of a mosquito.

When a dog gets bit by an infected mosquito, they become the definitive host, which means that the worm will mature and produce offspring while living inside the dog. Adult heartworms are typically found in the heart, pulmonary artery, and adjacent blood vessels of dogs, and they can live up to five years inside a dog.

As a dog owner, it’s crucial to be aware of the signs of heartworm in dogs so that you can catch it early on and get your pup the care they need. If heartworm disease in dogs goes untreated, it will progress and can damage a dog’s heart, liver, lungs, and kidneys, and can eventually result in death.

In this blog post, we’ll be talking about what causes heartworm disease in dogs, heartworm disease symptoms in dogs, how to treat heartworms in dogs, and more. Continue reading to find out more, or use the links below to jump to a section of your choice.

What Is Heartworm Disease?

Heartworm disease is a potentially fatal disease that is caused by a worm parasite known as Dirofilaria immitis. The parasite is transmitted by mosquitoes who carry the heartworm larvae into a dog who becomes the new host for the disease. Once a dog has been infected with heartworm larvae, they will grow into adult worms and live in the blood vessels in the heart and lungs of the dog.

Graphic displaying heartworm life cycle

The life cycle of a heartworm starts when a mosquito bites an infected dog and takes in the heartworm microfilariae as it feeds. In the span of about two weeks, the microfilariae will turn into infective larvae while living inside the mosquito. 

When a mosquito bites a healthy dog, it will transmit the infective larvae through the bite wound. The larvae will move through the dog's tissues until it reaches the pulmonary arteries and heart. It can take around six to seven months for the infective larvae to mature into adult heartworms. The life cycle of heartworms is complete once the adult heartworms mate and release their offspring into the dog’s bloodstream.

Heartworm disease in dogs is not contagious, and it can only be spread through the bite of a mosquito. A heartworm can live for up to seven years inside a dog. Adult heartworms can grow up to 12 inches in length, and there can be anywhere from 1 to 250 worms living inside a dog at any time.

Graphic listing heartworm symptoms in dogs

Heartworm Disease Symptoms In Dogs

Heartworm disease symptoms in dogs will vary depending on how severe their case is, how many worms are living inside of them, and how long they’ve been infected.

In the early stages of the disease, a dog may show few or even no symptoms, so it can be difficult to detect that the disease is present. But the longer the disease continues, the more noticeable the symptoms will get. Dogs who are active or heavily affected with heartworms will show the most noticeable signs. But if your dog is relatively inactive or has just been infected, symptoms may not be so obvious.

There are a couple of key signs of heartworm in dogs that you should keep an eye out for, such as:

  • Coughing
  • Exercise intolerance
  • Failure to grow
  • Labored breathing
  • Blue or purplish discoloration of the skin and gums
  • Spitting up gum
  • Fainting
  • Nose bleeding
  • Accumulation of fluid in the abdominal cavity
  • Lethargy
  • Weight loss
  • Decreased appetite

Graphic listing diagnostic tests used for heartworm


To diagnose a dog with heartworms, your vet will conduct a series of tests, including a blood test, X-ray, antigen test, and physical exam.

The antigen test detects adult heartworms in dogs and it’s performed on a blood sample. It's the preferred method for testing because it’s easy to perform and has the most accurate results. However, it can only detect adult, female worms, so it will not be able to identify early infections in dogs. Because of this, your vet may recommend getting the test done once a year. 

If your dog tests positive for heartworms, chest X-rays and blood tests may also be done to assess the extent of heart and lung damage and the presence of heartworm-associated organ damage prior to treatment. Your veterinarian will have the best idea of which tests to perform based on your dog’s health history.


Fortunately, there are a couple of ways you can go about treating heartworms in dogs. But first, your vet will need your dog’s comprehensive medical history so that they can choose the best treatment method based on the number of worms present, the extent of the damage caused, and if they have another disease. If your dog is severely affected, they may first need medication to stabilize their heart and lung disease before they can actually be treated for heartworms. 

The main treatment for adult heartworms in dogs is with an injectable drug called melarsomine dihydrochloride. This drug will kill heartworms in a dog’s heart and adjacent vessels. A dog can either be given two or three doses, depending on the severity of their case. Most dogs will receive one injection, wait a month, and then receive the second injection. If three doses are needed, they’ll receive the third injection 24 hours after the second. Your vet may also recommend an antibiotic to fight a potential infection with the bacteria that live inside the heartworm.

Once your dog receives their injections, it’s crucial to let them rest as the worms start to die and decompose.  It can take up to a couple of months for the worms to get reabsorbed by the dog’s body, so it’s crucial that your dog does not exercise for one month after their final injection. Six months after treatment is complete, you will need to bring your dog back to the vet so they can perform another heartworm test to make sure all the heartworms are gone.
As always, the best treatment for heartworm in dogs is prevention, and thankfully there are many ways you can prevent it! You can give your dog monthly chewable pills or topical medications which are meant to prevent heartworm. You can also give them an injectable medication that should be administered every 6 or 12 months. Be sure to talk to your vet about which preventative is best for your dog.

Vet holding dog

Heartworm In Dogs: Frequently Asked Questions

What are the first signs of heartworms in dogs?

The initial signs of heartworms in dogs will differ for each dog and their individual case. Most dogs will show no or few symptoms in the early stages of the disease, but the longer the infection continues, the more serious the symptoms get. Some of the most common signs of heartworm in dogs include a cough, reluctance to exercise, fatigue, decreased appetite, and weight loss. 

Can heartworms in dogs be cured?

Fortunately, most dogs infected with heartworms can be successfully cured. Treating heartworms in dogs will involve first stabilizing the dog if they’re showing serious symptoms and then killing the worms with an injectable drug.

What happens to a dog with heartworms?

When a dog gets infected with heartworms, they can develop heart failure, lung disease, and other organ damage. A dog with a lot of heartworms may also develop blockages of blood flow within the heart, which can lead to cardiovascular collapse. Heartworm disease in dogs can be fatal if untreated, so it’s crucial to bring your dog to the vet as soon as you notice any symptoms. 

How long can a dog live after heartworm treatment?

As long as a dog is given the proper heartworm treatment, they can continue on to live a very happy and healthy life. The chances of any long-term effects of the disease are slim as long as you follow your vet’s recommendations following treatment.

Final Notes

It’s definitely scary to hear that your dog has heartworms, but fortunately, heartworms in dogs can be treated with the right medication. And if you need help getting your dog the best heartworm medicine available, you can use is an online pet telehealth service that connects pet owners with licensed veterinarians. Dutch partners with a network of highly-trained vets who are available to help with a myriad of pet conditions and can get your dog the care they need.

To get started, all you have to do is sign up online and you’ll be connected with one of our licensed veterinarians within 24 hours. And the best part is– you’ll get any medication that you’re prescribed delivered directly to your door within 7 days, so you can get your pup back to their happy and energetic selves as quickly as possible.



  1. Atkins, Clarke E. “Heartworm Disease in Dogs - Dog Owners.” Merck Veterinary Manual, Merck Veterinary Manual, 28 Feb. 2022,

  2. Medicine, Center for Veterinary. “The Facts about Heartworm Disease.” U.S. Food and Drug Administration, FDA,

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Who is Dutch?

Dutch is an online veterinary pet telehealth service, created by pet parents and board-certified veterinary specialists. We use a science-backed approach to provide pets relief for their everyday physical and behavioral health issues. Dutch connects you with licensed veterinarians over video chat and messaging to help you get care for your dog or cat quickly wherever you are — without the stress or expense of a vet visit. We also partner with pharmacies who can deliver prescription medication (in applicable states only) and over-the-counter treatments directly to your door. Dutch isn’t a veterinary practice or pharmacy, but a company that helps facilitate these services for pet parents to make veterinary care more accessible to all.

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