Heartworms in Dogs: Your Guide to Causes, Symptoms, Prevention, and Treatment


Heartworms in Dogs: Your Guide to Causes, Symptoms, Prevention, and Treatment

Heartworm disease is a serious condition that can affect dogs and is spread through mosquitoes. It can be deadly, so treatment and prevention is critical for keeping your pet healthy. sarah-m-dowdy
sarah-m-dowdy By Sarah Mouton Dowdy Medically Reviewed by Michelle Moyal, DVM Updated March 27, 2023 Medically Reviewed by Michelle Moyal, DVM Advertisement Pin FB More Tweet Email Send Text Message Print little girl hugging dog in the grass; heartworm disease in dogs
little girl hugging dog in the grass; heartworm disease in dogs Ridofranz / Getty | Credit: charlene jones / Getty

On This Page

  • What are Heartworms?
  • Causes
  • Risk Factors
  • Symptoms
  • Diagnosis
  • Treatment
  • Prevention

According to estimates by the American Heartworm Society, more than one million dogs in the United States are infected with heartworms. These nasty parasites can cause serious, permanent problems in pets, including death. And while treatment is typically effective, it comes with a high price tag, some risks, and a lot of hassle. Year-round prevention, combined with annual heartworm testing, is by far the best option for the health of your dog (and your wallet).  

What are Heartworms?

Heartworms (also called Dirofilaria immitis) are parasites that infect the heart and lungs of affected animals, causing a condition called heartworm disease. The worms look similar to cooked spaghetti and can grow to be around 12 inches long. Dogs (as well as foxes, coyotes, and wolves) are natural hosts of heartworms, meaning the parasites can mature, mate, and reproduce while living inside them. However, other animals, including cats, ferrets, sea lions—and yes, even humans!—can also be infected with heartworms. 

How Do Dogs Get Heartworms?

Mosquitos play a key role in transmitting heartworms. Jackie Kleypas, DVM, a clinical instructor at the University of Missouri College of Veterinary Medicine in Columbia, Mo., describes the process as follows:

  1. Dogs, foxes, wolves, and coyotes infected with heartworms have baby worms in their bloodstream.
  2. When a mosquito bites and feeds on these infected animals, it picks up the baby worms in the blood. Once inside the mosquito, these baby worms develop into infectious worms.
  3. When the infected mosquito bites a dog, it leaves infectious worms on the dog's skin.
  4. The worms then crawl into the bite wound left by the mosquito, enter the dog's bloodstream, and eventually end up in its heart or the blood vessels leading from the heart into the lungs. 

The American Heartworm Society notes that adult heartworms can live up to seven years in a dog, meaning each mosquito season can increase the number of worms in an infected pet. If left untreated, dogs can end up having hundreds of heartworms.

RELATED: How to Keep Your Dog Safe From Mosquito Bites (and the Diseases They Carry)

Are Some Dogs More at Risk for Heartworm Infection?

Heartworm disease has been diagnosed in every state and is constantly spreading to new regions. The Companion Animal Parasite Council (CAPC) has parasite prevalence maps that can show you the number of positive canine heartworm tests in your area. It also has a 30-day parasite forecast map that rates infection risks from low to high by state and by county. 

But even if you don't live in a high-risk area, any dog that comes into contact with mosquitos is at risk. And because mosquitos can enter homes or bite dogs on bathroom breaks, even dogs who spend most of their time inside can get the parasite. 

What Are the Symptoms of Heartworm in Dogs?

According to Kleypas, most dogs diagnosed with heartworms don't show any signs (especially if they are inactive). Annual heartworm screenings during a pet's annual exam tend to catch the parasites before they're able to cause noticeable problems. The longer a pet is infected and the more worms they have, the more likely they are to have clinical signs. Kleypas says the most commons signs include:

  • Not wanting to exercise
  • Coughing
  • Vomiting

She also says more serious symptoms of heartworm in dogs occur in dogs with a lot of worms, including:

  • Weight loss
  • Severe coughing or coughing up blood/blood-tinged material
  • Difficulty breathing
  • Pale or grey/blue gums
  • Dark or bloody urine
  • Collapse
  • Death

How Is Heartworm Disease Diagnosed in Dogs?

Because the effects of heartworm disease get worse with time, it's important to have your dog screened for heartworms at least once a year. Kleypas explains that this screening typically occurs during your dog's annual wellness exam and involves two different tests: an antigen test and a microfilariae test. Neither test is 100 percent accurate, but using them in combination can yield better results:

  • Antigen test: detects a type of protein mainly found in female adult heartworms; infection may go undetected if there are few worms, if only male worms are present, or if the infection is recent.
  • Microfilariae test: detects the presence of microfilariae (baby heartworms) in the dog's bloodstream; not all heartworm-infected dogs have microfilariae.

Annual testing is necessary for all dogs—even those on year-round heartworm prevention medication because no heartworm preventative is 100 percent effective, and mosquitoes can still infect your dog if you forget to give him his monthly dose of preventative or give it late, your dog spits out the meds when you're not looking, or he rubs off a topical preventative. 

RELATED: Simparica Trio for Dogs Might Be the Easiest Way to Protect Your Pup From Parasites

How Are Heartworms Treated in Dogs?

If your dog tests positive for heartworms, your veterinarian will first closely evaluate your dog to make sure he's healthy enough to undergo treatment, Kleypas says. This will include blood tests and chest X-rays. Some dogs with serious infections need months of supportive therapies before they're stable enough to be treated for heartworms. Your veterinarian will also ask you to restrict your dog's exercise, as physical exertion can speed up heart and lung damage in infected dogs. 

Once your dog is determined to be healthy enough for heartworm treatment, the exact course will depend on the severity of the disease.


Treatment typically involves multiple injections of melarsomine, the only approved drug that kills adult heartworms. Additional medications may also be needed to kill microfilariae, reduce inflammation, control pain, or protect against bacterial infections. At the end of treatment, your dog will be placed on a monthly heartworm preventative medication. 

Melarsomine contains arsenic, and while that's effective at killing heartworms, it can also come with some serious side effects, including, but not limited to 

  • Swelling or pain at the injection site
  • Coughing or gagging
  • Lethargy/depression
  • Fever
  • Loss of appetite
  • Vomiting
  • Lung congestion
  • Pulmonary thromboembolism (blood clot in lungs)

It's very important for dogs to remain at rest before, during, and after melarsomine treatment to reduce the risk of blood clots forming in the lungs. Your vet will discuss exercise restriction and other care guidelines, so be sure to follow them for your dog's safety.


Dogs with caval syndrome, in which the heartworms physically block the blood vessels of the heart, will need to have the worms surgically removed before they can start the normal treatment regime. Sadly, dogs who have caval syndrome rarely survive.

Success of Treatment

Kleypas says that while most dogs test negative for adult heartworms after treatment, success can be costly in terms of time, money, and inconvenience (e.g. trying to keep your high-energy dog as calm as possible). It isn't easy on the patient either. Moreover, dogs can carry the consequences of infection for the rest of their lives.

"Dogs with no signs or mild signs of disease at the time of treatment will often only appear to fully recover," Kleypas explains. "But there are often microscopic signs of permanent damage done to the dog's blood vessels years after successful treatment. A dog can be heartworm-infection negative but forever have heartworm-associated disease."

RELATED: Heartgard for Dogs: Is This the Right Heartworm Preventative for Your Pup?

How Can Heartworms Be Prevented?

The good news: "Preventing heartworm disease is far easier and less expensive than treating it," Kleypas explains. There are several year-round preventative options available, and your veterinarian will work with you to find the best one for your pet. As an added bonus, many of these medications prevent other nasty parasites like fleas, hookworms, whipworms, and roundworms. 

Whether your dog is a puppy or an adult, you will need to talk with your veterinarian before starting your dog on a heartworm preventative. Dogs over seven months of age need to be tested for heartworms first, as preventative medications can cause harm if the dog is already infected. Dogs and puppies that have just started heartworm prevention will also need to be tested more frequently. In addition, it's important to test any dogs rescued from regions where heartworms are prevalent.

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