Heartworms in Dogs: Signs, Treatment and Prevention

By: Chewy EditorialUpdated:

Heartworms in dogs

Heartworms in Dogs: Signs, Treatment and Prevention

Heartworms in dogs can be a scary thing, but when the disease is caught early and treated with heartworm medicine, it’s not a death sentence for your beloved pooch.

If your dog has been diagnosed with heartworm disease, there are things you can to do help them through it and ways to prevent it from ever happening again.

What Are Heartworms?

Heartworm in dogs

Heartworms, Dirofilaria immitis, are worms that live in the heart, lungs and blood vessels of infected dogs, says Bianca Zaffarano, DVM, a member of the primary care service at the Hixson-Lied Small Animal Hospital and director of the Wildlife Care Clinic at the Iowa State University College of Veterinary Medicine in Ames, Iowa. The adult worms look like long, white threads, and they can grow up to a foot long, she says.

Heartworms affect many mammals, Dr. Zaffarano says, but dogs are the actual hosts for the filarial organism.

“Because dogs are the natural hosts, they’re where the heartworms live, mature into adults and reproduce,” she explains. “They can infect other mammals, including wolves, coyotes, foxes, sea lions, otters, jackals, hyenas, red pandas, cats, ferrets and even humans.”

Heartworms exist in every state in the U.S., but they thrive in certain geographic areas, says Bruce Gordon Kornreich, DVM, a board-certified veterinary cardiologist at Cornell University College of Veterinary Medicine in Ithaca, New York.

“Some areas have a higher incidence of heartworms, like the southeast Atlantic coast, places where it’s generally warm and moist,” he says. “Places that support populations of mosquitoes are more likely to have problems with heartworm being highly prevalent.”

That’s because mosquitoes play a big role in spreading the disease.

How Do Dogs Get Heartworms?

So, what causes heartworms in dogs? Mosquito bites, Dr. Zaffarano says, adding that at least 70 species of mosquitoes can serve as intermediate hosts. Here’s how the little bloodsuckers do their dirty work.

“A mosquito bites an infected dog and ingests the blood that has the microfilariae—or baby worms—circulating around in it,” she explains.

Those little babies grow into infective larva, at which point, they then go into the mosquito’s mouth parts.

“When [the mosquito] bites another dog or animal, the infective larvae are deposited on the surface of the animal’s skin and enter the new host through the bite wound,” Dr. Zaffarano says.

Once they’re inside the host, the pre-adult heartworms continue to grow—transitioning from larvae to juvenile—while they move through the subcutaneous tissue and muscle fibers and into the dog’s bloodstream toward the heart and lungs, Dr. Zaffarano says. They reach their destination as early as 67 days after transmission. At this point, the not-so-tiny worms measure 1-1½ inches.

By around day 120, the worms, which now measure a whopping 10-12 inches in length, have matured into adults, mate and start having babies, Dr. Zaffarano says. The entire process—from that initial mosquito bite to adult maturation and microfilariae-making—takes 6-9 months.

Signs of Heartworms in Dogs

So, what are the typical heartworm symptoms in dogs? In the disease’s early stages, dogs can remain asymptomatic for a time, Dr. Zaffarano says.

“When the heart/lung burden becomes large enough, dogs can have a mild or persistent cough, fatigue and resistance to walking and exercise because they get tired so fast,” she says. “They can have a diminished appetite and, as a result, weight loss. They can develop a large belly, and in really severe forms, there’s a syndrome called ‘caval syndrome’ where they can collapse and suddenly die.”

The Merck Veterinary Manual—the go-to guide for veterinarians and animal care professionals—outlines four classes of heartworm disease in dogs based on physical examination, imaging techniques, urinalysis and blood tests. We’ve outlined the heartworm symptoms in each class below.

Class I: Asymptomatic to mild heartworm disease

  • No clinical or radiographic signs
  • No laboratory abnormalities
  • Subjective signs, such as loss of condition, decreased exercise tolerance or occasional cough might be seen

Class II: Moderate heartworm disease

  • Occasional cough
  • Mild to moderate exercise intolerance
  • Slight loss of condition
  • Increased lung sounds
  • Mild to moderate radiographic changes, such as right ventricle enlargement, are present
  • Lab results may show anemia (low red blood cell count) and proteinuria (excess protein in the urine)

Class III: Severe heartworm disease

  • Anemia
  • Weight loss
  • Exercise intolerance
  • Tachypnea (rapid breathing) at rest
  • Severe or persistent coughing
  • Dyspnea (difficulty breathing)
  • Hemoptysis (coughing up blood)
  • Syncope (loss of consciousness due to a drop in blood pressure)
  • Ascites (abdominal swelling due to fluid in the peritoneal cavity)
  • Severely abnormal radiographs may show ventricular hypertrophy, enlargement of the main pulmonary artery and diffuse pulmonary densities
  • Lab results indicate marked anemia, thrombocytopenia (low platelets in the blood) and proteinuria
  • Electrocardiographic evidence of right ventricular hypertrophy often is present

Class IV: Caval syndrome

  • Sudden onset with collapse
  • Hemoglobinuria (bloody urine)
  • Respiratory distress
  • If surgery is not instituted immediately, this syndrome usually is fatal

Diagnosing Heartworms in Dogs

To diagnose heartworms in dogs who aren’t on heartworm preventives, have stopped taking their preventives for a time or are changing brands or type of preventives, veterinarians first perform two types of blood tests: an antigen test and a microfilaria test to confirm the findings, Dr. Zaffarno says.

“The antigen test checks for certain proteins produced by adult female heartworms,” she says. “The microfilaria test, also known as a modified Knott test, essentially looks for the babies of the adult heartworm on a slide.”

If the test is negative, the dog is put on a preventive (more on that, below). However, because the antigen and microfilaria tests don’t detect heartworms until 5-6 months post-infection, it’s important to remember that a negative diagnosis doesn’t necessarily mean the dog is heartworm-free, she cautions. The blood tests will need to be repeated six months later, and again after another six months, to ensure the dog indeed is clear of the disease. That’s three tests in one year. The dog then is tested annually thereafter, she says, on the anniversary date of the initial test.

If there’s a positive diagnosis, the veterinarian might confirm the finding with radiography, an ultrasound or an echocardiogram, Dr. Zaffarno says. These tools also help the dog’s vet stage the severity of the heartworm disease.

“The earlier it’s detected, the better the chances that the animal will recover,” she says. “The longer the infection goes, the greater the worm burden and the more difficult the recovery.”

Treatment of Heartworms in Dogs

Heartworm treatment in dogs is a long and complicated procedure that’s expensive and quite difficult on the dog, Dr. Zaffarno says. It’s centered on eliminating all life stages of the heartworm—microfilariae, larval stages, juveniles and adults—with minimal complications.

“Actual heartworm treatment—the injections and limiting the dog’s activity and the adjunct steroids—goes to day 91, and then there are follow-ups with testing through day 365,” Dr. Zaffarno says. “That’s a full year that you’re involved.”

Your dog’s veterinarian will determine the best course of action for the heartworm-infected patient, of course, but here is a look at what the American Heartworm Society recommends for heartworm treatment.

According to Dr. Zaffarno, the three-dose protocol described below, including the preventives, is 99.9-percent effective.

Day 0

  • The veterinarian will verify the positive antigen test with microfilaria test.
  • If no microfilariae are detected, the vet will confirm with a second antigen test from a different manufacturer.
  • Apply an EPA-registered canine topical product labeled to repel and kill mosquitoes.
  • Begin exercise restriction. The more pronounced the symptoms, the stricter the restriction.
  • If the dog is symptomatic, stabilize with appropriate therapy and nursing care, and give prednisone for four weeks.

Day 1

  • Administer heartworm preventive.
  • If microfilariae are present, the veterinarian will give the dog an antihistamine and glucocorticosteroids, if not already on prednisone, to reduce the risk of anaphylaxis.
  • The dog will be observed for at least eight hours for signs of reaction.

Days 1-28

  • Administer doxycycline for four weeks. This reduces the risks associated with dead heartworms and disrupts heartworm transmission.

Day 30

  • Administer heartworm preventive.
  • Apply an EPA-registered canine topical product labeled to repel and kill mosquitoes.

Days 31-60

  • Wait for one month to ensure pre-adult worms have been eliminated.

Day 61

  • Administer heartworm preventive.
  • The veterinarian will administer the first melarsomine injection deep into the dog’s back muscles.
  • Give prednisone for four weeks.
  • Decrease the dog’s activity level even further, which means cage restriction and staying on a leash when using the yard.

Day 90

  • Administer heartworm preventive.
  • The veterinarian will administer the second melarsomine injection.
  • Give prednisone for four weeks.

Day 91

  • The veterinarian will administer the third melarsomine injection
  • Continue exercise restriction for six to eight weeks following the last melarsomine injection

Day 120

  • The veterinarian tests for the presence of microfilariae. If positive, the dog will be treated with a microfilaricide and retested in four weeks.
  • Continue with a year-round heartworm prevention program.

Day 365

  • The veterinarian tests for heartworms with an antigen test nine months after the last melarsomine injection and screens for microfilariae.
  • If the dog still is positive for heartworms, the veterinarian will re-treat with doxycycline followed by two doses of melarsomine 24 hours apart.

Heartworm Prevention in Dogs

No one wants to put their canine pal through that kind of medical treatment nightmare, right? So, how do you prevent heartworms from infecting your dog in the first place?

It starts with keeping your four-legged friend on a safe, FDA-approved heartworm medicine for dogs that contains ivermectin, milbemycin oxime, moxidectin or selamectin, Dr. Zaffarano says. Your veterinarian can prescribe these products for you, and they are highly effective in killing heartworms in their active, pre-adult stage, she says.

“For dogs, you have three forms: a topical that goes on top of the dog’s shoulder blades, a chewy pill that looks like a treat, and an injectable dose that lasts for six months,” she says. “They have the same class of heartworm preventive drug, and some brands have other things in it, like drugs that will prevent intestinal parasites.”

The best way to treat this disease is to avoid it in the first place by using heartworm medicine for dogs year-round, no matter where you live, Dr. Kornreich says.

“Make sure you work very closely with your veterinarian to make sure your pet is tested and put on preventatives appropriately,” he says. “You definitely want to prevent this disease. You don’t want to have to treat it. It’s definitely preventable.”

Some heartworm medication brands for dogs include Trifexis, Heartgard and Interceptor Plus.

Common Questions about Heartworms in Dogs

Below are the common questions about heartworm in dogs, which are answered by Dr. Zaffarano.


Are heartworms in dogs contagious?

A: Yes, heartworms in dogs are contagious. Dogs are the natural hosts of heartworms, which means the worms live, mature and reproduce inside of them. The microfilariae—or baby worms—then are spread by mosquitoes to other mammals, including dogs and cats.


What causes heartworms in dogs?

A: Heartworms in dogs are caused by a bite from an infected mosquito. When the mosquito bites the dog, tiny baby worms are transmitted from the insect to the dog. The worms then grow into adults inside the dog’s heart, lungs and blood vessels.


How long can a dog live with heartworms?

A: Without heartworm treatment, a dog will die from heartworm disease. Once mature, heartworms live 5-7 years in dogs, and they continue mating and producing offspring, increasing their numbers and causing damage to the host dog’s heart, lungs and blood vessels.


Can people get heartworms?

A: Humans can get heartworms, but because we are not natural hosts for the worms, they will die off very quickly before maturing into adults and reproducing.


Are there home remedies for heartworms?

A: No, there are no home remedies for heartworms in dogs. Any concerned pet parent should consider treatments that center on eradicating mosquito populations, like eliminating standing water sources or using mosquito traps, or preventing mosquito bites, like limiting outdoor activities to times of the day when mosquitoes aren’t feeding.


Can heartworms in dogs be cured?

A: Yes, heartworms in dogs can be cured. However, the heartworm treatment procedure is complicated, expensive and difficult on the dog. It’s centered on eliminating all life stages of the heartworm—microfilariae, larval stages, juveniles and adults—with minimal complications.

A German Shepherd's Recovery From Heartworm Disease

Finn, a 2-year-old German Shepherd mix, is one example of a dog who made a complete recovery from heartworm disease. His pet parent, Monica Ericson of Cortland, New York, wasn’t expecting a heartworm diagnosis when she took Finn to the veterinarian for his annual exam and vaccines. She mentioned one strange symptom that recently had developed: a dry cough.

“I noticed Finn was coughing occasionally, but I thought it was being caused by his throat being irritated by his collar when he walked on his leash,” she says. “Other than that, he seemed completely healthy—nice coat, good appetite, great energy.”

Ericson’s veterinarian decided to run some bloodwork. The results came back positive for heartworm.

“I couldn’t believe it,” Ericson says. “I had him on flea and tick preventatives, but I wasn’t consistent in giving them to him. My vet said Finn was bitten by an infected mosquito when he wasn’t on his preventatives, and that’s how he contracted it.”

Thankfully, Finn’s case of heartworms was mild. The veterinarian put Finn on heartworm medicine for dogs, and he had no issues with it at all.

“After a year of treatments, Finn has been cleared of heartworms and he’s doing very well,” Ericson says. “I now keep him on heartworm prevention all year long—I learned my lesson!”

By: Wendy Wilson



By: Chewy EditorialUpdated: