Heartworms in dogs could 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 continues to be diagnosed with heartworm disease, there are things you can to complete help them through it and ways to prevent it from ever happening again.

What Are Heartworms?

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

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

“Because dogs would be the natural hosts, they're in which 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 within the U.S., but they thrive in a few geographic areas, says Bruce Gordon Kornreich, DVM, a board-certified veterinary cardiologist at Cornell University College of Veterinary Medicine in Ithaca, Ny.

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

That's because mosquitoes play a large role in spreading the disease.

How Do Dogs Get Heartworms?

So, what can cause heartworms in dogs? Mosquito bites, Dr. Zaffarano says, adding that at least 70 types 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 inside it,” she explains.

Those little babies come to be infective larva, after which, they then enter in the mosquito's mouth parts.

“When [the mosquito] bites another dog or animal, the infective larvae are deposited on the surface from 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 still grow-transitioning from larvae to juvenile-while they move through the subcutaneous tissue and muscle fibers and into the dog's bloodstream toward the center and lungs, Dr. Zaffarano says. They reach their destination as soon as 67 days after transmission. At this point, the not-so-tiny worms measure 1-1 1/2 inches.

Signs of Heartworms in Dogs

So, do you know the typical heartworm symptoms in dogs? Within the disease's early stages, dogs can remain asymptomatic for a time, Dr. Zaffarano says.

“When the heart/lung burden becomes large enough, dogs may have a mild or persistent cough, fatigue and resistance to walking and exercise simply because they get tired so fast,” she says. “They can have a diminished appetite and, as a result, weight loss. They are able to create a large belly, as well as 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 pet 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 every class below.

Class I: Asymptomatic to mild heartworm disease

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

Class II: Moderate heartworm disease

  • Occasional cough
  • Mild to moderate exercise intolerance
  • Slight lack 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 within 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 awareness 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 proof of right ventricular hypertrophy often is present

Class IV: Caval syndrome

  • Sudden onset with collapse
  • Hemoglobinuria (bloody urine)
  • Respiratory distress
  • If surgical treatment is not instituted immediately, this syndrome is frequently 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 kinds of blood tests: an antigen make sure 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 the slide.”

If the test is negative, your 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 remember this that the negative diagnosis doesn’t suggest your dog is heartworm-free, she cautions. The blood tests will need to be repeated 6 months later, and again after another six months, to guarantee the dog indeed is apparent of the disease. That’s three tests in a single year. Your dog then is tested annually thereafter, she says, on the anniversary date from the initial test.

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

“The sooner it’s detected, the better the probabilities the animal will recover,” she says. “The more the problem goes, the greater the worm burden and also the more difficult the recovery.”

Treatment of Heartworms in Dogs

Heartworm treatment in dogs is really a long and complex procedure that’s expensive and very difficult around the dog, Dr. Zaffarno says. It’s dedicated to eliminating all life stages from the heartworm—microfilariae, larval stages, juveniles and adults—with minimal complications.

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

Your dog’s veterinarian determines the best course of action for the heartworm-infected patient, obviously, but this is a take a look at exactly 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 the different manufacturer.
  • Apply an EPA-registered canine topical product labeled to repel and kill mosquitoes.
  • Begin exercise restriction. The greater pronounced the signs and symptoms, the stricter the restriction.
  • If the dog is symptomatic, stabilize with appropriate therapy and nursing care, and provide prednisone for four weeks.

Day 1

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

Days 1-28

  • Administer doxycycline for a month. This cuts down on the risks related to 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 just one month to ensure pre-adult worms happen to be eliminated.

Day 61

  • Administer heartworm preventive.
  • The veterinarian will administer the first melarsomine injection deep in to the dog's back muscles.
  • Give prednisone for four weeks.
  • Decrease the dog's level of activity 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 a month.

Day 91

  • The veterinarian will administer the third melarsomine injection
  • Continue exercise restriction for 6 to 8 weeks following a last melarsomine injection

Day 120

  • The veterinarian tests for the presence of microfilariae. If positive, the dog will be given a microfilaricide and retested in four weeks.
  • Continue having 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 your dog still is positive for heartworms, the veterinarian will re-treat with doxycycline accompanied by two doses of melarsomine Twenty four hours apart.

Heartworm Prevention in Dogs

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

It begins with keeping the four-legged friend on a safe, FDA-approved heartworm medicine for dogs that contains ivermectin, milbemycin oxime, moxidectin or selamectin, Dr. Zaffarano says. The vet can prescribe these products for you personally, and they're impressive in killing heartworms in their active, pre-adult stage, she says.

“For dogs, you have three forms: a topical which goes over the dog’s neck, a chewy pill that appears like a treat, as well as an injectable dose that can last for six months,” she says. “They've the same type of heartworm preventive drug, and some brands have other things inside it, like drugs that will prevent intestinal parasites.”

The best way to treat this disease is to avoid it to begin with by utilizing heartworm medicine for dogs year-round, wherever you live, Dr. Kornreich says.

“Make sure you work very closely with your veterinarian to make sure your dog is tested and put on preventatives appropriately,” he states. “You certainly want to prevent this ailment. You don’t wish 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, that 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 cats and dogs.


What causes heartworms in dogs?

A: Heartworms in dogs come from a bite from an infected mosquito. Once 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, your 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 harm to the host dog's heart, lungs and blood vessels.


Can people get heartworms?

A: Humans could possibly get heartworms, but because we're not natural hosts for that worms, they will die out very quickly before maturing into adults and reproducing.


Are there natural home remedies for heartworms?

A: No, there aren't any natural home remedies for heartworms in dogs. Any concerned pet parent should think about 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 hard around the dog. It's dedicated to eliminating all life stages from 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 designed a complete recovery from heartworm disease. His pet parent, Monica Ericson of Cortland, Ny, wasn’t expecting a heartworm diagnosis when she took Finn towards the veterinarian for his annual exam and vaccines. She mentioned one strange symptom that recently acquired: a dry cough.

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

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

“I couldn’t believe it,” Ericson says. “I had him on flea and tick preventatives, however i wasn’t consistent in giving them to him. My vet said Finn was bitten by an infected mosquito as 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 whatsoever.

“After a year of treatments, Finn continues to be 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!”