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Heartworm in Dogs

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Heartworm disease is caused by the heartworm parasite (Dirofilaria immitis). Canine heartworm infection was first described in Italy in 1626 and has been known to exist in the Americas since 1847 (1, 2).

Since then, heartworm in dogs has been well researched, and heartworm prevention and treatment have grown exponentially thanks to veterinary medicine. 

If you’re a dog owner, heartworm disease can be a serious and life-threatening condition for your beloved pet. Here is everything you need to know about heartworm, from signs of heartworm disease in dogs to treatment and prevention methods.

What is Heartworm in Dogs?

closeup of mosquito

Heartworm is caused by a parasitic worm, Dirofilaria immitis, which is spread through bites from infected mosquitoes. Mosquitoes get infected with heartworms when feeding on the blood of an infected dog, then serve as a host for the worms, which are then passed when they bite another dog.

“It is a parasite that actually lives in the [blood vessels of the] lungs and can ‘back up’ into the right side of the heart,” says Dr. Richard Meadows, professor at the University of Missouri’s College of Veterinary Medicine. “Many dogs hide any symptoms until they are in heart failure.” 

What Do Heartworms Look Like?

adult heartworms removed from dog

Its appearance has been compared to pale spaghetti. A heartworm looks like long, sinewy strings and can grow up to a foot long and can live in the heart and blood vessels of the lungs of an infected dog

What Causes Heartworm in Dogs?

Heartworms are contracted by dogs via mosquitoes. (It doesn’t occur directly from dog to dog.) Adult female heartworms release their larvae offspring, called microfilariae, into a dog’s bloodstream. In turn, mosquitoes ingest the microfilaria when feeding on an infected dog. 

“Baby worms (microfilaria) circulate in the blood stream of dogs and get sucked up by feeding mosquitoes,” says Meadows. “In a mosquito it matures for a period of time, and then goes back to a dog when the mosquito bites another (or the same) dog.”

Luckily, heartworm is not contagious from dogs to humans. It cannot be transmitted directly from your dog to you. Humans have been rarely infected with heartworms via bites from infected mosquitoes. 

“People very rarely get lumps in their lungs from heartworms,” says Meadows. “People, like dogs and cats, get them from mosquitoes, not directly from dogs.”

Symptoms of Heartworm in Dogs

Unfortunately, there are few, if any, early signs of disease when a dog is infected with heartworms, according to the American Heartworm Society. 

And in just a couple of months, heartworms can migrate from the site of the mosquito bite into the blood vessels of the lungs. Adult-stage heartworms cannot be effectively eliminated by heartworm preventives—which is why heartworm testing and preventives are key to keeping your dog healthy.

“[The time period required] from the mosquito bite until we can detect it in dogs with blood tests is at least six months,” says Meadows. “In dogs, clinical sign development depends on [many factors, including] the number of heartworms they contract.”

Following infection, most dogs will not show any symptoms for months. Once the worm burden becomes either too high or one worm has grown large enough to start issues, symptoms of heartworm include:

  • Mild persistent cough
  • Reluctance to exercise
  • Fatigue after moderate activity
  • Decreased appetite
  • Weight loss
  • Swollen belly due to excess fluid in the abdomen (caused by heart failure)

Heart failure is the final stage of heartworm symptoms, and may present itself as caval syndrome (CS), which blocks blood flow within the heart. This includes a sudden onset of labored breathing, pale gums, and dark bloody or coffee-colored urine and is an emergency situation that can be rapidly fatal (3).

Stages of Heartworm Disease in Dogs

Heartworm is an extremely progressive disease, so understanding the heartworm timeline and stages of the disease is important.

Timeline* Outcome
Day 1 An infected mosquito bites a dog.
1-2 months The larvae migrate through the dog’s tissues and increase in length
2-3 months Sexually immature adult heartworms enter a dog’s bloodstream, making their way to the heart and pulmonary arteries.
6-7 months Now fully mature, the adult heartworms are detectable in blood testing at this time. Female worms are approximately 12 inches (25 cm) in length at this time.
7 months + Adult heartworms mate and release immature heartworms into the bloodstream. Adult heartworms can live up to 7 years in a dog. If left untreated, dogs begin showing more severe signs of the disease.

*From the time of infection

Diagnosis of Heartworm in Dogs

veterinarian checks dog with stethoscope

If you suspect that your dog has heartworm, your veterinarian can confirm it with a couple of blood tests including: 

  • Antigen testing: This test looks for heartworm proteins called antigens that can be detected if your dog has been infected. (Currently available tests can only detect infections with at least one adult female worm—meaning it won’t work if your dog has an infection with only male worms.) 
  • Microfilaria testing: This is a secondary blood test that can also aid in the diagnosis of heartworm infections. It is examined under a microscope for movement of the microfilariae, which are produced by the female.

It’s important to note that a negative antigen test doesn’t necessarily mean that your dog is heartworm-free, and false-negatives do occur in some dogs infected with heartworms. For this reason, your veterinarian may recommend performing both antigen and microfilaria tests each time. 

Dogs who are on year-round heartworm prevention should be tested once annually.  The American Heartworm Society recommends starting puppies on heartworm prevention early in life, testing them for heartworms six months after your initial visit, again six months later, and annually thereafter (4). The same goes for adult dogs who were previously not on a preventive. 

Monthly preventives should be administered every 30 days. If you skip or delay giving your dog even one dose, she will be at risk of infection. 

How to Treat Heartworms in Dogs

If your dog tests positive for heartworm antigens, your vet will create an action plan to treat your dog, which may include a stay in a veterinary hospital (5).

Heartworm Treatment for Dogs

There is only one drug, called melarsomine, that is approved by the U.S. Food and Drug Administration (FDA) for treatment of heartworm infection in dogs. It is administered via injection in a veterinary hospital. 

Melarsomine is derived from an arsenic compound, so accurate dosing and proper administration are imperative to avoid potentially serious side effects. Your dog will also need to be on crate rest for about six weeks following injection. 

Your vet may also recommend steroids, aspirin, or doxycycline in conjunction with or prior to treatment with melarsomine (4). 

Surgical Treatment Options

Surgical extraction of heartworms is a rare treatment option reserved only for extreme emergencies. For dogs who present with caval syndrome, this may be the only option, as that is a life-threatening emergency. 

How to Care for a Dog Following Heartworm Treatment

dog rests in crate

During and following treatment of an adult heartworm infection, dogs will need to undergo cage rest and drastically restricted exercise. Physical exertion increases the rate at which the heartworms cause damage in the heart and lungs and may result in life-threatening complications (5).

Approximately nine months after treatment is completed, your veterinarian should perform a heartworm test to confirm that all heartworms have been eliminated. 

You will also need to administer heartworm prevention year-round for the rest of your dog’s life.

Cost to Treat Heartworms in Dogs

“It typically takes several months to complete heartworm treatment and can cost over $1,000 if done in a medically correct fashion,” says Meadows.

The American Heartworm Society estimates the average cost of heartworm treatment (for a 40-pound dog) to be between $1,200 and $1,800 (6). This cost range includes medications, veterinary fees, lab tests, X-rays, and post-treatment preventives. This compares to only $70 to $200 for 12 months of heartworm preventive.

Heartworm Prevention for Dogs

There are many ways that you can prevent heartworm disease in your dog.

Currently, there is no vaccine for heartworm prevention, which is why heartworm preventives are used. These medications work by eliminating the immature (larval) stages of the heartworm parasite, since heartworms must be eliminated before they reach their adult stage.

There are many FDA-approved heartworm preventives that belong to a class of drugs called macrocyclic lactone (ML). These include: ivermectin, milbemycin oxime, moxidectin, and selamectin.

Preventive medications are available as a once-a-month chewable, a once-a-month topical, and a twice-a-year injection. A once-a-year injection was also recently approved for dogs. Your veterinarian should determine the best option for your dog, as many medications have the added benefit of treating a variety of other common parasites as well. 

For example, Interceptor® Plus (milbemycin oxime/praziquantel) prevents heartworm disease and treats and controls adult hookworm, roundworm, whipworm and tapeworm infections in dogs.

Preventive heartworm medications should be used as part of a broad-spectrum parasite protection plan, which should include monthly medications for tick and flea treatment. Credelio® (lotilaner) is an example of a monthly chewable that provides protection against both ticks and fleas in dogs.


Credelio Indications

Credelio kills adult fleas and is indicated for the treatment and prevention of flea infestations, treatment and control of tick infestations (lone star tick, American dog tick, black-legged tick, and brown dog tick) for one month in dogs and puppies 8 weeks and older and 4.4 pounds or greater.

Credelio Important Safety Information

Lotilaner is a member of the isoxazoline class of drugs.  This class has been associated with neurologic adverse reactions including tremors, incoordination, and seizures. Seizures have been reported in dogs receiving this class of drugs, even in dogs without a history of seizures. Use with caution in dogs with a history of seizures or neurologic disorders. The safe use of Credelio in breeding, pregnant or lactating dogs has not been evaluated. The most frequently reported adverse reactions are weight loss, elevated blood urea nitrogen, increased urination, and diarrhea.  For complete safety information, please see Credelio product label or ask your veterinarian.

Interceptor Plus Indications

Interceptor Plus prevents heartworm disease and treats and controls adult roundworm, hookworm, whipworm, and tapeworm infections in dogs and puppies 6 weeks or older and 2 pounds or greater.

Interceptor Plus Important Safety Information

Treatment with fewer than 6 monthly doses after the last exposure to mosquitoes may not provide complete heartworm prevention. Prior to administration of Interceptor Plus, dogs should be tested for existing heartworm infections. The safety of Interceptor Plus has not been evaluated in dogs used for breeding or in lactating females. The following adverse reactions have been reported in dogs after administration of milbemycin oxime or praziquantel: vomiting, diarrhea, decreased activity, incoordination, weight loss, convulsions, weakness, and salivation. For complete safety information, please see Interceptor Plus product label or ask your veterinarian.


Disclaimer: The author received compensation from Elanco US Inc., the maker of Interceptor Plus and Credelio, for her services in writing this article. 



  1. Genchi, C., Bowman, D. & Drake, J. Canine heartworm disease (Dirofilaria immitis) in Western Europe: survey of veterinary awareness and perceptions. Parasites Vectors 7, 206 (2014) doi:10.1186/1756-3305-7-206
  2. Labarthe, Norma & Guerrero, Jorge. (2005). Epidemiology of heartworm: What is happening in South America and Mexico?. Veterinary parasitology. 133. 149-56. 10.1016/j.vetpar.2005.04.006. 
  3. Heartworm in Dogs. American Heartworm Society. Retrieved from https://www.heartwormsociety.org/heartworms-in-dogs
  4. American Heartworm Society. (2018). Current Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs [PDF file]. Retrieved from https://www.heartwormsociety.org/images/pdf/2018-AHS-Canine-Guidelines.pdf
  5. Heartworm Positive Dogs. American Heartworm Society. Retrieved from https://www.heartwormsociety.org/heartworm-positive-dogs
  6. American Heartworm Society. Weigh the Costs: Heartworm Treatment vs. Heartworm Prevention [PDF file]. Retrieved from https://d3ft8sckhnqim2.cloudfront.net/images/infographics/0010-weigh-the-costs.jpg


Credelio and Interceptor are trademarks of Elanco or its affiliates. 

© 2020 Elanco.  PM-US-19-2130

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