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Home > Ask A Vet > Heartworms in Dogs: Signs, Causes & Treatments (Vet Answer)

Heartworms in Dogs: Signs, Causes & Treatments (Vet Answer)

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Dr. Stacie Grannum

Veterinarian, DVM

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Foot-long spaghetti-like worms in your dog’s heart and lungs may sound like something from a science-fiction novel, but this parasite is real and dangerous. It is transmitted from one infected animal to another susceptible animal by a mosquito feeding on blood. When an adult worm, this parasite damages the heart and lungs of its host, which can lead to death. Dogs are quite susceptible to heartworm infections. Annual blood testing and year-round heartworm prevention remain the best measures that you can take to keep your dog safe from heartworm disease.


What Are Heartworms in Dogs?

Spread by mosquitoes, parasitic worms known as Dirofilaria immitis can cause severe and progressive damage to the heart, lungs, and blood vessels of susceptible animals. The worms can cause heart failure and lung disease, damage other organs like the liver and kidneys, and can be fatal. Heartworm disease affects pets like dogs, cats, and ferrets. Wild animals can be important carriers in the spread of heartworm disease, and it has been known to occur in foxes, coyotes, wolves, seals, sea lions, and raccoons.

Adult heartworm dirofilaria removed from a dog in a kidney dish
Image Credit: vitrolphoto, Shutterstock

What Are the Signs of Heartworms in Dogs?

There may be few to no signs of heartworm disease in dogs in the early stages of infection. Active dogs, dogs with a heavy worm burden, and those with concurrent health issues may show more severe signs.

As the disease progresses, dogs may show signs like:
  • Mild but persistent cough
  • Exercise intolerance
  • Reluctance to exercise
  • Fatigue after moderate exercise
  • Decreased appetite
  • Weight loss

Dogs with worsening signs of heartworm disease may develop a swollen belly from ascites (excess fluid in the belly) due to heart failure. If the heartworms become too numerous, they can block the flow of blood in the heart—similar to a hairball blocking a drain pipe—which causes a life-threatening loss of circulation known as caval syndrome. Signs include the sudden onset of labored breathing or respiratory distress, weakness, pale gums, and dark bloody urine. This blockage is fatal if it is not surgically removed as soon as possible.

What Are the Causes of Heartworms in Dogs?

While heartworms are not directly contagious from one animal to another, they are spread by an intermediate host: mosquitoes. Microfilaria, the microscopic immature (larvae) worms from adult females, circulate in the bloodstream. A mosquito feeding on the blood from an infected animal can pick up the microfilaria, which then mature in the mosquito over 10–14 days. The infected mosquito is capable of passing on the microfilaria to another susceptible animal during feeding. The microfilaria mature into adult worms over a 6-month period in dogs. Once mature, male and female worms mate and produce offspring. If your dog is not tested and treated, there could be hundreds of heartworms living inside your pet. Incredibly, adult heartworms can live for 5–7 years in dogs! Along with producing many offspring, this long lifespan can contribute to the increased heartworm burden that your dog can acquire each year during mosquito season.

a sick vizsla dog lying on a dog bed at home
Image Credit: Edgar Feliz, Shutterstock

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How Do I Prevent Heartworm Disease in My Dog?

The first step in heartworm prevention is having your dog tested. This can be done in the veterinarian’s office; the vet will take a small blood sample from your dog and use a SNAP test to detect adult heartworm proteins. Other veterinarians may prefer sending your dog’s blood sample to an outside diagnostic laboratory for testing. The sample from your dog can also be viewed under a microscope to look for circulating live microfilaria.

Since it takes approximately 6 months for the microfilaria to mature into adult worms and for the heartworm antigens to test positive in a blood test, dogs under 7 months of age can start heartworm prevention without being tested first. However, they will need to be tested 6 months after their first visit, tested again 6 months after that, and then annually thereafter. Dogs over 7 months of age will need to be tested before starting heartworm prevention and again 6 months later and then annually.

Annual testing is necessary to ensure that heartworm prevention is keeping your dog safe. Your veterinarian can help you decide which heartworm preventive is the safest and most effective for your dog. Some medications also contain compounds to kill fleas, ticks, and other gastrointestinal parasites. Heartworm preventives can be purchased from your veterinarian.

vet holding a papillon dog in his arms
Image By: Di Studio, Shutterstock

How Do I Care for a Dog With Heartworms?

If your dog tests positive for heartworm disease, your veterinarian may want to use a different test to confirm that your dog absolutely has heartworms. The treatment of heartworm disease can be expensive and may involve multiple medications and protocols, so your veterinarian wants to be sure treatment is needed. Your dog may have to be hospitalized and stabilized prior to starting treatment if they are showing clinical signs of heartworm disease. Restricting your dog’s exercise and activity will decrease the damage that the heartworms are causing to their heart and lungs. It will also help lessen the chance of a dangerous blood clot (pulmonary embolism) clogging the blood flow in your dog’s lungs when the worms die and break apart due to treatment.

Once your dog is stabilized, your veterinarian may decide that it’s time to start treatment. This may involve several steps. The American Heartworm Society recommends that vets administer a heartworm preventive for 2 months prior to starting treatment with the injectable drug melarsomine, the only FDA-approved drug for the treatment of heartworms in dogs.

Dogs with only mild clinical signs often recover quite well. Those with a more severe form of the disease can also be successfully treated, but they may be more prone to complications.

veterinarian checking up a golden retriever dog using stethoscope
Image By: Ground Picture, Shutterstock


Frequently Asked Questions (FAQs)

What Happens If I Forget to Give My Dog Their Monthly Dose of Heartworm Preventative?

Although heartworm prevention is effective, any missed doses can leave your dog vulnerable to infestation. If your dog misses a dose for one or more months, you should contact your veterinarian right away. Your dog should restart their monthly preventive, be tested for heartworm as soon as possible, and then be tested 6 months later to ensure that they are heartworm free. Testing is continued on an annual basis.

Can Cats Get Heartworms?

Yes, cats can get heartworms, but the disease occurs differently in cats than in dogs. Cats are not a natural host of heartworms like dogs are, and most worms do not survive long enough to mature. Therefore, cats may only have one to three adult heartworms at a time. With so few worms, many cats go undiagnosed. However, the larval stage of the worms can still cause damage to a cat’s lungs. This is known as heartworm-associated respiratory disease. Prevention is the only way to keep your cat safe from heartworms, as treatment modalities for dogs cannot be used in cats (may be fatal).

What Forms of Heartworm Preventives Are Available for Dogs?

Heartworm preventives are available orally, topically, or as an injectable. Oral and topical preventives may need to be given on a monthly schedule. Injectable preventives may need to be given every 6–12 months, depending on the medication.

dog with chewable to protect and treat from heartworm disease
Image By: ThamKC, Shutterstock

I Live in the Desert. Does My Dog Need Heartworm Prevention?

Yes, all dogs should be on a year-round prevention program, even those in desert regions. Heartworm disease has been found in all 50 states. Additionally, human-made water sources (fountains, golf courses, etc.) and wildlife like foxes and coyotes can be reservoirs for mosquitoes and thus, infection.

How Long Does It Take Melarsomine to Kill Adult Heartworms?

According to the American Heartworm Society, most adult worms may be killed in 1–3 months after starting treatment.



Heartworm disease is a dangerous and potentially fatal parasitic infection in dogs. The adult worms damage the cardiopulmonary system in a progressive fashion if the disease is not treated. All dogs in the United States should be on a year-round heartworm prevention program regardless of where they live. Annual blood testing and regular heartworm prevention remain the best way to keep your dog protected from the damaging effects of heartworm disease.

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Featured Image Credit: Lindsay Helms, Shutterstock

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