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Dog laying in grass

Heartworms, scientifically known as dirofilaria immitis, are harmful parasites transmitted by infected mosquitoes. While heartworm can affect many mammals, like cats and ferrets, dogs are often a favorite host for infected mosquitoes. Heartworms can grow up to 12 inches long and are potentially fatal if left untreated. Though a heartworm diagnosis is scary for any pet owner, there are steps you can take to help both prevent the disease and treat it if necessary.

The essentials

  • Heartworm is not contagious among dogs — Dogs cannot catch heartworm from one another. It’s only spread by mosquitoes most often to dogs, wolves, and coyotes but can also affect other mammals like cats.
  • Heartworms take about 6 months to mature and can live in a dog’s body for up to seven years — Worms cause inflammation and damage a dog’s lungs, arteries, and heart if left untreated.
  • Prevention is your best line of defense — While there are treatments to fight heartworms, it’s best to take preventative measures, like giving your dog monthly heartworm medication.

Symptoms of heartworm in dogs

Signs that your dog has heartworm disease may not be apparent until well after an infected mosquito has bitten them. This is because it takes about six months for infected larvae to fully mature and for your dog to start showing symptoms of an infection. Heartworm symptoms are broken up into four classes, including:

  • Class one. Early-stage clinical symptoms of heartworm disease are uncommon. At most, you may notice your dog has a mild cough in this phase.
  • Class two. As the infection progresses to class two, you may notice your dog has a more persistent cough, and their exercise tolerance may be limited.
  • Class three. More symptoms present themselves in this class, including increased exercise intolerance, decreased appetite, and weight loss, abnormal lung sounds, swollen belly, fainting, and weak pulse.
  • Class four: This is the most severe stage of the infection, often referred to as Caval syndrome. It often indicates a particle blockage due to worms in the heart and major blood vessels. Your dog may exhibit symptoms of a cardiovascular collapse, which include pale gums, dark urine, and severe difficulty breathing.

Veterinary exam fees are covered by pet insurance — find out what is (and isn’t) included by reading our pet insurance coverage guide.

Diagnosing heartworm

Diagnostic testing for heartworm is usually done once a year at your dog’s annual exam. If you suspect your dog is infected with heartworms, it’s essential to get them tested by your veterinarian as soon as possible for the best prognosis. Vets commonly run two blood tests to check if your dog is infected with heartworms. An antigen test can detect adult worms, while a microfilariae test identifies heartworm offspring.

An antigen test can see if there are any antigen proteins in your dog’s blood, which are released by adult female heartworms. This test can detect if your dog is infected with heartworms about five months after being initially infected. Since this test only identifies the presence of female adult heartworm, it can detect if your dog has heartworms before they start to reproduce, but cannot be used for early detection.

Another standard blood test to check for heartworms is to look for microfilariae in blood work. Microfilariae, the heartworm’s offspring, can’t be detected until about six months after your dog’s been bitten by an infected mosquito since it takes that long for heartworm larvae to fully mature and start to reproduce.

Additional testing

Depending on the stage of the infection and your dog’s medical history, veterinarians may use other tests to confirm and determine the severity of a heartworm diagnosis. These may include:

  • Echocardiography. This is an ultrasound of your dog’s heart. It allows vets to look at your dog’s heart and pulmonary artery to see if heartworms are present.
  • Chest x-rays or radiographs. X-rays of your dog’s heart will show enlargement or swelling in the pulmonary artery, suggesting heartworms.
  • Electrocardiogram (EKG/ECG). Electrocardiograms check the electrical current produced by the heart. While an EKG can help diagnose heartworm by detecting irregular heart rhythms, it’s also used to ensure your dog’s heart is strong enough to handle the treatment necessary to treat heartworm disease.
  • Additional blood tests. Besides testing the blood for antigens or microfilariae, your vet may also do a complete blood count test. Or, request a blood test to check on their kidney and liver, which can help deduce if heartworms are present.
How Soon it Detects Heartworms Cost (Private Animal Hospital) Cost (Shelter or Rescue)
Antigen Test 5 months after initial bite $35-$75 Included
Microfilariae Test 6 months after initial bite $20-$40 Included

Additional tests to determine the severity of a dog’s heartworm and stage vary in price. An echocardiography costs around $575 on average. X-rays cost anywhere from $75-$500, and additional blood tests range from $100-$200. If an electrocardiogram is necessary, this costs anywhere between $725 and $1,000.

Treatment and medications

There are typically four steps of treatment: weakening adult heartworms, eliminating their growth, restricting your dog’s activity, and then preparing the dog for the medication used to kill the adult heartworms. A veterinarian will determine the best course of treatment depending on the stage of the disease.


Your vet will create a treatment plan for your dog depending on the stage of the heartworm infection and your dog’s health history. Some of the most common medications used to treat heartworm disease include:

  • Melarsomine. ($500-$1,500) Melarsomine is an antiparasitic used to treat heartworm disease. Due to it being derived from an organic arsenic compound, this medication comes with increased risks. Still, the American Heartworm Society’s guidelines recommend it, since it kills more than 98 percent of worms during treatment.
  • Doxycycline. ($85-$110 for 30 pills) An antibiotic usually administered during the first month of treatment either with or to prepare for the use of melarsomine. It’s been proven to weaken adult heartworms.
  • Heartworm prevention. ($6-$18 per month) This will be administered on the first day and should be continued for the remainder of the dog’s life to stop the current infection and as a preventative measure later. This is also used to prevent further infection since heartworm positive dogs carry the infectious heartworm larvae that mosquitoes can carry and infect other dogs.
  • Steroids. ($40 for 30 day supply; $50-$150 single injection) Steroids are often prescribed during and after your dog’s treatment to help reduce inflammation and blood clots.
  • Surgery. In severe cases, surgery may be required to remove and treat heartworm disease.
Timing Cost (Private Animal Hospital) Cost (Shelter or Rescue)
Preventative Heartworm Pill Lifetime $6-$18 (per month) $6-$18 (per month)
Melarsomine Series of injections during treatment $500 - $1,500 $500 - $1,500
Doxycycline 1-28 days $85 - $110 $85 - $110
Steroids 1 month $40 for 30 day supply or $50-150 single injection $40 for 30 day supply or $50-$150 single injection

Keeping dogs comfortable during treatment

While your dog is being treated for heartworm, it’s important to keep them comfortable and relaxed. Rest is vital to your dog’s recovery, so you’ll want to have items on hand to restrict their exercise for four to six weeks after their final melarsomine injection. Some equipment you may want to have include:

  • Crate or playpen. Though it may seem like a punishment, using a crate or playpen to restrict your dog’s activity during and after treatment will help them heal and get stronger.
  • Thundershirt. Treatment may be stressful for both you and your pup. To help ease your dog’s anxiety, try using a thunder shirt to provide extra comfort and reassurance.
  • Puzzle toys. Dogs need to rest for weeks at a time while being treated for heartworm, so they’re bound to become restless. Activate their minds by treating them with a frozen Kong or serving their meals using snuggle mats or dog puzzles to keep them entertained while still stationary.
  • Bones and chews. In addition to dog puzzles and Kongs, try keeping your dog entertained with bones and chews while they recover.
  • Keep a leash on hand. While your pup recovers, always use a leash during the rest period to help limit their activity.


Your dog should be re-tested for heartworm disease six months after their final successful treatment. Like the tests used to detect a heartworm infection initially, your vet will conduct blood tests to make sure all of the worms were killed during treatment. If the initial test comes back positive, they may need to run a confirmatory test to determine a true positive. If your dog tests positive for heartworms, your dog will need to undergo treatment again using melarsomine.

Complications and long-term costs

Though there are treatments to fight heartworm disease, it’s not always easy to rid your dog of the infection. It’s estimated heartworm treatment costs anywhere between $600 to $6,000. Even more, heartworms cause long-lasting complications even after they’re gone. Some of the most common long-term complications include:

  • Caval syndrome. This usually occurs during the final, 4th class of heartworm infection and requires surgery. It’s brought on by a heavy concentration of worms obstructing blood flow back to your dog’s heart. If left untreated, it can lead to heart, liver, and kidney damage and potentially death.
  • Lasting damage. Studies have shown long-term damage in dogs from heartworms, even after successful treatment. Some of these findings include discovering mummified heartworm remnants and continued vascular and pulmonary issues.

Heartworm prevention

The best preventive medication to help protect your dog from contracting heartworm disease is monthly heartworm medication. In addition, make sure to take your dog to the vet once a year for their annual check-up, including heartworm testing. If you’re worried about veterinary or treatment costs, pet insurance is a wise investment to help ease the burden of unexpected vet bills.

Frequently asked questions

How much does a heartworm test normally cost?

On average, tests to detect heartworm disease cost $45-$50.

How much does it cost to treat a dog with heartworms?

Since treatment protocols for heartworm vary and depend on the severity of the infection, your dog’s medical history, and weight, the cost can vary. On average, it’s estimated heartworm treatments cost anywhere between $600 to $6,000.

Can I give my dog heartworm medicine without testing?

No. Heartworm treatment is a risky process that requires the guidance of a veterinarian. You will need to work with your veterinarian to diagnose and determine the best plan of action for treatment.

What do I do if I can’t afford heartworm treatment?

Vet bills can pile up, but don’t let this fear keep you from taking your pup to the vet. If you’re concerned about cost, talk with your vet to set up a payment plan. In addition, pet insurance can help offset vet and unexpected emergency bills.