Treating Heartworms in Dogs

Updated Apr. 27, 2022

Heartworms (Dirofilaria immitis) are parasitic roundworms that can infect both dogs and cats. If your pets are not on pet prescription heartworm prevention program, they may contract the parasite through the bite of an infected mosquito. Heartworm disease is preventable if you diligently use heartworm prevention prescribed by your veterinarian.

Dogs are “natural” hosts to heartworms, meaning that once they become infected, the juvenile heartworm parasites can complete their entire life cycle. As the heartworms mature, they work their way into your dog’s heart, lungs and associated blood vessels. Once there, they can grow to be up to a foot long. It is possible for one dog to be infected with hundreds of heartworms.

If not treated properly, heartworms in dogs will lead to serious health complications and death. This is why it is so important to keep your dog on continuous heartworm prevention.


Heartworm lifecycle in dogs

What to Expect at the Vet’s Office 

If your dog has been diagnosed with heartworm disease via a heartworm antigen test (the most common form of testing), your veterinarian will do a few more diagnostic tests to confirm diagnosis.

  • A test for microfilariae (juvenile heartworms in the blood stream) will be performed. If no microfilariae are found, a confirmatory test for adult heartworms should be run by sending a sample to an outside lab.

  • Complete blood cell count, blood chemistry tests, a urinalysis and chest X-rays to assess your dog’s overall condition and plan the safest way to move forward with treatment. An echocardiogram is recommended for moderate to severe cases.

  • Other tests may also be necessary based on a dog’s individual case.

Overview of Heartworm Treatment for Dogs    

  • Medications: Protocols for treating heartworms often call for the use of multiple medications including several injections to kill the adult heartworms, doxycycline and prednisone taken orally to reduce the chances of unwanted side effects, and heartworm preventative to kill the juvenile heartworms and prevent further infection. In some cases, other medications may be prescribed or different protocols recommended.

  • Surgery: Severe cases of heartworms in dogs may require surgery to remove the worms from the heart and vessels within the lungs, but many of these patients die regardless of treatment.

  • Exercise Restriction: Exercise restriction is a vital part of successful treatment for heartworms in dogs. This is required before, during and after treatment for a prolonged period of time.

Heartworm Treatment Steps 

Treatment protocols for heartworms are determined on a case-by-case basis, but most dogs are treated with some variation of the following, taking place over the course of several months:

  • Begin exercise restriction.

  • If the dog’s condition is especially severe, appropriate stabilization therapy is necessary.

  • Begin treatment with oral prednisone and doxycycline to reduce the chances of a bad reaction to the death of heartworms.

  • Hospitalize the dog for the day and give a heartworm preventative to kill juvenile heartworms in the bloodstream. This is often given in the clinic in case of a reaction. Continue to give heartworm preventatives monthly at home.

  • Give the first injection of melarsomine to kill adult heartworms 30 days after the end of the doxycycline treatment. Restrict exercise and closely monitor for side effects for the next 30 days.

  • Give the second injection of melarsomine 30 days after the first.

  • Give the third injection of melarsomine one day after the second.

  • Continue exercise restriction for another six to eight weeks.

  • Test for microfilariae (juvenile heartworms in the blood stream) 30 days after third melarsomine injection.

  • Test for adult heartworms and microfilariae approximately 9 months after the third melarsomine injection.

What to Expect at Home

The most important aspect of home care for dogs undergoing treatment for heartworms is exercise restriction. Dogs should be crated when a responsible adult is not able to prevent excess activity.

Dogs should only be allowed outside for short on-leash walks to urinate and defecate. Give your dog the full course of any prescription pet medications that have been prescribed, even if he or she appears to be healthy.

Questions to Ask Your Veterinarian

As with any type of laboratory test, false positive and false negative results on heartworm tests are possible. In particular, dogs who have been bitten by a heartworm-infected mosquito within the last six months will test negative until their heartworms mature. Retesting at an appropriate date will usually reveal that the dog does have heartworms.

If you have any doubts as to your dog’s diagnosis, you can ask that your dog be tested for heartworm disease at a later date or using a different type of test.

Dogs who have had heartworms are not immune to reinfection. Ask your veterinarian what schedule of testing and preventative administration is required to avoid future heartworm infections. Most dogs benefit from receiving year-round heartworm prevention.

Possible Complications With Heartworm Treatment for Dogs

Side effects are common with heartworm treatment. Many dogs experience soreness and swelling at the site of melarsomine injections (the muscles on either side of the spine). Abscesses can also form in these locations.

Talk to your veterinarian if your dog is very uncomfortable or becomes worse over time.

The most severe problems that are seen after heartworm treatment in dogs are related to the sudden death of large numbers of worms. Call your veterinarian immediately if you notice any of the following:

  • Your dog develops a cough or a preexisting cough becomes worse

  • Your dog has difficulty breathing or pants excessively

  • Your dog becomes weak or lethargic or collapses

  • Your dog’s appetite significantly decreases

  • Your dog begins to vomit or drool excessively or develops diarrhea

While the treatment of heartworms in dogs can cause serious side effects, heartworms kill dogs if left untreated.

Prevention, on the other hand, is easy and well-tolerated by most dogs. By preventing heartworms in the first place, you can save your dog from a long and difficult treatment process.

Featured Image: Jenkins

Jennifer Coates, DVM


Jennifer Coates, DVM


Dr. Jennifer Coates is an accomplished veterinarian, writer, editor, and consultant with years of experience in the fields of veterinary...

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