What is My Dog’s Life Expectancy After Heartworm Treatment? A Vet Weighs In

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This article was updated on July 15th, 2023

According to the American Heartworm Society, over one million pets have heartworms. So, it’s no surprise that I often encountered dogs with these deadly parasites when I was in practice. Fortunately, because we screened for the disease, we could catch the infestation early, eliminate the worms, and save dogs’ lives. 

Untreated, heartworm disease is often deadly. Once the parasites reach the heart and start growing, symptoms progress from mild to severe to life-threatening.

Because heartworm infection eventually causes severe health conditions, veterinarians recommend treating your dog with medicine designed to eliminate the parasites. However, heartworm treatments can also wreak havoc on your dog’s body – so what’s your dog’s life expectancy after a heartworm treatment? In this article, we will review treatment options and data on life expectancy.

How will heartworm treatment affect my dog?

Heartworm treatments kill the worms, but they can also wreak havoc on your dog’s body: the drugs (melarsomine and thiacetarsamide) your veterinarian may administer are poisons with potentially serious side effects, including:

  • Vomiting and diarrhea
  • Appetite loss
  • Liver damage and jaundice
  • Swelling or abscess at the injection site
  • Allergic reaction to the drugs

Besides the effects on your dog, the medicine kills worms. Depending on the worm burden, the worms’ death could cause:

  • New or worsening cough
  • Panting or difficulty breathing
  • Lethargy, weakness, or collapse
  • Vomiting, diarrhea, or drooling

If you notice any of these signs following heartworm treatment, contact your veterinarian immediately. There may be an artery blockage or other emergency condition.

What is the life expectancy with and without heartworm treatment?

For the life expectancy of dogs infected with heartworm, there isn’t a one-size-fits-all answer. Your dog’s age, health condition, and severity of infection all affect the length and quality of life. But leaving your dog untreated will likely shorten her lifespan by six or more years.

Naturally, dogs with mild or asymptomatic infections have a better prognosis than dogs with more advanced cases. That’s because heartworm treatment becomes increasingly invasive and takes a greater toll on your dog’s body.

Does life expectancy and survival rate vary depending on the stage of my dog’s heartworm infection?

Life expectancy and survival rates decrease as heartworm infection progresses. In more advanced stages, there is more damage to the organs and body systems, and this affects the quality and length of life.

Class I infection

In the first stage or class of the infection, dogs often show no symptoms or may have a mild cough. The adult worms are settling in the heart, but they haven’t produced microfilariae yet. The damage to tissues is still minimal, so the prognosis is highly favorable.
>>> Following treatment, most dogs enjoy a high quality of life for many years.

Class II infection

Dogs with moderate heartworm infections show signs that include exercise intolerance, fever, and a lingering cough. Clinical tests may also reveal anemia, enlargement of the right side of the heart, and severe protein loss in the urine. At this stage, the damage to the heart is more pronounced, and there may be a heart murmur. The adult worms are producing microfilariae. 
>>> Prognosis with treatment is still favorable, but there are increased risks of significant organ damage. The length and quality of life are lower than for Class I infections.

Class III infection

When dogs enter the third stage of infection, they show more obvious signs of the condition. Affected pups may show:

  • Chronic cough
  • Fatigue and difficulty breathing with or without exercise
  • Inappetence and weight loss
  • Coughing up blood
  • Ascites and abdominal distension
  • Anemia
  • Visible worms on chest x-rays
  • Enlarged right heart

By this stage, there is significant damage to your dog’s heart, lungs, and likely other body systems.
>>> While treatment can improve the quality of life for your pooch, he will have a shorter life expectancy than if the disease is treated at an earlier stage.

Class IV infection

Dogs with Class IV infections are very ill and have a condition known as Caval Syndrome. These dogs demonstrate signs of a Class III infection along with other severe symptoms.

  • Dark brown urine due to red blood cell rupture
  • Swollen liver
  • Weakness and collapse due to shock
  • Severe heart murmur

With a class IV infection, the heart has significant damage. Several worms fill the right side of the heart and the pulmonary artery and interfere with circulation. Treatment is more invasive at this point and may include surgical removal of the adult worms.
>>> Due to organ damage, survivability and life span are significantly reduced. Your dog’s quality of life will also be affected.

Impact to Life Expectancy – Summary Table

Infection classQuality of lifeTreatment’s impact to life expectancy
Class IMost dogs have a high quality of life after treatment; however, as many as 50% of dogs may exhibit gastrointestinal symptoms (vomiting/diarrhea) or behavioral changes (depression/lethargy)Dog should enjoy a normal lifespan after successful treatment
Class IIDogs have an increased risk of organ damage including valve damage; symptoms may include heart murmurs, exercise intolerance, and a coughLife expectancy is lower than for class I and varies based on the severity of organ damage
Class IIIDogs will have significant damage to heart, lungs, and probably other organ systems; Signs include chronic cough, fatigue, difficulty breathing, weight loss, fluid in the abdomen, anemia, enlarged right heart, coughing up bloodDue to significant organ damage, life expectancy is reduced. 
Class IVDogs are severely ill and have significant damage to the heart and other organs; symptoms include severe heart murmur, dark brown urine, swollen liver, weakness, and collapse from shockSurvivability and life span are significantly reduced due to extensive organ damage

Does heartworm treatment shorten a dog’s life span?

Heartworm treatment relies on adulticide drugs that can be toxic to your dog. How much the treatment affects your dog’s lifespan will depend on how many worms your dog has and how severe the infection is. However, your dog’s lifespan is likely to be longer if he’s treated than if untreated.

What percentage of dogs survive heartworm?

With the traditional, 3-dose adulticide treatment and supportive medications, the survival rate of dogs treated for heartworm is 98%.

Is a dog “cured” after heartworm treatment or are there long-term damages?

Dogs can fully recover from heartworm infection, but if they receive treatment after the worms cause damage to any of the organs, there can be lingering effects and symptoms.

If your dog’s heartworm infection is diagnosed in the first stage, your pup will probably make a complete recovery. Likewise, dogs with a Class II infection can survive treatment without permanent damage, but they may require more veterinary care during the treatment period.

Canines with Class III or IV infections usually experience damage to the heart, lungs, and possibly other organs such as the liver. Unfortunately, much of the damage is not reversible. Whether heartworm infection has long-term effects depends on the severity and class of the infection at the time of diagnosis. Class III and IV infections will likely result in long-term health effects.

Can my dog survive a heartworm infection without treatment?

Although it may take several months or years for severe signs to appear, untreated heartworm infections are often fatal and always devastating to your dog’s health. Adult worms and microfilaria affect different parts of the body and can cause damage to multiple organs.

Adult worms live in the heart or the large vessels of the heart and lung. A heavy burden can severely impact blood flow throughout the body. In the heart chamber, the adult worms may interfere with proper valve functions.

Microfilariae circulates mostly in the small blood vessels throughout the body. Although much smaller than the adults, they are wide enough to block blood flow. 

Whether adult worms reduce blood flow to the body or microfilariae block blood flow in smaller vessels, target tissues can’t get the oxygen and nutrients they need to live and function properly. The most commonly affected organs are the heart, lungs, and liver.

How can I make my dog’s life better after heartworm treatment?

At first, you need to restrict your dog’s activity and excitement. We recommend cage rest and no visitors for at least a few months after treatment. Limit exercise to short, low-impact walks on a leash.

While your pooch may not enjoy the restrictions, it’s for his long-term good. It takes time for enzymes in your dog’s body to break down the dead worms. Until the parasites are eliminated, vigorous exercise could force the dead worms into smaller vessels in the lungs and interfere with blood flow. This is a life-threatening condition.

Schedule regular appointments with your veterinarian to measure progress. Six months after treatment, the doctor should perform an antigen test to confirm your pup is heartworm-free. After that, return to annual screening and regular heartworm preventatives.

Frequently asked questions

How will my vet treat my dog’s heartworm?

If your pooch tests positive for heartworm, your veterinarian will use another test to confirm the diagnosis. This verification is crucial because the course of treatment is lengthy and can be quite expensive.

Once she verifies the positive diagnosis, your veterinarian will use a combination of drugs to treat heartworm. The treatment regimen is decided case-by-case. However, there are usually common elements:

  • Exercise restriction
  • Stabilizing health conditions
  • Treatment with oral antibiotics (doxycycline) and steroids (prednisone) to reduce the risk of side effects. Both drugs involve a four-week course of treatments.
  • Administration of heartworm preventative to kill juvenile worms – this should be done under hospitalization so that the veterinary staff can observe your dog for any adverse reactions.
  • Treatment with adulticide by deep muscle injection. The first dose is followed by a second injection 30 days later and a third one the following day.
  • Testing for heartworm larvae (microfilaria) one month after the last treatment. If the results are positive, your veterinarian will administer a microfilaricide treatment and retest in 4 weeks.

Treatment will take at least 6 months and could last longer if your pup has positive heartworm test results. With the cost of tests, drugs, and hospital care, heartworm treatment can become quite expensive. A full treatment course ranges in cost from $200 (at some discount clinics and shelters) to $1,000 or more. 

Are dogs more likely to be depressed after heartworm treatment?

 Between tissue damage caused by the worms and the adulticide effects, pups experience significant physical stress. As a result, some dogs show signs of depression like lethargy or withdrawal after heartworm treatment. 

How long does it take heartworms to dissolve after treatment?

Depending on the size and number of heartworms in a dog’s body, it can take from 6 to 8 weeks to several months.

Is heartworm infection or treatment painful for a dog?

The heartworm injections are an arsenic-based drug, and they are painful for dogs. This is one reason your dog gets additional medications during treatment.

Heartworm infection can also be painful for your pooch. If a dog has an advanced heartworm infection, the damage to the tissues can cause him considerable discomfort.

What are the symptoms of heartworms?

Most dogs do not show any symptoms or signs during the early stages of heartworm infection. After your dog is bitten by a mosquito, it takes the larvae about 7 months to migrate from the bloodstream to the heart and lungs. 

Symptoms include:

  • Cough
  • Exercise intolerance
  • Difficulty breathing
  • Abnormal heart sounds
  • Fluid accumulation in the abdomen(ascites)
  • Coma
  • Sudden death


  • Dr. Liz Guise, Veterinarian

    Dr. Liz (Elizabeth) Guise graduated from the University of Minnesota with a doctorate in Veterinary Medicine (DVM). She worked as a veterinarian for two years before working for the US Department of Agriculture for 13 years.

Disclaimer: This website's content is not a substitute for veterinary care. Always consult with your veterinarian for healthcare decisions. Read More.


  1. Thank you for the information and throrough instructions as well as what my dog will experience as far as pain and discomfort due to the injections. I was only given 5 gabapentin for pain. Being a Vet Tech myself and also a consumer of gabapentin i know that it takes a bit of time to reach a therapeudic level in the bloodstream so I’m a little skeptical about the efficacy of of gabapentin in an acute treatment setting. Also gabapentin is a neuropathic analgesic which also makes me wonder why not carprofen and tramadol? Thank you again for your time and article.

  2. Our dog is still showing a light positive after being diagnosed and treated before we adopted him. He was diagnosed in September 2021 and we adopted him in March, 2022. He is on year-round heartworm preventative and is not an outdoor dog. Yesterday when he got his shots, the heartworm test still showed a light positive. He is otherwise healthy. Heart,lungs, everything is fine. He wrestles and plays and eats well. Should we be worried?

    • This can happen. While most dogs test negative within 6 months after therapy, some dogs remain positive for longer. If not showing any signs, your vet will likely be content with just ensuring prevention is up to date.
      Dr. Linda Simon MVB MRCVS
      “The information on this website is not a substitute for in-person veterinary care. Always seek advice from your veterinarian if you have concerns about your pet’s medical condition.”

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