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Heartworm in Dogs: What You Need to Know

A dog running through a field
Dr. Erica Irish author of Heartworm in dogs: what you need to know

On previous trips to your veterinarian’s office, it is likely that you’ve had a conversation at some point about preventive care. This is an umbrella term that describes vaccines, deworming medications, and products that help to prevent fleas, ticks, and heartworms.

Heartworm disease is one of the most dangerous yet highly preventable diseases that your dog can develop, and in many cases, it can be very expensive to treat. 

For the average heartworm patient, you could purchase seven years-worth of heartworm prevention, yet it would still cost less than the total cost of heartworm treatment!

What is heartworm in dogs?

Heartworms are parasitic worms known asDirofilaria immitis that can live inside of your dog’s heart. Depending on your dog’s size, he can have anywhere from one to over two hundred worms within his heart and pulmonary arteries. When heartworms reside in these areas, they can cause significant and permanent damage.

The resulting heart and lung disease can eventually impact other internal organs and cause death in untreated dogs. Cats and ferrets can also become infected with heartworms, but at this time, there are no approved treatments for cats or ferret heartworm disease.

Heartworm disease occurs more frequently in the Southern United States because of the year-round warm and humid weather. The heaviest concentration of reported cases is near the Mississippi River delta. However, cases are reported all across the country and at any time of the year. Therefore, it is best to use heartworm prevention all year instead of only using it during the summer months.

Life Cycle of heartworms in dogs

Heartworms are acquired when an infected mosquito bites a susceptible animal. Mosquitoes can carry heartworm larvae known asmicrofilariae once the mosquito has had a blood meal from an infected animal. This same mosquito will then introduce microfilariae into the source of its next blood meal.

Microfilariae go through several stages before they develop into full-fledged adult heartworms. Because they spend most of their growth phase developing within the muscles of a dog, it can take almost six months for adult heartworms to form and then migrate to the heart and lungs. More microfilariae develop when male and female heartworms reproduce, thus propagating the lifecycle.

Once mature, heartworms can live for five to seven years. Dogs can only become infected via the bite of a mosquito. It is not possible for dogs to pass on heartworm disease to other animals or to people.  

Symptoms of heartworm in dogs

For dogs with early heartworm disease, there may be no obvious clinical signs at first. As the disease progresses, there can be structural changes within the heart and lungs. This can lead to coughing, weakness, exercise intolerance, and trouble breathing. In severe cases, heartworm disease can cause fainting or collapse, and dogs with signs of heart failure will appear underweight due to muscle wasting.

There are four classes of heartworm disease, progressing in severity as the class number goes higher:

  • Class 1 – asymptomatic or mild/intermittent coughing
  • Class 2 – mild to moderate signs like coughing and exercise intolerance
  • Class 3 – severe signs like trouble breathing, weight loss, anorexia, and heart failure
  • Class 4 – complete cardiovascular collapse due to worms completely obstructing blood flow, also known ascaval syndrome and is almost always fatal


Most veterinarians recommend screening for heartworm disease once a year. Even if your dog is on preventive medication, it is important to make sure that the preventive is working. When testing, a few drops of blood are necessary, and in-house blood tests can yield results in as little as ten minutes.

Heartworm tests have very sensitivity and specificity, which means that the likelihood of a negative being truly negative and a positive being truly positive is very high. If your dog’s heartworm test is positive, your vet may recommend confirmatory testing such as looking at a drop of blood on a microscope slide to detect microfilariae. 

Blood work, urine testing, and x-rays are recommended to evaluate the heart, lungs, and other internal organs. In some cases, echocardiograms are useful because heart damage can be evaluated, and sometimes heartworms can be seen!


Veterinarians used the “fast-kill” and “slow-kill” methods, the latter just involving long term use of a heartworm preventive to kill microfilariae and waiting for adult worms to die after five to seven years. The problem with this method is that worms can still continue to re-infect your dog, and significant damage can occur in a short amount of time. Therefore, the American Heartworm Society recommends the fast-kill method for dogs.

Dogs who are newly diagnosed need to start taking an oral heartworm preventive containing ivermectin, and they need to do this every 30 days while undergoing treatment to help prevent new infections. At the same time, an oral antibiotic known as doxycycline is prescribed for four weeks. It is used to kill tiny amoebae that live on the worms known asWolbachia.

During this time, you should begin trying to restrict some of your dog’s activity. After the first two months, your dog will be able to start theadulticide treatment. This is when your vet administers an arsenic-containing compound known asmelarsomine which kills adult heartworms.

Your dog’s class number will dictate if he requires the two-injection or three-injection protocol. Most dogs receive all three injections on the following days of treatment: day 60, day 90, and day 91. These injections can be extremely painful and cause lethargy for the first 24 to 48 hours. Your vet will recommend pain medications like NSAIDs or maybe steroids if your dog’s coughing is severe. Other pain medications like tramadol can be effective, and CBD oil may also be an option for comfort.

In rare cases, dogs can have an adverse reaction to treatment. Extreme exercise restriction is critical because fragments of dead heartworms can break up and act like blood clots, which can be life-threatening if they obstruct major arteries. Playing and running are not permitted, and any dog who goes outsidemust be on a leash at all times.

Follow up testing occurs around four to six months after the injection protocol is complete. However, veterinarians are always finding new information about variations to this protocol. For example, new evidence suggests thatmoxidectin, the active ingredient in topical Advantage Multi, may be an effective way to treat heartworm disease in dogs1.

Caval syndrome patients can only be treated via the emergency surgical removal of heartworms. The surgery is extremely risky, and unfortunately, most patients do not survive.

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Prevention is always in everyone’s best interests! Not only is heartworm prevention affordable, but it is also highly effective at preventing heartworm disease entirely. These preventives kill microfilariae before they can move to the muscle and develop into full-fledged worms, and so it is important to use preventives all year long for dogs, cats, and ferrets.

Most preventives are an oral chew or topical liquid that you give to your dog every 30 days. There is also an injectable preventive that can be given every six months if it is difficult for you to remember to give your pup a pill every 30 days.


Heartworm disease is a deadly and expensive illness that can affect dogs all across the country. Clinical signs can range from mild to severe, and heartworm disease can eventually lead to heart failure or the shutdown of other internal organs.

Treatment is long, difficult, expensive, and painful, but prevention is affordable, easy, and has little to no side effects. Remember to have your furry friend tested once a year, and make sure that he is up to date on prevention!

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Meet The Author 

Dr. Erica Irish

Erica has worked in the veterinary field since 2006, starting out as a veterinary technician before graduating from the UF College of Veterinary Medicine in 2013. As a general practitioner in an animal hospital, she has many interests and is especially interested in dermatology, cardiology, internal and integrative medicine

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