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  • Writer's pictureMispits & Friends

Heartworm Disease

The thought of worms, especially in the body makes most people cringe. There are many types of worms that effect dogs. The most serious, and most preventable type of worm, are heartworms.

What ARE Heartworms Exactly?

Heartworm disease is a serious and potentially fatal disease in pets in the United States and many other parts of the world. It is caused by foot-long worms (heartworms) that live in the heart, lungs and associated blood vessels of affected pets, causing severe lung disease, heart failure and damage to other organs in the body. Heartworm disease affects dogs, cats and ferrets, but heartworms also live in other mammal species, including wolves, coyotes, foxes, sea lions and—in rare instances—humans. Because wild species such as foxes and coyotes live in proximity to many urban areas, they are considered important carriers of the disease. (Source: American Heartworm Society)

Heartworm Life Cycle

Dogs are considered natural hosts of heartworms, meaning the worms live to adulthood, mate and produce offspring. The amount of heartworms inside a dog's body is called the worm burden.

How Do Dogs Get Heartworms?

MOSQUITOS! The mosquitos carry microscopic babies from one host to another. Once inside a new host, it takes approximately 6 months for the larvae to develop into mature adult heartworms. Once mature, heartworms can live for 5 to 7 years in dogs. Because of the longevity of these worms, each mosquito season can lead to an increasing number of worms in an infected pet. (Source: American Heartworm Society)

Heartworm disease has been reported in all 50 states. And the bite of just one mosquito infected with the heartworm larvae will give your dog heartworm disease. Heartworm disease has not only spread throughout the United States, but it’s also now found in areas where veterinarians used to say “Oh, we don’t have heartworm disease.” Areas like Oregon, California, Arizona, and desert areas -- where irrigation and building are allowing mosquitoes to survive. (Source: WebMD) The American Heartworm Society estimates more than a million pets in the US are infected.

Heartworm disease is not contagious. A dog cannot be contract the disease by being near an infected dog. It is spread only through the bites from mosquitos.

Symptoms of Heartworms in Dogs

The severity of heartworm disease is related to how many worms are living inside the dog (the worm burden), how long the dog has been infected, and how the dog’s body is responding to the presence of the heartworms. The dog’s activity level also plays a role in the severity of the disease and in when symptoms are first seen. Symptoms of heartworm disease may not be obvious in dogs that have low worm burdens, have been recently infected, or are not very active. Dogs that have heavy worm burdens, have been infected for a long time, or are very active often show obvious symptoms of heartworm disease. 

There are four classes, or stages, of heartworm disease. The higher the class, the worse the disease and the more obvious the symptoms.

  • Class 1: No symptoms or mild symptoms such as an occasional cough.

  • Class 2: Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.

  • Class 3: More severe symptoms such as a sickly appearance, a persistent cough, and tiredness after mild activity. Trouble breathing and signs of heart failure are common. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest x-rays.

  • Class 4: Also called caval syndrome. There is such a heavy worm burden that blood flowing back to the heart is physically blocked by a large mass of worms. Caval syndrome is life-threatening and quick surgical removal of the heartworms is the only treatment option. The surgery is risky, and even with surgery, most dogs with caval syndrome die. 

Not all dogs with heartworm disease develop caval syndrome. However, if left untreated, heartworm disease will progress and damage the dog’s heart, lungs, liver, and kidneys, eventually causing death. (Source: FDA)

Get Your Dog Tested!

All dogs should be tested annually, even those on preventative medications. A simple blood test detects the presence of the female heartworm in your dog's blood stream. If the blood test comes back positive, further testing, x-rays, blood profiles, and ultrasounds may be necessary to determine severity and develop a treatment plan.

Treatment for Heartworm Disease

The best treatment is prevention, but if a dog does become infected, heartworm disease IS treatable. The good news is that most dogs can be successfully treated for heartworms.

What to Expect

  • Confirm the diagnosis. Once a dog tests positive on an antigen test, the diagnosis should be confirmed with an additional—and different—test. Because the treatment regimen for heartworm is both expensive and complex, your veterinarian will want to be absolutely sure that treatment is necessary.

  • Restrict exercise. A dog’s normal physical activities must be restricted as soon as the diagnosis is confirmed, because physical exertion increases the rate at which the heartworms cause damage in the heart and lungs. The more severe the symptoms, the less activity your dog should have.

  • Stabilize your dog's disease. Before actual heartworm treatment can begin, your dog’s condition may need to be stabilized with appropriate therapy. In severe cases of heartworm disease, or when a dog has another serious condition, the process can take several months.

  • Administer treatment. Once your veterinarian has determined your dog is stable and ready for heartworm treatment, he or she will recommend a treatment protocol involving several steps. The American Heartworm Society has guidelines for developing this plan of attack. Dogs with no signs or mild signs of heartworm disease, such as cough or exercise intolerance, have a high success rate with treatment. More severe disease can also be successfully treated, but the possibility of complications is greater. The severity of heartworm disease does not always correlate with the severity of symptoms, and dogs with many worms may have few or no symptoms early in the course of the disease.

  • Test (and prevent) for success. Approximately 6 months after treatment is completed, your veterinarian will perform a heartworm test to confirm that all heartworms have been eliminated. To avoid the possibility of your dog contracting heartworm disease again, you will want to administer heartworm prevention year-round for the rest of his life. (Source: American Heartworm Society)


Heartworm disease is easily preventable! With testing and preventative measures you can keep your dog safe from heartworms. Heartworm preventives come in different forms: monthly chewables, topical medications, and injectable medications (6 month & 12 month). Heartworm preventives are available only by prescription from veterinarians.Owners should talk to their pet’s doctor about what product or products will be best for their pets.

  • Dogs don’t just need prevention during warm-weather months. Heartworm preventives work by treating heartworms that already infected the pet within the past month or longer; meanwhile, preventives need to be given on time, every time to be effective. The American Heartworm Society recommends year-round heartworm prevention for pets.

  • Heartworm preventives are safe, relatively inexpensive and easy to give, but if a dog becomes infected, heartworm treatment can be costly and difficult, requiring multiple veterinary visits and months of exercise restriction.

  • While there are drug-free strategies owners can put in place to reduce a pet’s exposure to mosquitoes, there’s no such thing as a “natural” heartworm preventives.

Treatment and Preventative Assistance ResourcesPAws4aCure

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