Heartworm Disease in Dogs (Dirofilariasis in Dogs)


Dogs suffering from heartworm disease are parasitized by the organism Dirofilaria immitis, a nematode (roundworm) commonly referred to as the heartworm. The severity of heartworm disease in dogs is directly dependent upon the number of worms present in the body, how long they’ve been there and the response of the host (the dog).

In regions where Dirofilaria immitis is endemic, dogs without proper preventative heartworm medicine for dogs are very likely to develop heartworm disease. The heartworm is mainly prevalent in geographic areas with tropical and subtropical climates. It is commonly found along the Atlantic and Gulf Coasts and up through the Ohio and Mississippi river basins.

The presence of Dirofilaria immitis is not limited to these areas, however. Dogs have been diagnosed with heartworm disease in all 50 US states.

Heartworm disease is preventable with the administration of a preventative heartworm medication, as recommended by a veterinarian. Your dog must be tested for heartworms before heartworm medication can be prescribed, especially if there is a lapse in administering the medication.

For dogs who do contract heartworm disease, the prognosis is good for mild to moderate cases with appropriate and timely treatment. Dogs with more severe cases may suffer from serious short- and long-term complications associated with the disease and its treatment.

Treatment of heartworms in dogs is expensive and always carries some risk to the dog. It is certainly better to prevent the disease than to deal with its consequences. It costs about the same to prevent heartworms for a dog’s entire life as it does to treat the disease just once.

Without treatment, most cases of heartworm disease are eventually fatal.

Types and Symptoms of Heartworms in Dogs

Common signs of heartworm disease in dogs include coughing, exercise intolerance and poor body condition, but symptoms can vary depending on the severity of the infection.

Heartworm disease is divided into classes. Dogs with Class I heartworm disease are often asymptomatic, meaning they exhibit no visible symptoms, or may only exhibit minimal signs such as an occasional cough.

Signs of heartworms in Class II patients typically include coughing and intolerance to a moderate level of exercise.

Class III cases may show a generalized loss of body condition (losing weight, greasy or dry hair, loss of muscle), more extreme exercise intolerance, labored breathing and a potbellied appearance associated with fluid accumulation in the abdomen as a result of right-sided heart failure.

Dogs with Class IV heartworm disease have a condition known as caval syndrome, which is caused by the presence of so many heartworms that they block the flow of blood into the heart. Treatment for dogs with Class IV heartworm disease is aimed at comfort, as the disease has progressed too far to treat.

Causes of Heartworms in Dogs

Heartworms are spread through the bites of mosquitos that carry the infective heartworm larvae. These larvae then migrate through the dog’s body until they reach the heart and blood vessels within the lungs, a process that takes approximately six months.

The larvae continue to mature in the dog’s heart and lungs—an adult heartworm can grow to be about 12 inches long. These adults reproduce and release immature heartworms, known as microfilariae, into the dog’s bloodstream.

When a mosquito bites an infected dog, the microfilariae can enter the mosquito’s body, mature, and then be passed on to another dog, thereby continuing the heartworm’s life cycle and spreading the disease to the next host.

Risk factors associated with heartworm disease include residence in endemic regions, exposure to mosquitos and lack of proper preventative heartworm medications. Most dogs in the US have the first two risk factors, making heartworm preventatives the only way to mitigate your dog’s risk.


A veterinarian can run a quick blood test to screen a dog for heartworms. These tests are routinely run both on dogs who are suspected of having heartworm disease and to monitor dogs who are on preventative heartworm medications. A positive screening test should be confirmed with another type of test before a definitive diagnosis is made.

Additional tests that are routinely run on dogs with heartworm disease include a blood chemistry panel, a complete blood cell count, a urinalysis and chest X-rays. These, and possibly other tests, are necessary to plan appropriate treatment of heartworms in dogs and to determine a dog’s prognosis.

Treatment for Heartworm Disease

Dogs with heartworm disease will initially receive any treatments needed to stabilize their condition. They will then be given medication to kill circulating microfilariae, and most will undergo a series of three injections over a month’s time to kill adult heartworms in the heart and lungs.

Hospitalization is necessary when these injections are given, and possibly at other times, so that your veterinarian can watch closely for side effects. Prescription pet medications like prednisone and doxycycline are also typically prescribed to reduce the chances that the dog will react badly to the death of the heartworms. Other treatments may be needed based on an individual dog’s condition.  

If a dog has caval syndrome, a surgical procedure will be necessary to remove adult worms from the right heart and pulmonary artery by way of the jugular vein. However, surgery is a real risk in these dogs due to their compromised heart and lung function. Most dogs with caval syndrome die regardless of treatment.

Living and Management

Restricting exercise before, during and after treatment of heartworms in dogs is absolutely vital to its success. Severely affected dogs may need to be kept in a cage to limit activity. A test for the presence of adult heartworms should be done approximately six months after treatment is complete to check for a continued presence of Dirofilaria immitis. If the test is positive, the treatment can be repeated.


Preventative heartworm medications should be given to all at-risk dogs as directed by your veterinarian. There are a number of heartworm preventatives that are safe, highly effective and commonly used. All products labeled to kill heartworms are available by prescription only, so you will need to talk with your veterinarian to find the best one for you.

Dogs who have been treated for heartworm disease also need to receive preventive medications since they can be reinfected. Heartworm preventatives are not 100 percent effective, particularly if they are not used per label instructions or doses are missed. Therefore, routine heartworm screening is recommended so that the disease can be caught early, when treatment is safest and most effective.

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