Heartworm Disease in Dogs



Dirofilariasis in Dogs (Heartworms 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 (dog).

In regions where Dirofilaria immitis is endemic, dogs without proper heartworm protection are very likely to develop heartworm disease. The heartworm is mainly endemic in geographic areas with tropical and subtropical climates. It is commonly found along the Atlantic and Gulf Coasts, and the Ohio and Mississippi river basins. The presence of Dirofilaria immitis is not limited to these areas, however. It is found worldwide. Dogs have been diagnosed with heartworm disease in all 50 U.S. states.

Heartworm disease is preventable with the administration of a heartworm prophylaxis (preventative medication), as recommended by a veterinarian. 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. Treating heartworms is expensive and always carries some risk to the dog. It is certainly better to prevent the disease than to deal with its consequences. Without treatment, most cases of heartworm disease are eventually fatal.

If you would like to read how heartworms affect cats, please click here and visit our article in the petMD health library.


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, more extreme exercise intolerance, labored breathing, and a pot-bellied 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 caused by the presence of so many worms that they block the flow of blood into the heart.

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 parasite’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 medications.


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 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, complete blood cell count, urinalysis, and chest X-rays. These, and possibly other tests, are necessary to plan appropriate treatment and to determine the dog’s prognosis. 

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