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Heartworm Disease in Dogs


Dirofilariasis in Dogs (Heartworms in Dogs)


Dogs suffering from heartworms are, in fact, infested by the organism Dirofilaria immitis, a parasitic nematode (roundworm) commonly referred to as the heartworm. The severity of this disease is directly dependent upon the number of worms present in the body, the duration of the infestation, and the response of the host (the infested dog is the host).


In regions where Dirofilaria immitis is endemic, dogs without proper heartworm protection are almost 100 percent likely to suffer from heartworm infestation. The heartworm is mainly endemic in geographic areas with tropical and subtropical climates, and is also 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 those dogs that do contract heartworm disease, the prognosis is good for mild to moderate cases, and such conditions can be relatively uneventful. Dogs with more severe cases may suffer from lung complications resulting from extreme medication given to kill serious infestations.


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


Symptoms and Types


Heartworm disease is defined in three classes, varying in severity. 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. Class II patients usually exhibit coughing and unusual intolerance to exercise. The most severe cases, defined as Class III, may show symptoms of anemia, exercise intolerance, fainting spells, and -- in severely affected dogs, right-sided chronic heart failure.


A physical examination may reveal further symptoms, especially in Class III cases. These include high blood pressure (hypertension), labored breathing, and extremely rapid heart beat (tachycardia).




Heartworms are spread through mosquitos that carry the infective heartworm larvae. These larvae migrate from the bite wound through the dog’s body until they reach the heart and blood vessels of the lungs, a process that takes approximately six months. The larvae mature in the dog’s body -- an adult heartworm can grow to be about 12 inches long. These adults reproduce and release immature heartworms, known as microfilariae, directly into the dog’s blood. When a mosquito bites an already infected dog, it may take in these microfilariae with the dog’s blood, and then pass on the infective heartworm larvae (the microfilariae develop once inside the mosquito) 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, such as tropical areas, outdoor habitation, and lack of the proper prophylaxis to prevent heartworm infestation.





If heartworm disease is suspected, an electrocardiograph (which monitors changes in the heart) may reveal heart rhythm disturbances and/or enlargement of the right ventricle of the heart (hypertrophy).


Additional tests may include a urine analysis, serologic tests that can identify the female heartworm antigen, and X-rays, which may reveal enlargement in key arteries associated with heartworm disease.