Heartworm disease is a parasitic infection of the heart and potentially a life-threatening condition that is caused by a parasitic nematode (roundworm) known as Dirofilaria immitis. These parasites live in the right side of the heart and blood vessels that supply blood to the lungs. Infestation by D. immitis can lead to severe lung disease, heart failure, other organ damage, and death in dogs which are natural hosts of the causative parasite. D. immitis or heartworms are long, spaghetti-like worms that can be anywhere from 6 to 12 inches in length . They are transmitted from dog to dog through mosquito bites. Although dogs are considered natural hosts of these parasites, more than 30 species are affected by D. immitis roundworm including cats, ferrets, horses, sea lions, coyotes, foxes, wolves etc.
Based on severity of infection, heartworm disease is categorized into three classes. Number of parasites in the affected dog, duration of the infestation and response of the host dog are factors that are directly linked to the severity of the condition. In most cases, signs begin to appear within 1-2 years after infection.
In class I cases, the dogs are mostly asymptomatic or show very mild and subtle clinical signs such as occasional cough.
Mild to moderate cases are placed in class II. They are characterized by coughing and abnormal exercise intolerance.
Class III cases are the most severe and they are characterized by anemia, exercise intolerance, fainting spells, and — in severely affected dogs, right-sided chronic heart failure. High blood pressure, rapid heart rate (tachycardia) and labored breathing are further signs of this condition, specially in class III cases.