Heartworm (Dirofilaria immitus) is a nematode parasite that resides within the host's large pulmonary arteries and right heart chambers. It primarily infests dogs but may also infest foxes, wolves, coyotes, ferrets, sea lions, horses, and cats. A dog can be infested with one to several hundred adult heartworms, which can grow to 12 in. (30 cm). During their 3–5-year life-span, heartworms can cause serious and often life-threatening damage to the heart and lungs. Endemic areas require a reservoir of infected animals (usually dogs) and the presence of mosquitoes, the intermediate host, which transmit the larval stages to a new host.
Adult female heartworms release their microscopic offspring called microfilariae into the bloodstream. A mosquito becomes infested with these circulating microfilariae while taking a blood meal from the dog. The microfilariae develop into mature larvae within the mosquito during the next 10–14 days. As the mosquito feeds again, the mature larvae are injected into the new host. Once in the dog, it takes approximately 6 months for these larvae to complete the cycle by migrating to the large arteries of the lung and right chambers of the heart.
Adult heartworms stimulate a progressive proliferation of the artery lining (endarteritis) that gradually restricts the blood flow to the lungs. The resulting increase in the pulmonary artery blood pressure (pulmonary hypertension) causes the right ventricle to pump harder, eventually leading to right heart failure. In advanced cases the lung fibrosis and heart changes may be permanent. When adult worms die either naturally or with treatment, their fragments become lodged distally in the smaller pulmonary arteries causing an exaggerated proliferation of the vessel lining, the formation of blood clots, and an intense local inflammatory reaction. Blood flow is severely restricted or totally blocked, resulting in severe coughing, coughing up blood, and difficulty in breathing (dyspnea).
Initial signs of disease include coughing, exercise intolerance, and weight loss. As the disease progresses, these symptoms become more pronounced. With advanced disease, dogs begin to exhibit progressive signs of pulmonary disease and associated heart failure, including fainting spells, collapse, difficulty in breathing, coughing up blood, and fluid accumulation around the lungs (hydrothorax) or within the abdominal cavity (ascites). Rarely, a rapidly fatal condition called vena caval syndrome may be observed in young dogs with massive heartworm infestation.
Heartworm infection can usually be determined by examining a blood sample for the presence of circulating microfilariae. Those infestations in which the adults produce no circulating microfilariae are termed occult infections, which can be accurately diagnosed by identifying specific circulating immunologic substances (uterine antigens) released into the blood by adult females.
All but the most advanced cases of heartworm disease can usually be treated successfully. Treatment involves multiple steps to eliminate both adult and microfilariae stages. Daily or monthly heartworm preventive medication is strongly recommended, especially during the mosquito season, in infested areas. The objective is killing the infectious larval stages before they develop into adults. Only dogs that test negative for heartworms should be placed on preventive medication because of the risk of serious reactions.
