Heartworm in Dogs (Dirofilaria immitis)

What is heartworm?

Dirofilaria immitis, the pathogen responsible for cardiopulmonary dirofilariasis (heartworm disease), is a roundworm (nematode) transmitted by mosquitoes. During its life cycle, the mosquito ingests microfilariae (early-stage larvae) circulating in the blood while feeding on an infected dog. Inside the mosquito, these larvae develop through several larval stages into the infectious third-stage larva, which is transmitted to a dog during a subsequent bite. In the dog, the larvae migrate and mature over several months until adult worms settle in the pulmonary arteries and the right side of the heart.

Transmission by Mosquitoes

Heartworm pathogens (Dirofilaria immitis) are transmitted by various species of mosquitoes that use dogs as hosts and reproduce with their help. The main areas of distribution are North America and the Mediterranean region (Italy, Spain, Greece, southern France). The infectious third-stage larvae (microfilariae) are transmitted to the dog by mosquitoes during the feeding process and enter its bloodstream, where they develop into adult heartworms (macrofilariae) over the course of several months. These grow to a length of up to 30 cm and settle in the dog’s pulmonary artery. The first symptoms appear after about 6 months. Typical symptoms include fatigue, rapid heartbeat, leg swelling, shortness of breath, coughing, heart failure, and high blood pressure. Affected animals initially lose weight and stamina; a cough then develops.

Prevalence and Risk for Traveling Dogs

The disease is most prevalent in North America and the Mediterranean region; in Central Europe, it occurs primarily in imported dogs or in dogs that have spent time abroad.

Symptoms of Heartworm Disease

The adult worms and their metabolic byproducts damage the pulmonary arteries and the surrounding lung tissue. Initial symptoms often do not appear until months later; typical signs include reduced stamina, coughing, shortness of breath, and, in advanced stages, signs of right-sided heart strain with signs of congestion.

Heartworm Testing in Dogs

Detection of circulating antigen—which originates primarily from adult female worms—is of central diagnostic importance. Since this antigen becomes detectable only as the adult worms mature, testing is most meaningful several months after infection at the earliest; a test performed too early may yield a negative result even though an infection is present.

The Fassisi® CanDiro detects Dirofilaria immitis antigen in serum, plasma, or whole blood. Dirofilaria immitis antigen is also detected in the CanVecto 4 and CanVecto Trio combination tests, which facilitate the simultaneous screening of multiple pathogens from a single sample in cases of vector-borne diseases and when a relevant travel history is present.

Evidence: Sources page.