The nematode Dirofilaria immitis, commonly called heartworm, belongs to the class Chromadorera, together with other important canine endoparasites, like roundworms, hookworms and whipworms. It further is classified as a member of the order Spirurida and within that of the family Onchocercidae. The main representative of the genus Dirofilaria is D. immitis, but another closely related Dirofilaria species, D. repens, is also known to infect dogs (subcutaneous dirofilariosis).

Like many other in his family, the development of D. immitis requires both an arthropod and a mammal host. First, adult female heartworms release their offspring, called microfilariae, into an animal's bloodstream. Then, mosquitoes become infected with microfilariae while taking a blood meal from the infected animal. During the next 10 to 14 days, the microfilariae mature to the infective larval stage L3 within the mosquito’s Malphigi vessels. After that time, the infective larvae enter the host during feeding of the mosquito e.g. on another dog, cat or other susceptible animal. Seventy days after infection migration and exsheathment to the adult stage is complete. Approximately 6 months after infection female adult worms contain mature microfilariae. These new offspring generation of microfilariae cannot mature into adult heartworms without first passing through a mosquito. Patent infections with microfilaremia in the dog are observed 7 to 9 months post infection.

The ubiquitous presence of one or more species of vector-competent mosquitoes makes transmission possible wherever a reservoir of infection and favorable climatic conditions coexist.

A pivotal prerequisite for heartworm transmission to occur is a climate that provides adequate temperature and humidity to support a viable mosquito population, and also sustain sufficient heat to allow maturation of ingested microfilariae to infective, third-stage larvae (L3) within this intermediate host. Laboratory studies indicate that development and maturation requires the equivalent of a steady 24-hour daily temperature in excess of 18°C (64°F) for approximately one month. Intermittent diurnal declines in temperature below the developmental threshold of 14°C (57°F) for only a few hours retard maturation, even when the average daily temperature supports continued development. At 27° C (80° F), 10 to 14 days are required for development of microfilariae to the infective stage within the mosquito.

The length of the heartworm transmission season in the temperate latitudes is critically dependent on the accumulation of sufficient temperature to incubate larvae to the infective stage in the mosquito. The peak months for heartworm transmission in the Northern Hemisphere are July and August.

In areas with low prevalence, a nidus of heartworm infection may be detected which usually represents both, a focal spread of infection and heightened awareness through increased testing. Once a reservoir of microfilaremic domestic and wild canids is established beyond the reach of veterinary care, eradication becomes improbable.