Dog or canine heartworm (Dirofilaria immitis) disease is increasingly prevalent in domestic dogs and wild canids (i.e. coyotes). Dog heartworm is endemic to Lake County where it is transmitted by mosquito vectors. This filarial nematode (roundworm) causes severe circulatory disease in dogs and coyotes, but also can dog heartworm cause respiratory disease in humans. Mosquitoes become infected by ingesting very small infective stages of the worm (microfilaria) while feeding on an infected dog. In the mosquito, the worm molts twice and moves to the mouthparts where they remain until the mosquito refeeds. During blood feeding, the worms move out of the proboscis and into the bite wound. In dogs and other canines, the worms molt to the adult stage and migrate to the large blood vessels and heart where mating and reproduction occur. Disease in dogs occurs when the number of worms becomes sufficiently large enough to impede blood flow and heart function. In humans, immature worms frequently become encapsulated in the lungs where they are detectable by chest x-ray.