Canine heartworm disease is caused by a parasite called Dirofilaria immitis. Dirofilaria immitis has a somewhat complicated lifecycle involving several larval stages and multiple hosts. Immature heartworms, referred to as microfilariae, circulate in the bloodstream of infected dogs. When an infected dog is bitten by a mosquito, the microfilariae enter the mosquito’s salivary glands and mature through several stages.