This slide shows a blood smear containing  two microfilariae of the dog heartworm (Dirofilaria immitis). Infection by this parasite occurs when the larval heartworms are transferred to the definitive host through the bite of a mosquito, the nematode's intermediate host.  Heavy infestations of adult worms (which live in the heart and large arteries of the lungs) can be fatal. The regular use of heartworm medication (obtained from a licensed veterinarian) can prevent the disease.