Microfilariae, the infective larvae stage of the heartworm, are visible under the microscope in a sample of Lillian’s blood. Microfilariae are transmitted from infected dog to mosquito upon feeding then from mosquito to uninfected dog. Once microfilariae enter a dog, they move towards the heart and its surrounding blood vessels. Six months later, the fully mature heartworms mate and reproduce more microfilariae, continuing the cycle of infection and transmission. This life-threatening infection