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

Microfilariae are heartworm larvae. Dogs (and cats) become infected for heartworms by a mosquito bite containing microfilariae, that grow into adult heartworms. When an animal tests positive for heartworms with a snap test, a vet will take a blood sample to confirm they see microfilariae under the microscope. That squiggly moving hair like thing is the microfilariae they are looking for to confirm the positive test result. The snap test itself is actually testing for the presence of heartworm an