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