After your pet has been treated for adult heartworms, the immature worms (microfilariae) must be removed from the blood to prevent your pet from serving as a source of infection for other dogs.
Microfilariae are eliminated with medication. After the treatment period, a blood sample will be checked. If microfilariae are still present, treatment is repeated until the blood is free of microfilariae.
After the doctor is certain that all microfilariae have been eliminated, a program of preventive medication is instituted so your dog does not become reinfected.