Does my pet need bloodwork before starting heartworm preventative?

Giving heartworm preventative to dogs or cats that are already infected with adult heartworms can be harmful and even fatal. A blood test before starting any medication will tell your veterinarian if your pet is already infected. Heartworm preventatives kill heartworm larvae, but the medicine does not kill adult heartworms. If your pet is heartworm-positive while on preventative, the disease will only worsen.