Preventative drugs can’t ever be given to dogs that already have heartworms. Only a veterinarian can determine if a dog is already infected with them. Blood tests can show if a dog has the problem and to what degree.
You should make sure that your dog has a protected place to sleep during mosquito season. Bring him or her into the house, garage, or a screened porch. Mosquito repellent is also available but it’s not very effective.
