The preventatives will not kill adult heartworms, so the only way to be sure your pet is free of heartworm disease is to have the blood test performed.
Do dogs need heartworm test before Heartgard?
It is recommended by most veterinarians and by the manufacturers of the Heartworm medications that each dog have a test prior to receiving the medications each year, to ensure that the pet is free from Heartworm disease.
Can I give my heartworm positive dog Heartgard?
Heartgard is the safest heartworm prevention for heartworm positive dogs; it is important to remain on heartworm prevention throughout your dog's life. Your dog is on Heartgard for 2-3 months prior to initiating treatment because it kills some of the circulating larval forms before they become adult worms in the heart.
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