Last week we took a look at some of the basic facts about heartworm disease—what it is and how your pet can contract it. This week we will conclude our discussion of heartworm disease by focusing on how heartworm preventatives work in your dog’s body.

Here’s how Heartworm Preventatives work:

Heartworm preventatives are larvaecides—they act by killing the still-developing heartworms before they are able to fully mature. While this medication is extremely effective against the larval form of heartworm, it is ineffective against, and will not kill, any adult heartworms which are already present and fully grown in your pet. Since it is ineffective to administer the preventative to a pet which already has adult heartworm, before prescribing heartworm preventatives to any dog which has not previously received heartworm protection, we perform a heartworm test. In the unfortunate event that your pet has the disease, we will have detected it and can begin treating it. Because heartworm larvae take 6 months to fully develop, it is safe for puppies less than 7 months old to begin taking a heartworm preventative without a test— any older and they should be tested first.