Got my new Lab , and was going over the prior owners vet records he had on the Lab. It was last on a Flea and heartworm preventative back in April of this year. It was supposed to have it done last Month, but from the looks of the records, it was not done. They said it tested negative for Heartworm at it's last checkup. I checked with the Vet , and they confirmed it.

Question:

When would it be safe to start it up again ?