Resuming heartworm preventative without a test ?

It depends on the preventive being used :). Heartgard and generics(ivermectin) are often given to heartworm positive dogs before treatment. So while there is a chance of a reaction, it's probably safe. Others can cause such a serious reaction that it would probably be best to test before you go.

If you keep him on a preventive from here on out, you won't need another test this year.

Doug, DVM