By Dr. Ken McMillan
DTN Contributing Editor
I give my dog heartworm preventive every month, but my vet told me he's seen cases where consistently-treated dogs still develop heartworms. How is this possible? I've been told we are to use this preventive every month of the year, but I wonder if that can actually cause resistance in these worms? Using antibiotics too often can create resistant bacteria, so why doesn't heartworm preventive do the same thing? Please explain the difference.