This is probably a dumb question, but I've never had a dog get fleas before. (I guess there are advantages to having a dog that cannot be around other dogs.) Kabota got fleas last month, I switched him to Vectra and the fleas went away. We're now entering winter, in PA, and I'm wondering how much longer do I have to give him Vectra? Is all winter necessary, just until it's regularly below freezing at night, or what?