My dogs are on heartworm prevention meds year round. All I know is the mosquito's are going to be really bad this year and soon. My vets office has been sending reminders and facebook updates about the bad mosquito season and making sure people are giving heartworm prevention meds, as well as local rescues. I probably see at least a post a day about it. I'm in Michigan so not too far from you, and if it were me I would start now. We already have mosquito's hatching here.