I give Heartgard year round to my boys. We have milder winters here than you have and all it takes is one warmish day -- I had a mosquito land on my hand this past December.
There is also the matter of the life cycle of eggs and heartworms. I am not well versed enough to explain it properly, hopefully someone else will. If a dog is infected at the end of the last dose and then taken off the preventative for 2 - 3 months, that is very bad news.
__________________ Kristin, Max and Teddy  |