If it gets cold during the winter months in Edmonton, you don't need to use the product for those months as there are no mosquitoes, it is spread by mosquitoes. Tiny heart worm larvae are ingested when a mosquito bites an infected dog. Usually treatment is from May-October. (That's if your in a area that gets very cold/snow etc) I am in Montreal and only treat 6 months out of the year. I am using heartgard now, so far so good. I am not treating for fleas and ticks as I have never seen a flea in my life and ticks are not so common here, I check the dogs after their walks. Here is the link for heartgard:
http://heartgard.ca/faq