Purrley, I can promise you that anyone who has watched a dog suffer through heartworm treatments will caution you to use the preventative 12 months out of the year. One of our new members here, Quanzhan, is going through it with her rescued Golden Retriever, Zack, right now. Please, use the preventative. I'm with Souraya, on Tess, who will be so small, it will cost you much less than it does those of us with large dogs.