Here's another article from PetMD recommending year-round heartworm prevention as well:
https://www.petmd.com/blogs/fullyvet...n_winter-11867
"I generally recommend that owners give heartworm prevention 12 months a year, and here’s why:
In many parts of the country, mosquitoes remain active year-round. Unless you live in an extremely cold and/or dry region, your dog or cat may be bitten during any month of the year. Travel can also complicate the situation. When I lived in western Wyoming, many of my clients did not give heartworm prevention during the winter, which was defensible in that extreme client. But, a lot of folks also liked to escape the frigid winter temperatures or the slop of spring and would forget to protect their pets when they traveled.
Keep in mind also that most preventatives do more than protect against heartworms. Some prevent flea and tick infestations, mange, lice, or intestinal worms, and stopping will leave your pet open to these problems during the winter months. This is especially important if your pet has contact with other animals in boarding facilities, doggy day care centers, at the groomers, etc."