Heartworm prevention time is quickly approaching, don’t let the cold temperatures lull you into a false sense of safety! I know some people keep dogs on heartworm prevention meds year-round — and many veterinarians recommend this. Because climate change is warming winter, and mosquitoes season starts earlier and stays later each year, timing is getting too difficult to predict. A week or more of warm weather in the middle of winter can create the right conditions for a few to hatch. It only takes one to bite.