Why should you continue heartworm prevention through the winter if there are no mosquitos? Great question! The heartworm preventives that we use here at Northwood also contain intestinal worm prevention and flea prevention. We can honestly say that this past month has been the worst month for fleas. So yes, they are still around, and yes, they can still infest your pet. Intestinal worms are outside every month, all year round. Even in the winter. Several of these worms are contagious to YOU. Do what’s best for your pet (and your family) and use these products every month of the year.