You should give your dog heartworm prevention all year round. Heartworm prevention medication works by intermittently killing the larvae of heartworm which may have transmitted to your dog by infected mosquitoes. And heartworm prevention are killing larvae that may have infected a dog in the previous two months. The larvae will mature and cause serious disease if you don’t give your dog the consistent administrating of preventive medications. In order to avoid this serious and life-threatening infection, it is important and vital to give your dog heartworm prevention medication consistently all year round.