We are currently recommending that heartworm preventive medications be given monthly all year long. The last few winters in New England haven't been cold enough to kill all mosquitos. It is so much easier to prevent the disease than it is to treat-today's monthly heartworm preventives also help to protect against many intestinal parasites. Some intestinal parasites can be transmitted from pets to people. By giving the medication monthly, you are protecting both your pet and your family.