This week we've had a larger than normal number of biters in my back yard, but the wind has been very still, too. They aren't strong fliers, so a good breeze literally blows them away. This is way too late to start medication. The treatment for adult heartworms can be very deadly. I just would not dream of not having my dogs on preventative if I were in affected areas.
Most areas do have the disease.
Please don't wait until your dog tests positive - this is far to late. It does not detect the larval stage injected by the mosquito. It takes about 6 months from infection to detection. Since it's mid winter, everyone except those living in very warm climates has time to make decisions for the coming year. For BCCA members I had an article on heartworm preventives in the August 96 issue.