Coming in late on the heartworm thing, but I seem to have been "tagged" so you all get my two cents worth whether you like it or not.
The reason for heartworm testing is that you can't simply start a preventative if your dog actually has heartworms. The preventive will kill the immature heart worms circulating in the blood stream and that can be fatal if it causes anaphylaxis. And the preventative doesn't kill adult heart worms anyway. So the testing determines the presence of heart worms, allows time for treatment, and then the dog goes on preventative after that. Preventative without testing first can kill the dog, and since heartworm is transmitted by mosquitos, and adult worms continue to live in the heart, the lengthy and sometimes risky treatment has to be completed before preventatives can work.
And Bruce is right - I insisted on my dog being tested when we moved here, and after a couple of years the vet finally convinced me that heartworms just weren't a big issue here. However, that has changed in the last 20 years. There have now been reports of dogs with heartworm close to the Big Horn Basin, so guess what I'm back to doing, even though my vet rolls her eyes at me when I ask for the test? Mosquitoes fly, doggone it! Back many years ago when I was a vet-tech, the treatment for heartworms was basically IV arsenic and some really bad years in the Chicago area we lost almost as many dogs as we saved. I don't know exactly what it is now, and I have no intention of finding out! Yep. Test.