Are you sure of the data you got (in DC area)
Washington DC Arthropod Biodiversity Project Colubridae......
The copperhead is not normally a lethal bite to a GSD. Several of our dogs on the team have been bit by a copperhead. We normally give a large dose of oral benedryl immediately and take the dog to the vet. If you did not see the dog get bit you will see the swelling and the tooth marks. Any swelling like that on a dog would get the same treatment.
When we go up into the mountains, there actually ARE timber rattlers. I carry injectable benedryl and dexamethasone as some areas are much greater than 30 minutes to get to a vet and the Timber Rattler can kill a dog or a person.
There are ways to snakeproof a dog using an ecollar. Not too common in the east and very common out west. I am frequently in the woods off trail in snake habitat with my dog and just use common sense.