Had a lab get bit once during early duck season by a cottonmouth. His entire head swole up, eyes swole shut, I took him to the vet and the Dr gave him a shot (I don't know what it was but it wasn't anti-venom more like an antihistamine) when he needed to go outside he would walk with the wall on his shoulder for feel to the door because he couldn't see. About three days later he was fine.