JOEB, dogs of course can be bitten by snakes at anytime. This would occur more readily out in the field while the dogs are in grassy areas etc. where snakes like to sun themeselves and look for food. You wouldn't find one down an inhabited earth or any earth that is used by game for that matter. They would more likely be in secluded places under fallen trees, under rocks etc.
Snakes are clever and can sneak away undetected. I never had a terrier bit by a snake, but have happened on them a few times.