Another possibility (which needs to be ruled out) would be intestinal worms. Since that wasn't part of the discussion, I'm going to assume that she's on appropriate anthelmintics for your area — especially given the possibility of her picking up parasites and parasite eggs in the wild.
She is not on any worm medicines. I always thought they were used if there was evidence in the stool, and hers has never shown anything. It sounds like I should be changing my approach. How much exposure does she need to a carrier to acquire something?
If she hasn't been dewormed, given her lifestyle, I would bet that she's got at least roundworms at home in her gut which could in turn be causing perianal irritation.
As suggested in my previous comment, her outdoor activities dictate regular prophylactic use against roundworms especially which can be found in the soil everywhere. There is no evidence of eggs shed in the stool because they're microscopic; they can be tested for, but the cost of the various tests is way more expensive than the relatively cheap and effective medications (eg, pyrantel pamoate) which have a wide margin of safety.
Tapeworms will usually make themselves known by excreted segments adhering to the perineal area/fur and in the dog's bedding; fresh they look like cucumber seeds and are motile, dessicated like toasted cumin seeds. They can be treated as necessary.
In all cases there is zoonotic potential. So if you entertain small children or immunocompromised persons, you'd do well to act on the cautious side.
