hi-
my third pig was a rescue from animal control. when the chief officer telephoned me [she's an acquaintance and knows i love pigs], i was only going to help with transporting the pig, initially, to a vet who is about 90 minutes away, and is a guinea pig rescuer of ten years.
i decided to keep him because after quarantine was over, they all got on famously. well- a few weeks later, i noticed very DARK brown flecks in the fur of the tortie and white; they were located in abundance on his rump area. the recent rescue, perhaps a carrier of mites, and my other pig were not showing any dead skin flakes. i've read that mites are really common in pigs, can actually be present for a long time without any problems, then a trigger- stress, environmental change, something- can turn asymptomatic to symptomatic.
the vet said mites, and they were all three treated. it sort of freaked me out at first- never having dealt with said issue. the main thing was to simply treat them for mites, and it was fairly simple.
good luck to you.