Oh, poor soul. It's in the commonly found area (on the rump) and ger hair looks 'snapped'
Your vet might do a scraping to look at, you'll most likely get Ivermectin. Have they both got it? They'll treat both of them to be safe. Can you scrub out their cage (steam if you have a steamer) then all fresh bedding too. Good Luck sleepypigs
Another injection in 2 weeks time. Vet did say though that she was surprised how early we recognised it as normally she sees piggies in a really bad condition before anyone notices so I am pleased about that.
If it recurs, it's possible to treat mites yourself with oral ivermectin - the Guinea Lynx forums have the dosage instructions.
When I had pigs, I bedded mine on fleece as it took away any wetness from the surface, rather than having soggy patches like towels will. In a large cage with 3 pigs I could change every 2 days on fleece.
Keep an eye on them though - one of mine used to suffer with mites flare ups before the onset of illnesses - she was subject to respiratory problems throughout her life and mites were usually the first indicator she was about to get ill.