My new boar picked up mites from the girls when I was treating them although in a separate cage as he was waiting after surgery to be with them. He either got them from my clothes or the sofa or from a fleece snuggy eventhough I wash them at 60 degrees.
I have sprayed the cages with F10 insecticide (not the disinfectant) that I got from Amazon, and removed all the fleece and houses. I have changed back to finacard and plastic houses while I treat them (I don't think they're impressed). They're going back to fleece again soon. It would be too expensive to buy all new fleece as I have so many tunnels etc so I have my fingers crossed all the mites in them are now dead and no eggs are there waiting for a piggie to come by. As I understand it, all the eggs would be in the piggie's skin.
Once you have got an acute outbreak, mites jump from piggy to piggy. You always have to treat all guinea pigs in direct or indirect contact with each other.
Unfortunately, what ivermectin can't do, is kill any dormant mite eggs in the skin, but before ivermectin, there was nothing to treat mange at all. But with prompt treatment of all piggies in the house, you should be able to get on top of it fairly easily. I get on average one or two mites outbreaks per year with mine; most of them are residual mite eggs that come up in the same place, often after a bereavement, a stressful situation in a group or an illness.
Unless it is a severe outbreak (which can happen especially with frail older, pregnant or young piggies with a niot yet fully developed immune system, it is usually enough to treat all companions with three rounds of weaker easimec if you treat promptly in a home where you have a good number of piggies, and only do the full high strength course with any acutely affected or frail piggies that don't have the support of fully functional immune system to get on top of the mites. The trick is to treat promptly and to treat all piggies.
If in any doubt, please see a vet for a diagnosis and do not home treat on spec. I have had a very similar looking bare patch in the same location in sisters with only three weeks apart, and one turned out to be mange mites and the other ringworm. And yes, I see a vet if I am in any slightest doubt whatsoever, too!
The tips I have given you are enough to get on top of your current mites outbreak.