Sadly, this has been going on for years.
I haven’t located my old records but I believe this started approximately 3-4 years ago and concerned 11 guinea pigs. I was busy. I was working enough overtime at the office and using my tax return to take all the sows in for scurfy skin. I spent about $400 and saw extremely mild results.

I don’t see them excessively biting or losing hair but I did notice heavy flakes on my TSW satin sow right along the axis of color over her flank. I saw heavy black flakes moving over into her bright, white fur from the opposite side. I inspected the other girls and found that Coriander was not the only one suffering from this. As I scratched gently along the flakes and they were wildly upset with me.

Two laundry baskets of guinea pigs go to the veterinarian’s office. I had 4 girls in 1, 5 girls in the other and the 2 boys had to be separate because they hadn’t had their neuters yet.

We did hair plucks to test for mites and scrapings for fungus and nothing came back with a positive reading. From trolling around the internet and talking to my vet, sometimes the tests can be inconclusive so as a precaution we treated the entire herd with Revolution. It’s normally for fleas on dogs and safe for guinea pigs or at least it was years ago. We had to weigh all the guinea pigs and dose them between the shoulders accordingly by syringe and CCs.
This required 2 courses a month apart.

It seemed to lessen for a while but didn’t clear up. Fruitless. Expensive. Piggie Sympathy.

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I was going to get a hairless guinea pig and read on the internet that the products at Gorgeous Guineas in the UK were highly recommended for Skinny and Baldwin hairless guinea pigs. Chrissie has items to naturally treat parasites, fungal infections, dry skin, bumblefoot, sensitive skin of nekkid piggehs, etc. I got the manuka and neem melt and shampoo and shaved my guinea pigs down and followed the instructions.

Instructions do not include shaving the pigs. That was for me. It allowed me to know that the melts were firmly to the skin and that I could monitor their progress. It really helped.
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My hairless is always ashy because of the lack of humidity. I use Gorgeous Guinea’s calendula lotion for her and occasionally an aloe melt. My youngest furry one seems fine. My two old ladies have really bad dandruff that has chunky pieces and feels like scabs, but there aren’t any. Just before I left for vacation I have used what I had on hand: Lice n Easy shampoo, incase of any cooties that I didn’t see and gave them a swabbing of diluted microneem, which will at least start on a fungal infection until I get my supplies reordered.