Guinea Pig dry skin

Question
Hello, Sam. I have 2 guinea pigs. One is a girl name Snickers she's pretty old(she turned 6 on April 20th) and I had noticed that she had something that looked like dandruff mainly towards her backside and she has been losing a lot of hair. So when I examined that back i noticed that she has a little round patch of hair that is missing. I had checked her and there are no bugs(lice and fleas) that I can see on her. And I also checked my other guinea pig(Toshi) I'm not too sure how old he is, I had gotten him from a friend's friend who wasn't able to take care of him anymore. But anyway when I checked him his hair is perfectly fine no dandruff, no missing hair patches no nothing. I have had both for over a year now and I know that they are not aggressive with each other or anything. Could it be possible that this has something to do with old age? Or do you know of anything that I can treat dry skin with on a guinea pig? Thank you very much for your help.

Answer
it will be mange, the bugs are too small to see.

get some mange spray and tea tree oil cream. spray all over the guinea pigs body, them apply the cream to the areas missing hair, after a week or two the piggy will be fine, i suggets you treat the other guine apig with the spray too just in case.