black dust coming off of skin when rubbed

Question
Hi Abi,
  We have a male guinea pig about 11 months old.  In the last day or so we have noticed that when holding him and petting him, he lost some hair and saw white and black dust on us or wherever he was(tonight I held him over the sink to look at what fell off on a white surface). I don't see anything moving but could this be a sign of a bug, like mites?  How do you know if they have mites?  He seems upset over the rubbing but I don't know if it hurts or what.
  So what would you suggest?
  Thank you!
  Sincerely, Gennifer

Answer
i think its dander, but it could be a mite of some sort. you can basically tell because they'll seem more aggitated, may not be acting the same (loss of appitite, playfulness, etc...), or you may find little scabs in their fur. i suggest going to the vet because you'll get a definate answer.