Mites may be present without causing itching and then when the pig is stressed e.g. pregnancy, weaning, fighting etc.; the mites overcome the pig's resistance and cause disease. Normal washing of the hands with soap and water after handling cavies will prevent transfer of mites from one group to another, but infestations are easily acquired when introducing new guinea pigs from another group.
Treatment should ideally be given to all guinea pigs in a group at one time, whether they are showing symptoms or not.
I am actually doing a lot of research on mites right now for gl.