Barbering

Barbering means a mouse grooms its conspecifics so intensively that they have light or even bald parts in the fur or their whiskers. Sometimes you can hear the groomed mice cheeping.
Contrary to funghi or parasites which can also cause fur loss, the skin is bald but it looks normal and healthy without any changes.
Individuals barbered around their nose are usually almost completely bald there and the whiskers, necessary for orientiation, are missing.

Barbered Indian gerbil

This behaviour usually occurs all of a sudden. It is difficult and sometimes impossible to determine an exact cause.
Usually the mice reduce social pressure via barbering. The mouse in the higher position grooms one in a lower position. Some people think about a congenital reason for barbering. Against this assumption stands that the disorder occurs suddenly and that it often disappears after some changes. Above that barbering can be seen with mice from too small and boring cages more often. That's why it is regarded as a behaviour disorder.

Barbering is most common with fancy mice barbering each other, but there are also fancy mice barbering spiny mice at their back. Seldom you can watch the behavior among gerbils.
On other species I still haven't observed barbering at all.

Barbering is usually stopped quite easily. Sometimes offering more space or keeping mice more busy is all you have to do.
If that doesn't work, you have to change the group structure which will probably lead to a success. If a new mouse is introduced to the group or one dies, the structure of the group changes. Nearly always barbering disappears after that.

Translation
Angelus Noctis
Proofreading
Jedediah