[Pebbles82]

It is basically an infected abscess and will most likely need antibiotic treatment. It is caused by tiny cuts and scrapes which become infected. So if you don't have mesh shelves then look at what else in the cage could be causing it. Does the wheel have a rough surface? Ideally a wheel should have a smooth, solid surface. Some have joints or bumps on them. It is usually a wheel or a shelf as that is what is most walked on.