Causes
Almost always due to unsuitable vivarium conditions: insufficient humidity, lack of objects to rub against, dehydration, poor nutrition or an underlying illness.
Diagnosis
Examination by an exotics vet plus a review of the vivarium conditions.
Treatment
Humidity baths and careful removal of the retained skin (NEVER pull it off dry). The key is to correct the humidity and the vivarium conditions. Rings that constrict the toes must be removed promptly.
Prevention
Humidity appropriate to the species, rough objects for the reptile to rub against, good hydration and nutrition.

