Illness:
For degus, having orange teeth is a sign of heath.This orange colour come for vitamin C. If they becaume really white, it can be mouth problem. But if your octodon dosent seem tierd or sick, maybe he didnt eat green tings for some time.
This is the only illness I am documented on, if you have a question contact me or u can always go see a veterinary.
If your degu seems in good healt, his cage is always clean and he eats well but he still seems depressive, its because he maybe want a compagnion.
Degus in wild life live with compagnions. If your degu is a female, I recomand you to buy an other female as a friend for her. If its a male his compagnion and him might fight. If you dont want an other degu you can at least spend more time with your small friend.