It's crucial to take your mouse to a veterinarian as soon as possible. A vet can properly diagnose the problem and provide appropriate treatment.
Here are some possible reasons for a purple tail and tail loss in mice:
* Necrotizing Dermatitis: This is a bacterial infection that can cause tissue damage and discoloration.
* Trauma: A tail injury could lead to bruising, inflammation, and even necrosis.
* Tail Slip: This condition can cause the tail to detach at the base.
* Nutritional Deficiency: Certain deficiencies can affect skin and tissue health.
Please note that I am not a veterinarian and this is not a complete list of possible causes. It's vital to consult a professional for accurate diagnosis and treatment.