Causes
Infectious (bacteria, viruses, fungi, protozoa) or immune-mediated (the immune system attacks the nervous system — common in young dogs of certain breeds).
Diagnosis
Neurological examination, blood tests, cerebrospinal fluid analysis, MRI.
Treatment
Depending on the cause: antibiotics/antifungals if infectious, immunosuppressants if immune-mediated. Early treatment greatly improves the prognosis.
Prevention
Difficult to prevent. Prompt detection and treatment are the most important thing.

