ICD-10 Code A83.3 – St Louis encephalitis (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code A83.3 – St Louis encephalitis
What it is
A83.3 identifies St Louis encephalitis, a viral infection of the central nervous system caused by the St. Louis encephalitis virus. Use it when the record documents this specific encephalitis rather than a nonspecific viral brain infection.
Clinical signs
Clinical features vary; refer to documentation. Typical encephalitis findings may include fever, headache, altered mental status, confusion, or neurologic deficits, but the diagnosis should be supported by provider documentation of St Louis encephalitis.
When to use this code
Use A83.3 when the clinician explicitly diagnoses St Louis encephalitis, whether suspected, confirmed, or listed as the reason for admission. It is appropriate for documented cases involving CNS infection from the St. Louis encephalitis virus. If the chart only says “viral encephalitis,” check documentation before assigning this code.
Do not use for
Do not use A83.3 for other viral encephalitides, meningitis, or unspecified encephalitis. If the organism or diagnosis is unclear, check documentation and code the documented condition instead.
Coding tip
Code A83.3 only when St Louis encephalitis is specifically named in the record; otherwise, query for clarification.