ICD-10 Code A84.9 – Tick-borne viral encephalitis, unspecified (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code A84.9 – Tick-borne viral encephalitis, unspecified
What it is
A84.9 identifies tick-borne viral encephalitis when the specific virus or subtype is not documented. Use it for an unspecified encephalitis acquired through a tick-borne viral infection.
Clinical signs
Clinical features vary; refer to documentation. Typical encephalitis findings may include fever, headache, altered mental status, confusion, seizures, or focal neurologic deficits.
When to use this code
Use A84.9 when the record states tick-borne viral encephalitis but does not name the exact organism or subtype. It also fits when the clinician documents encephalitis linked to tick exposure and confirms a viral cause without further specificity.
If the chart later identifies a more specific tick-borne viral encephalitis, code the documented specific diagnosis instead. Check documentation for site, organism, and any associated neurologic manifestations.
Do not use for
Do not use this code for nonviral meningitis, bacterial encephalitis, or tick-borne illnesses without encephalitis. Check documentation if the diagnosis is only suspected or if a different neurologic infection is named.
Coding tip
Capture the most specific tick-borne viral diagnosis available; if the provider leaves it unspecified, A84.9 is the appropriate fallback.