ICD-10 Code A83.9 – Mosquito-borne viral encephalitis, unspecified (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code A83.9 – Mosquito-borne viral encephalitis, unspecified
What it is
A83.9 identifies an unspecified mosquito-borne viral encephalitis. Use it when the record confirms a mosquito-transmitted viral infection of the brain, but does not name the exact virus.
Clinical signs
Typical findings include fever, headache, altered mental status, confusion, seizures, or other neurologic deficits. Clinical features vary; refer to documentation for the specific encephalitis presentation.
When to use this code
Use A83.9 when the provider documents mosquito-borne viral encephalitis and no more specific organism is stated. It also fits cases where the chart supports arboviral encephalitis, but the virus remains unconfirmed. Check documentation if the illness is only suspected.
Do not use for
Do not use this code for meningitis without encephalitis, or for encephalitis from a non-mosquito source. If the virus is identified, code the more specific diagnosis instead.
Coding tip
Look for the exact wording “mosquito-borne” and “encephalitis”; if either is missing, verify the documentation before assigning A83.9.