ICD-10 Code A83.1 – Western equine encephalitis (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code A83.1 – Western equine encephalitis
What it is
A83.1 identifies western equine encephalitis, a mosquito-borne viral infection that causes inflammation of the brain. Use it when the diagnosis is documented as western equine encephalitis.
Clinical signs
Clinical features vary; refer to documentation. Typical encephalitis findings may include fever, headache, altered mental status, seizures, or neurologic deficits, but coding depends on the documented diagnosis.
When to use this code
Use A83.1 when the provider documents western equine encephalitis as the confirmed or assessed diagnosis. It may appear in inpatient, emergency, or neurologic evaluations where the infection is the reason for care.
If the record only notes suspected viral encephalitis, check documentation before assigning this code. Code from the clinician’s final diagnosis rather than from exposure history alone.
Do not use for
Do not use this code for other arboviral or unspecified encephalitis diagnoses. Avoid it when the chart lacks a clear statement of western equine encephalitis; check documentation.
Coding tip
Confirm the exact encephalitis type in the assessment, discharge summary, or problem list before coding A83.1.