A83.9
BillableMosquito-borne viral encephalitis, unspecified
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is A83.9 an HCC code?
No. A83.9 is a billable ICD-10-CM code but does not map to any HCC category in V28, V24, ESRD, or RxHCC.
This code does not map to an HCC category in any model (V28, V24, ESRD, RxHCC).
What This Code Means
A83.9 is the ICD-10-CM diagnosis code for mosquito-borne viral encephalitis, unspecified. This code describes inflammation of the brain caused by a virus transmitted through mosquito bites, but the specific type of mosquito-borne virus has not been identified. It is used when a patient has symptoms of viral encephalitis from a mosquito-borne source, but testing has not determined which specific virus is responsible. A83.9 sits in the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99), within the section covering viral and prion infections of the central nervous system (a80-a89).
A83.9 is a billable ICD-10-CM code but does not map to a payment HCC under the CMS-HCC V28, V24, ESRD, or RxHCC risk adjustment models. It can be reported on Medicare Advantage encounter data submissions but it does not contribute to a beneficiary's RAF score and therefore does not affect risk-adjusted payments to the plan.
Use this code only when the specific mosquito-borne encephalitis virus (such as West Nile, Japanese encephalitis, or dengue) cannot be determined; if the specific virus is identified, use the more specific A83.0-A83.8 codes instead.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for A83.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Use this code only when the specific mosquito-borne encephalitis virus (such as West Nile, Japanese encephalitis, or dengue) cannot be determined; if the specific virus is identified, use the more specific A83.0-A83.8 codes instead
- •Ensure documentation supports mosquito transmission and encephalitis diagnosis; coordinate with laboratory results and physician documentation to determine if a more specific code becomes available