2026 ICD-10-CM Diagnosis Code A83.1Western equine encephalitis

ICD-10-CM Codes›A00–B99›A80-A89›A83

ICD-10-CM A83.1
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

A83.1 is a billable ICD-10-CM diagnosis code for Western equine encephalitis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026). Coders also document this condition as encephalitis caused by Alphavirus. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Encephalitis and Viral infection.

Code Identity

ICD-10-CM Code
A83.1
Billable Status
Yes — Valid for Submission
Code Describes
Western equine encephalitis
Short Description
Western equine encephalitis
Same as the full description in the CMS dataset.
Parent Code
A83
Mosquito-borne viral encephalitis

Code Classification

ChapterA00–B99Certain infectious and parasitic diseases
SectionA80-A89Viral and prion infections of the central nervous system
CategoryA83Mosquito-borne viral encephalitis
This CodeA83.1Western equine encephalitis

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Encephalitis caused by Alphavirus
  • Invasive viral infectious disease
  • Neuroinvasive Western equine encephalitis virus infection
  • Non-neuroinvasive Western equine encephalitis virus infection
  • Western equine encephalitis

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • equine (acute) (infectious) (viral)
        • Western
    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • viral, virus
        • arthropod-borne NEC
          • mosquito-borne
            • Western equine
    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • Western equine
    • Encephalomyelitis
      • equine
        • Western

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NVS002
Encephalitis
Default principal diagnosis: inpatient Yes · outpatient Yes
CCSR INF008
Viral infection
Default principal diagnosis: inpatient No · outpatient No

Patient EducationClinical

Encephalitis

Encephalitis is inflammation (swelling) of the brain. It can happen when an infection or medical condition that affects the brain activates your immune system. The inflammation can cause a wide range of symptoms. In extreme cases, it can lead to brain damage, stroke, or even death.

The full article covers:

  • What is encephalitis?
  • What causes encephalitis?
  • Who is more likely to get encephalitis?
  • What are the symptoms of encephalitis?
  • How is encephalitis diagnosed?
  • What are the treatments for encephalitis?
  • Can encephalitis be prevented?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert A83.1 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
062.1 West equine encephalitis
Exact Match The mapping is direct, with no qualifiers.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About A83.1Overview

Is A83.1 (Mosquito-borne viral encephalitis) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report Western equine encephalitis on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What is the ICD-9 equivalent of A83.1?

Under the General Equivalence Mappings, Western equine encephalitis converts to ICD-9-CM 062.1 (West equine encephalitis). The mapping is a direct match.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.