2027 ICD-10-CM Diagnosis Code A83.3St Louis encephalitis
ICD-10-CM Codes›A00–B99›A80-A89›A83
- Billable — Valid for Submission
- MCC — Major Complication or Comorbidity
- Not Chronic
A83.3 is a billable ICD-10-CM diagnosis code for st Louis encephalitis. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 23 through 24, 97 through 99. As a secondary diagnosis, it counts as a major complication or comorbidity (MCC) and places an inpatient stay in the highest severity level of its MS-DRG family. It does not count, however, when the principal diagnosis is one of 174 closely related codes. Coders also document this condition as arbovirus encephalitis. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Encephalitis and Viral infection.
Code Identity
Code Classification
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Arbovirus encephalitis
- Disease caused by Flaviviridae
- Invasive viral infectious disease
- Neuroinvasive St. Louis encephalitis virus infection
- St. Louis encephalitis virus infection
- St. Louis encephalitis virus non-neuroinvasive disease
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to A83.3: its own notes plus those printed at A83, block A80-A89, and Chapter 1. A note printed at a category, block or chapter applies to every code under it.
Includes
Further defines, or gives examples of, the content of the category.
- mosquito-borne viral meningoencephalitis
- diseases generally recognized as communicable or transmissible
Excludes1
Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.
- carrier or suspected carrier of infectious disease (Z22.-)
- certain localized infections - see body system-related chapters
- infectious and parasitic diseases complicating pregnancy, childbirth and the puerperium (O98.-)
- infectious and parasitic diseases specific to the perinatal period (P35-P39)
- influenza and other acute respiratory infections (J00-J22)
Use Additional Code
Add a second code, after this one, to fully describe the condition.
- code to identify resistance to antimicrobial drugs (Z16.-)
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Encephalitis, St. Louis
a viral encephalitis caused by the st. louis encephalitis virus (encephalitis virus, st. louis), a flavivirus. it is transmitted to humans and other vertebrates primarily by mosquitoes of the genus culex. the primary animal vectors are wild birds and the disorder is endemic to the midwestern and southeastern united states. infections may be limited to an influenza-like illness or present as an aseptic meningitis or encephalitis. clinical manifestations of the encephalitic presentation may include seizures, lethargy, myoclonus, focal neurologic signs, coma, and death. (from adams et al., principles of neurology, 6th ed, p750)
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 A83.3 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About A83.3Overview
What is the ICD-10 code for st Louis encephalitis?
The ICD-10-CM code for st Louis encephalitis is A83.3 (sometimes written as A833). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is A83.3 (Mosquito-borne viral encephalitis) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report st Louis encephalitis on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What MS-DRG does A83.3 group to?
When st Louis encephalitis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 24, 98, 99, 23, 97, with relative weights from 1.5249 to 5.7422 depending on complications. Higher weights mean higher Medicare reimbursement.
Is A83.3 a CC or MCC?
CMS lists A83.3 as an MCC (major complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it places the inpatient stay in the highest-weighted DRG of its severity family. It does not count when the principal diagnosis is one of the 174 closely related codes in its exclusion list.
Can A83.3 be reported with G14?
Not as a rule. The Excludes1 note at block A80-A89, which applies to A83.3, lists "postpolio syndrome (G14)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).
