A84.0
Far Eastern tick-borne encephalitis [Russian spring-summer encephalitis]
A84.0 (also written A840) is a billable ICD-10-CM diagnosis code for far Eastern tick-borne encephalitis [Russian spring-summer encephalitis]. It is part of the FY2027 code set, effective October 1, 2026. It is one of the 5 codes in category A84 (Tick-borne viral encephalitis). The Alphabetic Index lists it under 5 entries. Its coding notes include a Use additional code instruction, 3 Excludes1 entries and 5 Excludes2 entries.
Use additional code
Add the listed code as a secondary code, if known.
Excludes1
Not coded here. The listed conditions generally should not be reported together.
Excludes2
Not included here. Both codes may be reported if the patient has both conditions.
Also from chapter/block level (5 notes)
- 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)
Care settings
How CMS programs treat this code. These are CMS claim edits and list entries, not coverage or eligibility rules.
Home Health Primary
Accepted as the primary (principal) diagnosis on a home health claim. The Home Health Grouper assigns it to the Neuro Rehabilitation clinical group.
Source: CMS Home Health Grouper Software v07.2.26, effective October 1, 2026IRF 60% Rule
On the CMS list of diagnoses used to determine presumptive compliance with the IRF 60 percent rule.
Source: CMS IRF Presumptive Compliance-3, FY 2027 IRF PPS final rule files, effective for discharges on or after October 1, 2026Hospital MCC
Counts as a major complication or comorbidity (MCC) when reported as a secondary diagnosis on an inpatient claim. It does not count when the principal diagnosis is one of the 174 codes in its exclusion list. Some MS-DRGs also ignore a CC or MCC that defines the DRG.
Source: CMS MS-DRG Definitions Manual v44, effective October 1, 2026, Appendix CIncludes
Further defines or gives examples of the content of the category.
- tick-borne viral meningoencephalitis
Also from chapter/block level (1 note)
- diseases generally recognized as communicable or transmissible
Alphabetic Index terms
Terms the official Alphabetic Index lists for this code. 5 entries
- Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute), Far Eastern (tick-borne)
- Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute), Russian, spring-summer (taiga)
- Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute), viral, virus, arthropod-borne NEC, tick-borne, Far Eastern
- Encephalitis (chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute), viral, virus, arthropod-borne NEC, tick-borne, Russian spring-summer (taiga)
- Russian spring-summer type encephalitis