2026 ICD-10-CM Diagnosis Code A77.0Spotted fever due to Rickettsia rickettsii

ICD-10-CM Codes›A00–B99›A75-A79›A77

ICD-10-CM A77.0
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

A77.0 is a billable ICD-10-CM diagnosis code for spotted fever due to Rickettsia rickettsii. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 867 through 869. Coders also document this condition as Eastern Rocky Mountain spotted fever. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Parasitic, other specified and unspecified infections.

Code Identity

ICD-10-CM Code
A77.0
Billable Status
Yes — Valid for Submission
Code Describes
Spotted fever due to Rickettsia rickettsii
Short Description
Spotted fever due to Rickettsia rickettsii
Same as the full description in the CMS dataset.
Parent Code
A77
Spotted fever [tick-borne rickettsioses]

Code Classification

ChapterA00–B99Certain infectious and parasitic diseases
SectionA75-A79Rickettsioses
CategoryA77Spotted fever [tick-borne rickettsioses]
This CodeA77.0Spotted fever due to Rickettsia rickettsii

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Eastern Rocky Mountain spotted fever
  • Rocky Mountain spotted fever
  • Western Rocky Mountain spotted fever

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Inclusion Terms

  • Rocky Mountain spotted fever
  • Sao Paulo fever

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.

    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • American
        • spotted
    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • Bullis
    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • Lone Star
    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • mountain
        • meaning Rocky Mountain spotted fever
    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • Rocky Mountain spotted
    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • Sao Paulo
    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • spotted
        • American
    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • spotted
        • Brazilian
    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • spotted
        • Colombian
    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • spotted
        • due to Rickettsia
          • rickettsii
    • Fever(inanition) (of unknown origin) (persistent) (with chills) (with rigor)
      • spotted
        • Rocky Mountain
    • Lone Star fever
    • Rocky Mountain(spotted) fever
    • Sao Paulo fever or typhus
    • Typhus(fever)
      • Sao Paulo

Clinical ClassificationClinical

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

CCSR INF009
Parasitic, other specified and unspecified infections
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Rocky Mountain Spotted Fever

    an acute febrile illness caused by rickettsia rickettsii. it is transmitted to humans by bites of infected ticks and occurs only in north and south america. characteristics include a sudden onset with headache and chills and fever lasting about two to three weeks. a cutaneous rash commonly appears on the extremities and trunk about the fourth day of illness.

  • Rickettsia rickettsii

    a species of gram-negative, aerobic bacteria that is the etiologic agent of rocky mountain spotted fever. its cells are slightly smaller and more uniform in size than those of rickettsia prowazekii.

  • Rocky Mountain Spotted Fever

    an infection that is caused by rickettsia rickettsii, which is transmitted to humans from infected ticks; it is characterized by the sudden onset of fever, headache, and myalgia, followed by rash that usually begins peripherally.

Patient EducationClinical

Bacterial Infections

Bacteria are tiny organisms (living things) that have only one cell. Under a microscope, they look like balls, rods, or spirals. They can be found almost everywhere on Earth. There are lots of bacteria in and on your body. In fact, your body has about 10 times more bacteria cells than human cells.

The full article covers:

  • What are bacteria?
  • How do bacterial infections spread?
  • What are the treatments for bacterial infections?
  • Can bacterial infections be prevented?

Read the full article at MedlinePlus

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

Convert A77.0 to ICD-9-CMHistory

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

ICD-9-CM
082.0 Spotted fevers
Approximate The match is approximate rather than exact.

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 A77.0Overview

Is A77.0 (Spotted fever [tick-borne rickettsioses]) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report spotted fever due to Rickettsia rickettsii on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does A77.0 group to?

When spotted fever due to Rickettsia rickettsii is the principal diagnosis on an inpatient stay, it groups to MS-DRG 867, 868, 869, with relative weights from 0.7297 to 2.0932 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of A77.0?

Under the General Equivalence Mappings, spotted fever due to Rickettsia rickettsii converts to ICD-9-CM 082.0 (spotted fevers). The mapping is approximate, so confirm the match fits the documentation.

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.