Microbiology · Intracellular and atypical bacteria
Rickettsia rickettsii / Rickettsia typhi !
- Bacterium
- Gram-negative
- Exam priority: medium
Bacteria that live inside blood vessels; spread by ticks or fleas, they cause fever with spots that can be fatal.
Updated 2026-11-01
Also called: spotted fever, Tobia fever, murine typhus
At a glance
- Gram
- Gram-negative
- Shape
- Small coccobacilli
- Oxygen
- Obligate intracellular (endothelium)
- Family
- Rickettsiaceae
- Stain
- Stain poorly with Gram; Giemsa or Giménez in tissue
Morphology
- Live free in the cytoplasm of endothelial cells
- R. rickettsii moves cell to cell with actin tails
Transmission
- R. rickettsii: bite of hard ticks (Amblyomma, Rhipicephalus, Dermacentor)
- R. typhi: rat or cat flea faeces rubbed into scratched skin
- No person-to-person spread
Reservoir: Ticks (transovarial transmission), rodents, dogs and other mammals; R. typhi: rats and opossums
Vector: Hard ticks (spotted fever); rat and cat fleas (murine typhus)
Isolation precautions
What the care team adds to standard precautions.
- Standard
Virulence factors
- Infect endothelium and cause widespread vasculitis
- Increased vascular permeability: oedema, hypotension, multi-organ failure
- Damage to small vessels of skin, brain, lung and kidney
Diseases it causes
- R. rickettsii spotted fever (Rocky Mountain spotted fever, Tobia fever)
- Murine (endemic) typhus from R. typhi
Clinical picture
- High fever, severe headache and myalgia after a tick bite
- Rash after a few days: starts at wrists and ankles, moves to the trunk and involves palms and soles; may turn petechial
- Murine typhus: milder illness, rash spreading outward from the trunk
- Low platelets, low sodium and raised transaminases
Diagnosis
- Clinical-epidemiological diagnosis: treatment does not wait for the lab
- Indirect immunofluorescence (IFA) with paired samples: fourfold titre rise
- PCR or immunohistochemistry on skin biopsy
- Weil-Felix: historical test, poorly sensitive
Treatment
Doxycycline at any age, started on clinical suspicion, because delay raises mortality; chloramphenicol is an alternative in special situations such as pregnancy. Sulfonamides can worsen the illness.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Long clothing, repellents and body checks after time in rural areas
- Tick control on dogs and horses; rodent and flea control
- Remove ticks with tweezers without crushing them
Test yourself
Think of the answer, then tap the question to see it.
Which antibiotic is used for spotted fever, even in young children?
Doxycycline, started on clinical suspicion without waiting for lab results.
How does the R. rickettsii spotted fever rash spread?
It starts at wrists and ankles, spreads to the trunk and includes palms and soles.
Which cell does Rickettsia infect, and with what result?
The endothelial cell; it causes vasculitis, capillary leak, oedema and hypotension.
Which vector transmits R. typhi?
Rat and cat fleas; their faeces enter through scratched skin.
Related
← Back to the group: Intracellular and atypical bacteria
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: About Rocky Mountain Spotted Fever | Rocky Mountain Spotted Fever (RMSF) | CDC (Centers for Disease Control and Prevention (CDC), 2024); Rocky Mountain spotted fever: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2024); Typhus Fevers Basics | Typhus Fevers | CDC (Centers for Disease Control and Prevention (CDC), 2024).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.