Rickettsia USMLE Notes & Mnemonics contains all the high-yield points you need to know.
- Rickettsia: A genus of Gram-negative, obligate intracellular bacteria, causing various tick-borne diseases.
Pathophysiology:
- Transmitted primarily by arthropod vectors (ticks, fleas, lice). (Ref)
- Infects endothelial cells → vascular injury → widespread vasculitis.
Classification:
- Spotted Fever Group (SFG):
- Rickettsia rickettsii: Rocky Mountain Spotted Fever (RMSF).
- Rickettsia conorii: Mediterranean Spotted Fever.
- Typhus Group:
- Rickettsia prowazekii: Epidemic typhus.
- Rickettsia typhi: Murine (endemic) typhus.
- Scrub Typhus Group:
- Orientia tsutsugamushi (formerly Rickettsia): Scrub typhus.
Clinical Features:
- Rocky Mountain Spotted Fever:
- Fever, headache, myalgia.
- Rash (wrists and ankles → trunk, palms, and soles).
- Can progress to severe complications (encephalitis, ARDS).
- Typhus:
- High fever, headache, rash (trunk → extremities, sparing palms and soles).
- Epidemic typhus more severe than murine typhus.
- Scrub Typhus:
- Fever, chills, headache, myalgia.
- Eschar at the site of chigger bite.
- Rash similar to typhus.
Diagnosis:
- Clinical Presentation: History of exposure, characteristic rash.
- Serology: Weil-Felix test (less specific), ELISA.
- PCR: For specific Rickettsial DNA.
Treatment:
- Doxycycline: First-line treatment for all Rickettsial diseases.
- Chloramphenicol: Alternative in pregnancy.
Prevention:
- Avoiding vector exposure (tick and insect repellent, protective clothing).
- Prompt removal of ticks.
Mnemonic: “RICKETTSIA”
- Rash
- Intracellular (obligate)
- Chills and fever
- Known vectors (ticks, fleas, lice)
- Endothelial cells targeted
- Typhus and Spotted Fever
- Travel history important
- Serology for diagnosis
- Intravenous Doxycycline treatment
- Ankle-wrist rash in RMSF
Chart: Rickettsia Disease Comparison
| Disease | Vector | Rash Location | Complications |
|---|---|---|---|
| Rocky Mountain Spotted Fever | Ticks | Wrists, ankles → trunk, palms, soles | Encephalitis, ARDS |
| Epidemic Typhus | Lice | Trunk → extremities (sparing palms/soles) | Severe, multiorgan involvement |
| Murine Typhus | Fleas | Trunk → extremities (sparing palms/soles) | Less severe than epidemic typhus |
| Scrub Typhus | Chiggers (mites) | Eschar at bite site, trunk rash | Similar to typhus |
Note: On the USMLE, it’s important to recognize the clinical presentation of Rickettsial diseases and remember that Doxycycline is the treatment of choice. Be familiar with the epidemiology and the different vectors associated with each disease. The characteristic rash patterns and potential complications of each illness are also commonly tested.
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