Related Subjects:
|Rickettsia (General Principles)
|Rickettsia africae (Tick Bite Fever)
|Rickettsia akari (Rickettsial pox)
|Rickettsia conorii (Mediterranean Spotted Fever)
|Rickettsia prowazekii (Epidemi/Louse-borne Typhus)
|Rickettsia rickettsii (Rocky Mountain spotted fever)
|Rickettsia tsutsugamushi (Scrub typhus)
|Rickettsia typhi (Murine/Endemic typhus)
📖 About
- Rickettsia rickettsii → causative agent of Rocky Mountain spotted fever (RMSF), a severe tick-borne rickettsial infection.
🔬 Characteristics
- Small, Gram-negative bacterium (hard to see on Gram stain).
- Obligate intracellular parasite → requires host ATP & machinery.
- Only grows in tissue culture (like other rickettsiae).
🏠 Source
- Reservoirs: Rodents, dogs, small mammals.
- Vector: Dermacentor ticks → American dog tick (D. variabilis) & Rocky Mountain wood tick (D. andersoni).
- Transmission risk highest in spring & summer.
🌍 Geography & Epidemiology
- Despite the name, most US cases occur in the southeast & mid-Atlantic states.
- One of the most lethal rickettsial diseases in the Americas if untreated.
⚠️ Pathogenicity & Clinical Presentation
- Incubation: ~2–14 days after tick bite.
- Early symptoms: sudden fever 🌡️, chills, severe headache, myalgia, vomiting, fatigue.
- Rash: appears day 2–5:
- Starts on wrists & ankles → spreads to trunk, palms & soles (classic clue ✋🦶).
- Progresses from macular → petechial.
- Pathogenesis: Invades vascular endothelium → vasculitis → vascular leakage, oedema, multi-organ dysfunction.
- Complications: pneumonia, myocarditis, renal failure, CNS involvement (confusion, seizures, coma).
🔎 Investigations
- 🧪 Weil–Felix test: historical, poor specificity.
- 🧪 Skin biopsy + immunofluorescence: shows rickettsiae in rash lesions.
- 🧪 PCR: early, sensitive diagnostic tool (blood/tissue).
- 🧪 Serology: confirms by paired IgM/IgG titres (often retrospective).
💊 Management
- First-line: Doxycycline → start empirically (do NOT wait for confirmation).
- Safe even in children & pregnancy when benefit outweighs risk.
- Supportive: IV fluids, electrolytes, monitoring for organ dysfunction.
- Mortality: up to 25% untreated → drops to ~5% with early therapy.
📊 Prognosis
- Early treatment → usually complete recovery within 1–2 weeks.
- Delayed treatment → ↑ risk of permanent neurological sequelae or death.
🛡️ Prevention
- Avoid tick habitats during peak season.
- Repellents (e.g. DEET), long sleeves/trousers, permethrin-treated clothing.
- Perform thorough tick checks after outdoor exposure.
- Prompt removal of attached ticks reduces risk.
⚠️ Exam pearl: RMSF → fever, headache, rash starting wrists/ankles → spreading to palms/soles. Treat immediately with doxycycline (do not delay for tests).
