Makindo Medical Notes

📖 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).