A patient from a forested area of Himachal Pradesh presents with fever, eschar at site of scrub tick attachment, headache and rash. IgM by ELISA for Orientia tsutsugamushi returns positive. Which antibiotic is the drug of choice and why?
- A Azithromycin 500 mg daily for 3 days — macrolide active against intracellular pathogens
- B Chloramphenicol 500 mg four times daily for 7 days — bacteriostatic against O. tsutsugamushi
- C Doxycycline 100 mg twice daily for 7 days — inhibits protein synthesis at 30S ribosome, obligate intracellular organism ✓
- D Ciprofloxacin 500 mg twice daily — broad-spectrum fluoroquinolone
Explanation
Doxycycline is the drug of choice for all rickettsial infections including scrub typhus (Orientia tsutsugamushi), Indian tick typhus (Rickettsia conorii), and murine typhus (R. typhi). It inhibits protein synthesis at the 30S ribosomal subunit and is active against obligate intracellular bacteria. Treatment is for 7 days, or until 3 days after fever resolution. Azithromycin is the preferred alternative in pregnancy or paediatric patients <8 years where doxycycline is contraindicated. Chloramphenicol was historically used but is now second-line due to side effects. Fluoroquinolones are not reliably effective.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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