A 30-year-old man from Himachal Pradesh develops acute febrile illness, severe headache, rash starting from wrists/ankles spreading centripetally with petechiae, confusion and thrombocytopenia after a tick bite. Which Rickettsia species is responsible, and what is the specific laboratory test to confirm the diagnosis?
- A Rickettsia conorii — confirmed by Weil-Felix test (OX-19 positive)
- B Rickettsia rickettsii — confirmed by immunofluorescence assay (IFA) for IgM/IgG against R. rickettsii; Weil-Felix also positive (OX-19, OX-2) ✓
- C Rickettsia prowazekii — confirmed by OX-K positive Weil-Felix reaction
- D Orientia tsutsugamushi — confirmed by Weil-Felix OX-K positive; spreads by tick bite
Explanation
Rocky Mountain Spotted Fever (RMSF), caused by Rickettsia rickettsii, is transmitted by Dermacentor ticks. The centripetal rash starting on wrists/ankles (acral distribution) moving centrally with palm and sole involvement is characteristic. RMSF is the most lethal Rickettsia. Gold standard diagnosis is IFA (immunofluorescence assay), which shows positive IgM/IgG from day 7–10. Weil-Felix (agglutination of Proteus OX-19, OX-2) is a cross-reaction-based screening test (sensitivity ~50%). OX-K positivity is for Orientia tsutsugamushi (scrub typhus), transmitted by trombiculid mites (not ticks). Ricketsia conorii causes Indian tick typhus.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.