Scrub typhus infections poorly responsive to antibiotics in northern Thailand

Rickettsia tsutsugamushi, the aetiological agent of scrub typhus, is common in Asia and readily infects visitors to areas where disease transmission occurs. Rapid defervescence after antibiotic treatment is so characteristic that it is used as a diagnostic test for R tsutsugamushi infection. Reports from local physicians that patients with scrub typhus in Chiangrai, northern Thailand responded badly to appropriate antibiotic therapy prompted us to do a prospective clinical evaluation and antibiotic susceptibility testing of human rickettsial isolates.

The clinical response to doxycycline treatment in patients with early, mild scrub typhus in northern Thailand was compared with the results of treatment in Mae Sod, western Thailand. Prototype and naturally occurring strains of R tsutsugamushi were tested for susceptibility to chloramphenicol and doxycycline in mice and in cell culture.

By the third day of treatment, fever had cleared in all seven patients from Mae Sod, but in only five of the 12 (40%) from Chiangrai (p < 0.01). Median fever clearance time in Chiangrai (80 h; range 15-190) was significantly longer than in Mae Sod (30 h; range 4-58; p < 0.005). Conjunctival suffusion resolved significantly more slowly in Chiangrai (p < 0.05). Antibiotics prevented death in mice infected by Chiangrai strains of R tsutsugamushi less often than after infection by the prototype strain (p < 0.05). Only one of three Chiangrai strains tested in cell culture was fully susceptible to doxycycline.

Chloramphenicol-resistant and doxycycline-resistant strains of R tsutsugamushi occur in Chiangrai, Thailand. This is the first evidence of naturally occurring antimicrobial resistance in the genus Rickettsia.