PREVALENCE OF ANTIBODIES TO RICKETTSIAE IN THE HUMAN-POPULATION OF SUBURBAN BANGKOK

Following a report of three cases of scrub typhus in suburban Bangkok, we performed a serosurvey in the patients' communities. Both IgG and IgM antibodies were measured in an indirect immunoperoxidase assay, using separate spots of antigen from Rickettsia tsutsugamushi (scrub typhus), R. typhi (murine typhus), and TT-118 spotted fever group rickettsiae. Of 215 people donating blood, antibody levels indicative of most recent exposure to R. tsutsugamushi were most prevalent (21%), followed by R. typhi (8%), and TT-118 (4%). Seroprevalence suggesting most recent exposure to R. tsutsuga-mushi varied by location (range 13-31%), gender (26% of females and 13% of males), age (61-80-year-old individuals were the highest, with a prevalence of 38%), and contact with orchards and orchid farms (29% of those with extensive contact, 38% of those with occasional contact, and 10% of those with no contact). These patterns indicated that exposure to R. tsutsugamushi was related to occupation and behavior, as has been observed in areas of rural transmission. Expansion of metropolitan Bangkok has created a situation in which people employed in agriculture live with people employed in the city. As a result, a serosurvey in suburban Bangkok reveals evidence of murine typhus, which is usually transmitted in urban areas, as well as scrub and tick typhus, which are usually transmitted in rural areas.