Antibodies to Orientia tsutsugamushi in Thai soldiers

Thai soldiers who were conscripted, Royal Thai Army forces, professional Border Patrol Police, or local militia (Thai Rangers) located in any of seven provinces of Thailand were bled in April and again, four months later, in July 1989. In 1991, soldiers from five different locations in southern Thailand were bled once, in July. Serum samples were tested by indirect fluorescent antibody assay for antibody to Orientia (formerly Rickettsia) tsutsugamushi, etiologic agent of scrub typhus, with any titer greater than or equal to 1:50 considered positive. Prior to field exercises, prevalence of antibody varied significantly between different types of units, ranging between 18.6% for Thai Rangers and 6.8% for the Royal Thai Army. The April prevalence, July prevalence, and incidence varied significantly by province in 1989, with highest incidence being 14.5% in Kanchanaburi and the lowest 0% in Utraladit. The prevalence in southern Thailand in 1991 varied between 1.6% and 6.8%. The data demonstrate that O. tsutsugamushi is widely distributed in Thailand and that military activity consisting of field exercises that simulate combat conditions significantly expose soldiers to infection.