Scrub typhus is an acute infectious disease caused by an obligate intracellular bacterium Orientia tsutsugamushi following the bite of infected trombiculid mites of the genus Leptotrombidium. This zoonotic disease is a major cause of febrile illness in the Asia-Pacific region, with a large spectrum of clinical manifestations from unapparent or mild disease to fatal disease. O. tsutsugamushi is characterized by a very high genomic plasticity and a large number of antigenic variants amongst strains. The 56-kDa type specific antigen (TSA) gene, encoding the major antigenic protein, was used as reference to investigate the genetic relationships between the strains and to genotype O. tsutsugamushi isolates. The open reading frame of the 56-kDa TSA gene of 41 sequences (28 Cambodian and 13 Vietnamese strains) from patient samples were sequenced and used for genotyping. The 28 Cambodian isolates clustered into 5 major groups, including Karp (43.5%), JG-v (25%), Kato/TA716 (21.5%), TA763 (3.5%) and Gilliam (3.5%). Karp (77%), TA763 (15.5%) and JG-v (7.5%) strains were identified amongst the 13 Vietnamese isolates. This is the first countrywide genotyping description in Cambodia and in Central Vietnam. These results demonstrate the considerable diversity of genotypes in co-circulation in both countries. The genotyping result might raise awareness amongst Cambodian and Vietnamese clinicians of the high genetic diversity of circulating O. tsutsugamushi strains and provides unique and beneficial data for serological and molecular diagnosis of scrub typhus infections as well as raw materials for future studies and vaccine development.
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