Orientia tsutsugamushi, the cause of scrub typhus, is a major pathogen in the Asia-Pacific region. The severity of infection ranges from mild features to multiorgan failure and death. The aim of this prospective study was to define the O. tsutsugamushi loads in the blood samples of patients with scrub typhus on the day of hospital admission and to determine whether this was associated with disease severity. Quantitation was performed using a real-time PCR Orientia tsutsugamushi, the cause of scrub typhus, is a major pathogen in the Asia-Pacific region, where it accounts for up to 23% of all febrile episodes (4). Infection commonly presents as an acute febrile illness 7 to 10 days after the bite of an infected larval trombiculid mite (chigger). The major presenting features are fever, severe headache, and myalgia. Other signs and symptoms include rash, lymphadenopathy, hepatosplenomegaly, cough, sore throat, abdominal pain, and central nervous system involvement. The severity of infection ranges from mild features to multiorgan failure and death, which occurs for around 4% of patients presenting to a hospital (10, 21). Severity of illness has been reported to show geographical variability (10), although the basis for this is unknown and has multiple explanations, including differences in host genetic susceptibility or response to infection and variability in the virulence of the organism. A relationship between magnitude of bacteremia and severity of clinical picture has been reported for a range of bacterial infections in both adults and children, based on quantitative bacterial culture (25) and, more recently, molecular techniques (5, 8). A previous study reported the O. tsutsugamushi DNA loads in 7 patients with scrub typhus, as defined by quantitative real-time PCR targeting a gene encoding a 47-kDa protein (20). This ranged from 1,076 to 28,812 DNA copies/l in blood, but clinical features and outcomes were not described (20).The aim of this study was to define the O. tsutsugamushi loads in blood samples taken on the day of hospital admission from a large, unselected population of patients with scrub typhus in northeast Thailand and to compare this with disease severity. MATERIALS AND METHODSPatients with scrub typhus were identified during a prospective study of acute febrile illness in Udon Thani hospital, northeast Thailand, between October 2000 and December 2001, as described previously (21). In brief, the inclusion criteria were an age of Ն15 years, the presence of a fever (Ͼ37.8°C) of unknown cause, and agreements to participate and to attend an outpatient follow-up for a convalescent-phase serum sample at 2 weeks. The exclusion criterion was a blood smear positive for malaria parasites or the presence of other definable infections, such as pneumonia or urinary tract infection. Blood was drawn on admission for aerobic blood culture, serological testing, and molecular diagnostics. The duration of symptoms prior to admission, antibiotic treatment at presentation, and clinical features, including the prese...

Orientia tsutsugamushi is an intracellular α-proteobacterium which resides in trombiculid mites, and is the causative agent of scrub typhus in East Asia. The genome sequence of this species has revealed an unprecedented number of repeat sequences, most notably of the genes encoding the conjugative properties of a type IV secretion system (T4SS). Although this observation is consistent with frequent intragenomic recombination, the extent of homologous recombination (gene conversion) in this species is unknown. To address this question, and to provide a protocol for the epidemiological surveillance of this important pathogen, we have developed a multilocus sequence typing (MLST) scheme based on 7 housekeeping genes (gpsA, mdh, nrdB, nuoF, ppdK, sucD, sucB). We applied this scheme to the two published genomes, and to DNA extracted from blood taken from 84 Thai scrub typhus patients, from 20 cultured Thai patient isolates, 1 Australian patient sample, and from 3 cultured type strains. These data demonstrated that the O. tsutsugamushi population was both highly diverse [Simpson's index (95% CI) = 0.95 (0.92–0.98)], and highly recombinogenic. These results are surprising given the intracellular life-style of this species, but are broadly consistent with results obtained for Wolbachia, which is an α-proteobacterial reproductive parasite of arthropods. We also compared the MLST data with ompA sequence data and noted low levels of consistency and much higher discrimination by MLST. Finally, twenty-five percent of patients in this study were simultaneously infected with multiple sequence types, suggesting multiple infection caused by either multiple mite bites, or multiple strains co-existing within individual mites.

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