Incidence of rickettsial infection in patients with acute fever in provincial Thai army hospitals.

Scrub typhus is common among patients with acute fever in rural areas of Thailand. The authors prospectively recruited patients with acute fever from provincial Thai army hospitals. Dot-ELISA test for scrub typhus was done in hospitals and then compared with standard immunofluorescent assay for diagnosis of scrub typhus. Among 178 patients, scrub typhus was diagnosed by immunofluorescent assay in 10 patients (5.61%). The incidence was high in the northeastern and northern regions. Dot-ELISA gave positive results in 4 of 115 patients, while immunofluorescent assay gave positive results in 6 patients (sensitivity = 66.7%). No false positive results of Dot-ELISA were found among 109 patients (specificity = 100%). All patients gave negative results for murine typhus and Thai tick typhus using immunofluorescent assay. Regarding this present study, Dot-ELISA for scrub typhus has a good sensitivity and specificity and can be used in rural hospitals. This test could be useful for diagnosis of scrub typhus in hospitals where immunofluorescent assay is not available.

Dot-ELISA Rapid Test Using Recombinant 56-kDa Protein Antigens for Serodiagnosis of Scrub Typhus.

We developed a rapid dot-enzyme-linked immunosorbent assay (dot-ELISA) using the combination of recombinant 56-kDa protein antigens that exhibited broad reactivity with serum antibodies against the four most prevalent strains (Karp, Kato, Gilliam, and TA763) of Orientia tsutsugamushi. The assay is rapid (30 minutes), and can be done at room temperature, and results can be read by the naked eye. Only a simple shaker is required to wash the membrane. Sera from 338 patients suspected of being ill with scrub typhus from rural hospitals around Thailand were tested using this dot-ELISA. Seventy-five (22.2%) patients were found to be positive. The sensitivity and specificity of dot-ELISA were determined using the indirect immunofluorescent assay (IFA) test as the gold standard, with the cutoff titer of immunoglobulin peroxidase conjugate M (IgM)/G (IgG) greater than 1:400/1:400. The dot-ELISA had a sensitivity of 98.5%, a specificity of 96.3%, a positive predictive value of 86.7%, and a negative predictive value of 99.6% for the acute-phase specimens. The results indicate that dot-ELISA rapid test using recombinant 56-kDa protein antigen was comparable with the IFA test and may be very useful for the diagnosis of scrub typhus in rural hospitals, where IFA is not available.

Isolation and characterization of Orientia tsutsugamushi from rodents captured following a scrub typhus outbreak at a military training base, Bothong district, Chonburi province, central Thailand.

Orientia tsutsugamushi, an obligate intracellular Gram-negative bacterium, is the causative agent of scrub typhus, a vector-borne disease transmitted by infected chiggers (trombiculid mite larvae). In 2002, an outbreak of scrub typhus occurred among Royal Thai Army troops during the annual field training at a military base in Bothong district, Chonburi province, central Thailand. This report describes the outbreak investigation including its transmission cycle. Results showed that 33.9% of 174 trained troops had scrub typhus-like signs and symptoms and 9.8% of those were positive for O. tsutsugamushi-specific antibodies by indirect fluorescence antibody assay. One hundred thirty-five rodents were captured from this training area, 43% of them had antibodies against O. tsutsugamushi. Six new O. tsutsugamushi isolates were obtained from captured rodent tissues and successfully established in cell culture. Phylogenetic studies showed that these six isolates were either unique or related to a native genotype of previously described isolates from Thailand.