Genetic typing, based on the 56-kilodalton type-specific antigen gene, of Orientia tsutsugamushi strains isolated from chiggers collected from wild-caught rodents in Taiwan

Abstract

Orientia tsutsugamushi is the etiological agent of scrub typhus, a mite-borne, febrile illness that occurs in the Asia-Pacific region. We conducted strain characterization of O. tsutsugamushi isolates from chiggers obtained from rodents based the nucleotide sequence of the 56-kDa outer membrane protein gene. With the use of PCR, a total of 68 DNA sequences of 56-kDa antigen genes were amplified. Phylogenetic analysis revealed that there were at least six definable clusters among the 68 isolates: 37% Karp-related strains (25/68), 27% TA763 strains (18/68), 12% JG-related strains (8/68), 19% Kato-related strains (13/68), 4% divergent strains (3/68), and 1% representing a Gilliam prototype strain (1/68). Overall, the O. tsutsugamushi genotypes exhibited a high degree of diversity, similar to that seen in strains from the rest of the areas where scrub typhus is endemic. Moreover, the 56-kDa protein sequence similarity between O. tsutsugamushi isolates from mites and those from human patients (H. Y. Lu et al., Am. J. Trop. Med. Hyg. 83:658-663, 2010) were striking, thus highlighting potential risk factors for this emerging zoonotic disease.

Fig. 1.

Map of the geographic areas in Taiwan showing O. tsutsugamushi strains isolated in this study and from a human scrub typhus patient. For each study site, the number of captured rodents and the prevalence of O. tsutsugamushi DNA fragment in mite-hosting rodents are indicated.

Fig. 2.

Phylogenetic tree of O. tsutsugamushi based on the nucleotide sequences of the 56-kDa cell surface antigen gene. The subset of the phylogenetic tree is made up of isolates (strains from this study are indicated by open circles) associated with clades with sequence divergence from the reference strains (filled circles), such as Karp, Gilliam-related strains, and Kato, and strains isolated from humans (no mark). Isolates are identified by their GenBank accession numbers. Phylogenetic analyses were conducted using MEGA4. The evolutionary history was inferred using the neighbor-joining method and a bootstrap test on 1,000 replicates.