Diversification of <i>Orientia tsutsugamushi</i> genotypes by intragenic recombination and their potential expansion in endemic areas

다국어 초록 (Multilingual Abstract)

<▼1><P><B>Background</B></P><P>Scrub typhus is a mite-borne febrile disease caused by <I>O</I>. <I>tsutsugamushi</I> infection. Recently, emergence of scrub typhus has attracted considerable attention in several endemic countries in Asia and the western Pacific. In addition, the antigenic diversity of the intracellular pathogen has been a serious obstacle for developing effective diagnostics and vaccine.</P><P><B>Methodology/Principal findings</B></P><P>To understand the evolutionary pathway of genotypic diversification of <I>O</I>. <I>tsutsugamushi</I> and the environmental factors associated with the epidemiological features of scrub typhus, we analyzed sequence data, including spatiotemporal information, of the <I>tsa56</I> gene encoding a major outer membrane protein responsible for antigenic variation. A total of 324 <I>tsa56</I> sequences covering more than 85% of its open reading frame were analyzed and classified into 17 genotypes based on phylogenetic relationship. Extensive sequence analysis of <I>tsa56</I> genes using diverse informatics tools revealed multiple intragenic recombination events, as well as a substantially higher mutation rate than other house-keeping genes. This suggests that genetic diversification occurred via frequent point mutations and subsequent genetic recombination. Interestingly, more diverse bacterial genotypes and dominant vector species prevail in Taiwan compared to other endemic regions. Furthermore, the co-presence of identical and sub-identical clones of <I>tsa56</I> gene in geographically distant areas implies potential spread of <I>O</I>. <I>tsutsugamushi</I> genotypes.</P><P><B>Conclusions/Significance</B></P><P>Fluctuation and diversification of vector species harboring <I>O</I>. <I>tsutsugamushi</I> in local endemic areas may facilitate genetic recombination among diverse genotypes. Therefore, careful monitoring of dominant vector species, as well as the prevalence of <I>O</I>. <I>tsutsugamushi</I> genotypes may be advisable to enable proper anticipation of epidemiological changes of scrub typhus.</P></▼1><▼2><P><B>Author summary</B></P><P>Scrub typhus, caused by <I>Orientia tsutsugamushi</I> infection, is a mite-borne febrile illness endemic in the Asia-Pacific region. Recent emergence and continuous local outbreaks in many of the endemic countries make it a serious public health issue. In addition, the antigenic diversity of the <I>tsa56</I> gene, encoding a major outer membrane protein, hampers the development of effective diagnostics and vaccine. Here, we extensively analyzed <I>tsa56</I> sequences and their spatiotemporal information to elucidate the evolutionary pathway of genotypic diversification, as well as the environmental basis associated with the epidemiological changes of scrub typhus. Based on various informatics analyses, we found that genetic diversification of <I>tsa56</I> might have been attained via frequent point mutations and subsequent genetic recombination among diverse genotypes. Prevalence of numerous bacterial genotypes and dominant vector species in Taiwan also suggest that the subtropical area located at the center of endemicity, may serve as a local mixing ground for genotype diversification. In addition, detection of identical and sub-identical clones of <I>tsa56</I> genes in geographically distant countries indicates a potential spreading of bacterial genotypes. Continuous monitoring of dominant vector species and the associated <I>O</I>. <I>tsutsugamushi</I> genotypes might be required for developing better diagnostics and an effective vaccine for scrub typhus.</P></▼2>