Genomic and comparative genomic analyses of Rickettsia heilongjiangensis provide insight into its evolution and pathogenesis

Isolation of Rickettsia heilongjiangensis (Strain AH-19) from Haemaphysalis longicornis on a Wild Hedgehog in Anhui Province, China

Background: Spotted fever group rickettsioses, caused by rickettsiae of the spotted fever group, pose a significant zoonotic threat to public health. In endemic areas of Anhui Province, China, the ecology and transmission dynamics of these pathogens remain under investigation. Methods: We isolated a rickettsial strain from Haemaphysalis longicornis ticks collected from a wild hedgehog in the Dabie Mountain area. Molecular identification and phylogenetic analysis were employed to confirm the strain. Pathological examinations of the hedgehog's tissues were conducted to assess the potential impact of the infection. Results: The isolated strain was identified as R. heilongjiangensis strain AH-19. Pathological examination revealed significant tissue alterations, including cellular vacuolization, necrosis, and disarray of tissue architecture. It remains uncertain whether these changes were directly attributable to the rickettsial infection or other factors. Conclusions: The identification of R. heilongjiangensis strain AH-19 in H. longicornis suggests that hedgehogs may serve as reservoir hosts in the Dabie Mountain area.Further research is needed to clarify their role in the ecology and epidemiology of this pathogen, which could inform future public health strategies.

Genomic Features of Rickettsia heilongjiangensis Revealed by Intraspecies Comparison and Detailed Comparison With Rickettsia japonica

Rickettsia heilongjiangensis is the causative agent of Far-Eastern spotted fever (FESF). In Japan, a human case of FESF was identified in Sendai in Miyagi Prefecture in 2008, and R. heilongjiangensis bacteria were isolated from Haemaphysalis concinna ticks collected in the suspected geographical area of infection. Although the intraspecies genome diversity of Rickettsia has been poorly investigated, our recent analysis revealed extremely low genomic diversity of R. japonica, the agent of Japanese spotted fever, which is a close relative of R. heilongjiangensis. In this study, to investigate the genomic diversity of R. heilongjiangensis and understand the genetic relationship between Japanese and Chinese isolates, we sequenced three isolates from H. concinna ticks collected in Sendai and one isolate from a H. concinna tick collected in Inner Mongolia, China, and performed genomic comparisons between these isolates and strain 054, the type strain isolated from a Dermacentor silvarum tick in Heilongjiang Province, China. Although the three Japanese strains were isolated in 2008, 2009, and 2012, their genome sequences were identical, indicating that H. concinna ticks carrying a single R. heilongjiangensis clone have been distributed in Sendai, Japan. Among the five R. heilongjiangensis isolates, only 81 SNPs and 13 insertion/deletion sites were identified, despite the significant differences in these isolates both geographically and temporally. A significant portion of the 81 SNPs (16/81) were found to be recombinogenic. These results indicate low genomic diversity of R. heilongjiangensis, as observed in R. japonica. We further performed a detailed genomic comparison of R. heilongjiangensis and R. japonica to accurately define conserved and species-specific genes. This analysis revealed that although notable variations were found in the genomic loci encoding RelA/SpoT family proteins and tandem repeats in major surface proteins, there was only a small difference in the gene repertoire between the two species, suggesting that SNPs and small InDels are responsible for the functional or physiological differences between the two species, if present. Through this analysis, several species-specific genomic regions that can serve as ideal PCR targets for distinguishing R. heilongjiangensis and R. japonica were also identified.

'Candidatus Rickettsia mendelii', a novel basal group rickettsia detected in Ixodes ricinus ticks in the Czech Republic

A novel rickettsial sequence in the citrate synthase gltA gene indicating a novel Rickettsia species has been detected in 7 out of 4524 Ixodes ricinus ticks examined within several surveys performed in the Czech Republic from 2005 to 2009. This new Candidatus Rickettsia sp. sequence has been found in 2 nymphs feeding on wild birds (Luscinia megarhynchos and Erithacus rubecula), in a male tick from vegetation, and 4 ticks feeding on a dog (3 males, 1 female tick). Portions of the ompA, ompB, sca4, and htrA genes were not amplifiable in these samples. A maximum likelihood tree of rickettsiae based on comparisons of partial amino acid sequences of citrate synthase and nucleotide sequences of 16S rDNA genes and phylogenetic analysis revealed a basal position of the novel species in the proximity of R. bellii and R. canadensis. The novel species has been named 'Candidatus Rickettsia mendelii' after the founder of genetics, Gregor Mendel.

Strategies for detecting rickettsiae and diagnosing rickettsial diseases

Rickettsial diseases and scrub typhus constitute a group of the oldest known vector-borne diseases. The cosmopolitan distribution of the vectors that transmit rickettsiae and orientiae leads to a worldwide prevalence of these diseases. Despite their significant historical status, detection and diagnosis of these diseases are still evolving today. Serological methods remain among the most prevalent techniques used for the detection/diagnosis of rickettsial diseases and scrub typhus. Molecular techniques have been instrumental in increasing the sensitivity/specificity of diagnosis, identifying new Rickettsia and Orientia species and have enhanced epidemiological capabilities when used in combination with serological methods. In this review, we discuss these techniques and their associated pros and cons.