Complete genomic DNA sequence of the East Asian spotted fever disease agent Rickettsia japonica

Rickettsia-Host-Tick Interactions: Knowledge Advances and Gaps

Ticks are hematophagous ectoparasites capable of transmitting multiple human pathogens. Environmental changes have supported the expansion of ticks into new geographical areas that have become the epicenters of tick-borne diseases (TBDs). The spotted fever group (SFG) of Rickettsia frequently infects ticks and causes tick-transmitted rickettsioses in areas of endemicity where ixodid ticks support host transmission during blood feeding. Ticks also serve as a reservoir for SFG Rickettsia. Among the members of SFG Rickettsia, R. rickettsii causes Rocky Mountain spotted fever (RMSF), the most lethal TBD in the United States. Cases of RMSF have been reported for over a century in association with several species of ticks in the United States. However, the isolation of R. rickettsii from ticks has decreased, and recent serological and epidemiological studies suggest that novel species of SFG Rickettsia are responsible for the increased number of cases of RMSF-like rickettsioses in the United States. Recent analyses of rickettsial genomes and advances in genetic and molecular studies of Rickettsia provided insights into the biology of Rickettsia with the identification of conserved and unique putative virulence genes involved in the rickettsial life cycle. Thus, understanding Rickettsia-host-tick interactions mediating successful disease transmission and pathogenesis for SFG rickettsiae remains an active area of research. This review summarizes recent advances in understanding how SFG Rickettsia species coopt and manipulate ticks and mammalian hosts to cause rickettsioses, with a particular emphasis on newly described or emerging SFG Rickettsia species.

Virulence potential of Rickettsia amblyommatis for spotted fever pathogenesis in mice

Rickettsia amblyommatis belongs to the spotted fever group of Rickettsia and infects Amblyomma americanum (Lone Star ticks) for transmission to offspring and mammals. Historically, the geographic range of A. americanum was restricted to the southeastern USA. However, recent tick surveys identified the progressive northward invasion of A. americanum, contributing to the increased number of patients with febrile illnesses of unknown etiology after a tick bite in the northeastern USA. While serological evidence strongly suggests that patients are infected with R. amblyommatis, the virulence potential of R. amblyommatis is not well established. Here, we performed a bioinformatic analysis of three genome sequences of R. amblyommatis and identified the presence of multiple putative virulence genes whose products are implicated for spotted fever pathogenesis. Similar to other pathogenic spotted fever rickettsiae, R. amblyommatis replicated intracellularly within the cytoplasm of tissue culture cells. Interestingly, R. amblyommatis displayed defective attachment to microvascular endothelial cells. The attachment defect and slow growth rate of R. amblyommatis required relatively high intravenous infectious doses to produce dose-dependent morbidity and mortality in C3H mice. In summary, our results corroborate clinical evidence that R. amblyommatis can cause mild disease manifestation in some patients.

Obligate intracellular bacteria diversity in unfed Leptotrombidium scutellare larvae highlights novel bacterial endosymbionts of mites

It is well known that the mite Leptotrombidium scutellare carries the pathogen of scrub typhus, Orientia tsutsugamushi. However, our understanding of other bacterial endosymbionts of mites is limited. This study investigated the diversity of the obligate intracellular bacteria carried by L. scutellare using 16S rRNA gene amplicon analysis with next-generation sequencing. The results showed that the detected bacteria were classified into the genera Rickettsia, Wolbachia, and Rickettsiella and an unknown genus of the order Rickettsiales. For further classification of the detected bacteria, a representative read that was most closely related to the assigned taxonomic classification was subjected to homology search and phylogenic analysis. The results showed that some bacteria of the genus Rickettsia were identical or very close to the human pathogens Rickettsia akari, Rickettsia aeschlimannii, Rickettsia felis, and Rickettsia australis. The genetic distance between the genus Wolbachia bacteria in the present study and in previous reports is highly indicative that the bacteria in the present study can be classified as a new taxon of Wolbachia. This study detected obligate intracellular bacteria from unfed mites; thus, the mites did not acquire bacteria from infected animals or any other infectious sources. Finally, the present study demonstrated that various and novel bacterial endosymbionts of mites, in addition to O. tsutsugamushi, might uniquely evolve with the host mites throughout overlapping generations of the mite life cycle. The roles of the bacteria in mites and their pathogenicity should be further examined in studies based on bacterial isolation.

Rickettsia japonica Infections in Humans, Zhejiang Province, China, 2015

We investigated 16 Japanese spotted fever cases that occurred in southeastern China during September–October 2015. Patients had fever, rash, eschar, and lymphadenopathy. We confirmed 9 diagnoses and obtained 2 isolates with high identity to Rickettsiajaponica strain YH. R. japonica infection should be considered for febrile patients in China.

Extremely Low Genomic Diversity of Rickettsia japonica Distributed in Japan

Rickettsiae are obligate intracellular bacteria that have small genomes as a result of reductive evolution. Many Rickettsia species of the spotted fever group (SFG) cause tick-borne diseases known as “spotted fevers”. The life cycle of SFG rickettsiae is closely associated with that of the tick, which is generally thought to act as a bacterial vector and reservoir that maintains the bacterium through transstadial and transovarial transmission. Each SFG member is thought to have adapted to a specific tick species, thus restricting the bacterial distribution to a relatively limited geographic region. These unique features of SFG rickettsiae allow investigation of how the genomes of such biologically and ecologically specialized bacteria evolve after genome reduction and the types of population structures that are generated. Here, we performed a nationwide, high-resolution phylogenetic analysis of Rickettsia japonica, an etiological agent of Japanese spotted fever that is distributed in Japan and Korea. The comparison of complete or nearly complete sequences obtained from 31 R. japonica strains isolated from various sources in Japan over the past 30 years demonstrated an extremely low level of genomic diversity. In particular, only 34 single nucleotide polymorphisms were identified among the 27 strains of the major lineage containing all clinical isolates and tick isolates from the three tick species. Our data provide novel insights into the biology and genome evolution of R. japonica, including the possibilities of recent clonal expansion and a long generation time in nature due to the long dormant phase associated with tick life cycles.

Molecular Testing in Emerging Infectious Diseases

It was widely believed in the late 1960s that infectious diseases had been conquered by vaccines and antibiotics and humans were no longer under threat by microbial pathogens. Yet, since that time more than 60 pathogens have been discovered that can cause serious emerging infectious diseases. Molecular methods have played critical roles in the discovery, monitoring, and clinical diagnostics of emerging pathogens. In this chapter, we present well-recognized emerging pathogens. We provide examples of the utility of molecular assays in research and clinical care of emerging infectious diseases. We also discuss some theoretical and practical limitations of molecular tests and the future prospects of expanding molecular diagnostics for emerging pathogens based on new advances of knowledge and technologies.

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.

Genomic diversification in strains of Rickettsia felis Isolated from different arthropods

Rickettsia felis (Alphaproteobacteria: Rickettsiales) is the causative agent of an emerging flea-borne rickettsiosis with worldwide occurrence. Originally described from the cat flea, Ctenocephalides felis, recent reports have identified R. felis from other flea species, as well as other insects and ticks. This diverse host range for R. felis may indicate an underlying genetic variability associated with host-specific strains. Accordingly, to determine a potential genetic basis for host specialization, we sequenced the genome of R. felis str. LSU-Lb, which is an obligate mutualist of the parthenogenic booklouse Liposcelis bostrychophila (Insecta: Psocoptera). We also sequenced the genome of R. felis str. LSU, the second genome sequence for cat flea-associated strains (cf. R. felis str. URRWXCal2), which are presumably facultative parasites of fleas. Phylogenomics analysis revealed R. felis str. LSU-Lb diverged from the flea-associated strains. Unexpectedly, R. felis str. LSU was found to be divergent from R. felis str. URRWXCal2, despite sharing similar hosts. Although all three R. felis genomes contain the pRF plasmid, R. felis str. LSU-Lb carries an additional unique plasmid, pLbaR (plasmid of L. bostrychophila associated Rickettsia), nearly half of which encodes a unique 23-gene integrative conjugative element. Remarkably, pLbaR also encodes a repeats-in-toxin-like type I secretion system and associated toxin, heretofore unknown from other Rickettsiales genomes, which likely originated from lateral gene transfer with another obligate intracellular parasite of arthropods, Cardinium (Bacteroidetes). Collectively, our study reveals unexpected genomic diversity across three R. felis strains and identifies several diversifying factors that differentiate facultative parasites of fleas from obligate mutualists of booklice.