Variations of plasmid content in Rickettsia felis

Rickettsia felis is an emerging human pathogen associated with cat fleas: A review of findings in Taiwan

Rickettsia felis is an emerging rickettsial agent principally associated with cat fleas (Ctenocephalides felis), formerly discovered in 1990. Since then, clinical cases of R. felis infection have been identified globally by specific DNA sequences in patients with undifferentiated febrile illness, including in Taiwan, but such evidence is limited. R. felis rickettsiosis is self-limiting and easily treated with doxycycline, but its diagnosis remains a challenge. Environmental risk factors for R. felis rickettsiosis have yet to be clearly demonstrated, and its transmission biology is incompletely understood. Cat fleas are naturally infected with R. felis at varying rates, and vector competence in the transmission of R. felis has been demonstrated in animal models, including dogs, which may serve as reservoir hosts. In northern Taiwan, despite ∼20% of cat fleas infesting companion animals consistently found to be infected with R. felis, only a few cases of potential R. felis infection have been identified through a retrospective serological investigation, though without molecular confirmation. Ecological studies have identified divergent R. felis-like organisms in different arthropod hosts, but these strains appear to serve as nonpathogenic endosymbionts. Although its association with disease is limited, we believe cat flea-borne R. felis warrants increased recognition in an aging population due to immunosenescence and the proximity of companion animals to the elderly. Adopting a One Health approach involving collaboration and communication between clinicians, veterinarians, public health practitioners, and environmental scientists will improve our knowledge about this neglected pathogen and promote the prevention and control of vector-borne diseases.

Culture Isolate of Rickettsia felis from a Tick

Although the cat flea, Ctenocephalides felis, has been identified as the primary vector of Rickettsia felis, additional flea, tick, mite, and louse species have also been associated with this bacterium by molecular means; however, the role of these arthropods in the transmission of R. felis has not been clarified. Here, we succeeded in culture isolation of R. felis from a host-seeking castor bean tick, Ixodes ricinus, the most common tick in Slovakia. The bacterial isolation was performed on XTC-2 cells at 28 °C using the shell-vial technique. An evaluation of the growth properties was performed for both the XTC-2 and Vero cell lines. We observed R. felis in the infected host cells microscopically by Gimenez staining and immunofluorescence assay. The R. felis isolate was purified by gradient ultracentrifugation and visualized by electron microscopy. Fragments of the genes gltA, ompA, ompB, htrA, rpoB, sca4, rffE, and rrs were amplified and compared with the corresponding sequences of the type strain URRWXCal2 and other R. felis culture -isolated strains. We did not detect any nucleotide polymorphisms; however, plasmid pRFδ, characteristic of the standard strain, was absent in our isolate. Herein, we describe the first successful isolation and characterization of a tick-derived R. felis strain “Danube”, obtained from an I. ricinus nymph.

Genomic evolution and adaptation of arthropod-associated Rickettsia

Rickettsia species are endosymbionts hosted by arthropods and are known to cause mild to fatal diseases in humans. Here, we analyse the evolution and diversity of 34 Rickettsia species using a pangenomic meta-analysis (80 genomes/41 plasmids). Phylogenomic trees showed that Rickettsia spp. diverged into two Spotted Fever groups, a Typhus group, a Canadensis group and a Bellii group, and may have inherited their plasmids from an ancestral plasmid that persisted in some strains or may have been lost by others. The results suggested that the ancestors of Rickettsia spp. might have infected Acari and/or Insecta and probably diverged by persisting inside and/or switching hosts. Pangenomic analysis revealed that the Rickettsia genus evolved through a strong interplay between genome degradation/reduction and/or expansion leading to possible distinct adaptive trajectories. The genus mainly shared evolutionary relationships with α-proteobacteria, and also with γ/β/δ-proteobacteria, cytophagia, actinobacteria, cyanobacteria, chlamydiia and viruses, suggesting lateral exchanges of several critical genes. These evolutionary processes have probably been orchestrated by an abundance of mobile genetic elements, especially in the Spotted Fever and Bellii groups. In this study, we provided a global evolutionary genomic view of the intracellular Rickettsia that may help our understanding of their diversity, adaptation and fitness.

Epidemiological serosurvey of vector-borne and zoonotic pathogens among homeless people living in shelters in Marseille: cross-sectional one-day surveys (2005-2015)

Homeless people are often exposed to unhygienic environments as well as to animals carrying arthropods which both transmit zoonotic infections and human louse-borne pathogens. We attempted to determine the prevalence of antibodies against several vector-borne and zoonotic pathogens among homeless adults living in Marseille. During the 2005-2015 period, we collected sera samples from 821 homeless adults living in shelters. Antibodies against Bartonella quintana, Bartonella henselae, Borrelia recurrentis, Coxiella burnetii, Francisella tularensis (with a cut-off of 1:100), Rickettsia akari, Rickettsia conorii, Rickettsia felis, Rickettsia prowazekii, and Rickettsia typhi (with a cut-off of 1:64) were searched by microimmunofluorescence (MIF). MIF-positive serum samples were confirmed by cross-adsorption to characterise cross-reacting antigens and immunoblotting. Positive sera by Western blot were further tested using qPCR. We evidenced a prevalence of 4.9% seroreactivity to at least one pathogen including phase II C. burnetii (2.1%), B. quintana (1.7%), R. conorii (0.4%), R. prowazekii (0.4%), R. typhi (0.1%), B. recurrentis (0.1%), and F. tularensis (0.1%). No DNA from any pathogens was detected. A comparison with studies conducted prior to the 2000-2003 period showed a decrease in the overall seroprevalence of several vector-borne and zoonotic infections.

Identification of Rickettsial Infections (Rickettsia sp. TH2014) in Ctenocephalides orientis Fleas (Siphonaptera: Pulicidae)

Rickettsia felis (Rickettsiales: Rickettsiaceae) is an emergent human pathogen that causes febrile illnesses in various parts of the world. This study describes the identification and growth characteristics of a R. felis-like organism (designated as Rickettsia sp. TH2014) cultured from Ctenocephalides orientis fleas in rural Malaysia. In this study, culturing of rickettsiae from filtered triturated flea lysates was performed in Aedes albopictus C6/36 cells. Cytopathic effects were observed from one of the samples 4 d post-inoculation. Electron microscopy revealed actively replicating intracytosolic coccobacillary organisms in the rickettsia-infected cells. Sequence analysis of amplified citrate synthase (gltA) gene fragment shows complete match of the rickettsia with Rickettsia sp. Rf31 in Southeast Asia, and 'Candidatus Rickettsia senegalensis' strain PU01-02 in Africa. The whole-genome sequence of Rickettsia sp. TH2014 was determined and assembled. The estimated genome size and guanine + cytosine content of the rickettsia are 1.37 Mb and 32.9%, respectively. The high values of average nucleotide identity and tetra-nucleotide signature correlation index obtained from pairwise genome comparison study suggest the identification of the rickettsia as R. felis. The whole-genome single-nucleotide polymorphism analysis demonstrates close genetic relatedness of the rickettsia with R. felis and Rickettsia asemboensis. However, based on sequence analyses of rickettsial genes (16S rDNA, gltA, ompB, and sca4), Rickettsia sp. TH2014 is found to be distinct from R. felis and R. asemboensis. The sequence analyses reveal that Rickettsia sp. TH2014 is highly similar to 'Ca. Rickettsia senegalensis' detected in fleas from Africa, Asia, and North America. Further investigation to provide insights on pathogenic potential and transmission dynamics of the rickettsia is warranted.

Phylogenetic Variants of Rickettsia africae, and Incidental Identification of "Candidatus Rickettsia Moyalensis" in Kenya

Background

Rickettsia africae, the etiological agent of African tick bite fever, is widely distributed in sub-Saharan Africa. Contrary to reports of its homogeneity, a localized study in Asembo, Kenya recently reported high genetic diversity. The present study aims to elucidate the extent of this heterogeneity by examining archived Rickettsia africae DNA samples collected from different eco-regions of Kenya.

Methods

To evaluate their phylogenetic relationships, archived genomic DNA obtained from 57 ticks a priori identified to contain R. africae by comparison to ompA, ompB and gltA genes was used to amplify five rickettsial genes i.e. gltA, ompA, ompB, 17kDa and sca4. The resulting amplicons were sequenced. Translated amino acid alignments were used to guide the nucleotide alignments. Single gene and concatenated alignments were used to infer phylogenetic relationships.

Results

Out of the 57 DNA samples, three were determined to be R. aeschlimanii and not R. africae. One sample turned out to be a novel rickettsiae and an interim name of “Candidatus Rickettsia moyalensis” is proposed. The bonafide R. africae formed two distinct clades. Clade I contained 9% of the samples and branched with the validated R. africae str ESF-5, while clade II (two samples) formed a distinct sub-lineage.

Conclusions

This data supports the use of multiple genes for phylogenetic inferences. It is determined that, despite its recent emergence, the R. africae lineage is diverse. This data also provides evidence of a novel Rickettsia species, Candidatus Rickettsia moyalensis.

Rickettsia africae is a bacterium mainly vectored by Amblyomma and Rhipicephalus species of ticks. It is the etiological agent of African tick bite fever (ATBF), a spotted fever rickettsiosis that presents as an acute febrile illness characterized by petecheal skin hemorrhages, from which the name is derived. This bacterium is probably the most important in sub-Saharan Africa, including Kenya, in terms of incidence and prevalence. This notwithstanding, the disease is poorly understood and is often mistreated as malaria, and therefore qualifies as a highly neglected disease. This study examined the genetic relationships of R. africae collected from diverse eco-regions of Kenya. We present data that indicate high genetic diversity in Kenya’s R. africae and corroborate a recent study that reported similar genetic diversity in R. africae samples collected from a localized area in western Kenya. Importantly, we describe a divergent lineage and propose the name Candidatus Rickettsia moyalensis.

Rickettsia felis, an Emerging Flea-Borne Rickettsiosis

Rickettsia felis is an emerging insect-borne rickettsial pathogen and the causative agent of flea-borne spotted fever. First described as a human pathogen from the USA in 1991, R. felis is now identified throughout the world and considered a common cause of fever in Africa. The cosmopolitan distribution of this pathogen is credited to the equally widespread occurrence of cat fleas (Ctenocephalides felis), the primary vector and reservoir of R. felis. Although R. felis is a relatively new member of the pathogenic Rickettsia, limited knowledge of basic R. felis biology continues to hinder research progression of this unique bacterium. This is a comprehensive review examining what is known and unknown relative to R. felis transmission biology, epidemiology of the disease, and genetics, with an insight into areas of needed investigation.

Origin and Evolution of Rickettsial Plasmids

Background

Rickettsia species are strictly intracellular bacteria that have undergone a reductive genomic evolution. Despite their allopatric lifestyle, almost half of the 26 currently validated Rickettsia species have plasmids. In order to study the origin, evolutionary history and putative roles of rickettsial plasmids, we investigated the evolutionary processes that have shaped 20 plasmids belonging to 11 species, using comparative genomics and phylogenetic analysis between rickettsial, microbial and non-microbial genomes.

Results

Plasmids were differentially present among Rickettsia species. The 11 species had 1 to 4 plasmid (s) with a size ranging from 12 kb to 83 kb. We reconstructed pRICO, the last common ancestor of the current rickettsial plasmids. pRICO was vertically inherited mainly from Rickettsia/Orientia chromosomes and diverged vertically into a single or multiple plasmid(s) in each species. These plasmids also underwent a reductive evolution by progressive gene loss, similar to that observed in rickettsial chromosomes, possibly leading to cryptic plasmids or complete plasmid loss. Moreover, rickettsial plasmids exhibited ORFans, recent gene duplications and evidence of horizontal gene transfer events with rickettsial and non-rickettsial genomes mainly from the α/γ-proteobacteria lineages. Genes related to maintenance and plasticity of plasmids, and to adaptation and resistance to stress mostly evolved under vertical and/or horizontal processes. Those involved in nucleotide/carbohydrate transport and metabolism were under the influence of vertical evolution only, whereas genes involved in cell wall/membrane/envelope biogenesis, cycle control, amino acid/lipid/coenzyme and secondary metabolites biosynthesis, transport and metabolism underwent mainly horizontal transfer events.

Conclusion

Rickettsial plasmids had a complex evolution, starting with a vertical inheritance followed by a reductive evolution associated with increased complexity via horizontal gene transfer as well as gene duplication and genesis. The plasmids are plastic and mosaic structures that may play biological roles similar to or distinct from their co-residing chromosomes in an obligate intracellular lifestyle.

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

Rickettsia heilongjiangensis, the causative agent of far eastern spotted fever, is an obligate intracellular gram-negative bacterium that belongs to the spotted fever group rickettsiae. To understand the evolution and pathogenesis of R. heilongjiangensis, we analyzed its genome and compared it with other rickettsial genomes available in GenBank. The R. heilongjiangensis chromosome contains 1333 genes, including 1297 protein coding genes and 36 RNA coding genes. The genome also contains 121 pseudogenes, 54 insertion sequences, and 39 tandem repeats. Sixteen genes encoding the major components of the type IV secretion systems were identified in the R. heilongjiangensis genome. In total, 37 β-barrel outer membrane proteins were predicted in the genome, eight of which have been previously confirmed to be outer membrane proteins. In addition, 266 potential virulence factor genes, seven partially deleted antibiotic resistance genes, and a genomic island were identified in the genome. The codon usage in the genome is compatible with its low GC content, and the amino acid usage shows apparent bias. A comparative genomic analysis showed that R. heilongjiangensis and R. japonica share one unique fragment that may be a target sequence for a diagnostic assay. The orthologs of 37 genes of R. heilongjiangensis were found in pathogenic R. rickettsii str. Sheila Smith but not in non-pathogenic R. rickettsii str. Iowa, which may explain why R. heilongjiangensis is pathogenic. Pan-genome analysis showed that R. heilongjiangensis and 42 other rickettsiae strains share 693 core genes with a pan-genome size of 4837 genes. The pan-genome-based phylogeny showed that R. heilongjiangensis was closely related to R. japonica.

Use of the plaque assay for testing the antibiotic susceptibility of intracellular bacteria

The plaque assay was first described for titration of bacterial inoculums and clonal isolation, and was later adapted for testing antibiotics susceptibility and to study virulence factors and motility of bacteria. Over time, the sensitivity and reproducibility of the technique has been improved. Usually, the number of plaques is counted; however, the recent development of informatics tools has stimulated interest in the quantification of plaque size. Owing to this new approach, the plaque assay has been used to characterize the host cell response when infected cells are treated with antimicrobial agents. It was found that statins prevented cell lesions following rickettsial infection; in other studies, some antibiotics were found to cause apoptosis of host cells, suggesting a toxic activity. Here, we present an overview of the plaque assay as it has been used to investigate intracellular bacteria.

Update on tick-borne rickettsioses around the world: a geographic approach

Tick-borne rickettsioses are caused by obligate intracellular bacteria belonging to the spotted fever group of the genus Rickettsia. These zoonoses are among the oldest known vector-borne diseases. However, in the past 25 years, the scope and importance of the recognized tick-associated rickettsial pathogens have increased dramatically, making this complex of diseases an ideal paradigm for the understanding of emerging and reemerging infections. Several species of tick-borne rickettsiae that were considered nonpathogenic for decades are now associated with human infections, and novel Rickettsia species of undetermined pathogenicity continue to be detected in or isolated from ticks around the world. This remarkable expansion of information has been driven largely by the use of molecular techniques that have facilitated the identification of novel and previously recognized rickettsiae in ticks. New approaches, such as swabbing of eschars to obtain material to be tested by PCR, have emerged in recent years and have played a role in describing emerging tick-borne rickettsioses. Here, we present the current knowledge on tick-borne rickettsiae and rickettsioses using a geographic approach toward the epidemiology of these diseases.

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

Rickettsia japonica is an obligate intracellular alphaproteobacteria that causes tick-borne Japanese spotted fever, which has spread throughout East Asia. We determined the complete genomic DNA sequence of R. japonica type strain YH (VR-1363), which consists of 1,283,087 base pairs (bp) and 971 protein-coding genes. Comparison of the genomic DNA sequence of R. japonica with other rickettsiae in the public databases showed that 2 regions (4,323 and 216 bp) were conserved in a very narrow range of Rickettsia species, and the shorter one was inserted in, and disrupted, a preexisting open reading frame (ORF). While it is unknown how the DNA sequences were acquired in R. japonica genomes, it may be a useful signature for the diagnosis of Rickettsia species. Instead of the species-specific inserted DNA sequences, rickettsial genomes contain Rickettsia-specific palindromic elements (RPEs), which are also capable of locating in preexisting ORFs. Precise alignments of protein and DNA sequences involving RPEs showed that when a gene contains an inserted DNA sequence, each rickettsial ortholog carried an inserted DNA sequence at the same locus. The sequence, ATGAC, was shown to be highly frequent and thus characteristic in certain RPEs (RPE-4, RPE-6, and RPE-7). This finding implies that RPE-4, RPE-6, and RPE-7 were derived from a common inserted DNA sequence.

An update on the detection and treatment of Rickettsia felis

Rickettsia felis was described as a human pathogen almost two decades ago, and human infection is currently reported in 18 countries in all continents. The distribution of this species is worldwide, determined by the presence of the main arthropod vector, Ctenocephalides felis (Bouché). The list of symptoms, which includes fever, headache, myalgia, and rash, keeps increasing as new cases with unexpected symptoms are described. Moreover, the clinical presentation of R. felis infection can be easily confused with many tropical and nontropical diseases, as well as other rickettsial infections. Although specific laboratory diagnosis and treatment for this flea-borne rickettsiosis are detailed in the scientific literature, it is possible that most human cases are not being diagnosed properly. Furthermore, since the cat flea infests different common domestic animals, contact with humans may be more frequent than reported. In this review, we provide an update on methods for specific detection of human infection by R. felis described in the literature, as well as the treatment prescribed to the patients. Considering advances in molecular detection tools, as well as options for as-yet-unreported isolation of R. felis from patients in cell culture, increased diagnosis and characterization of this emerging pathogen is warranted.

Rickettsia felis: from a rare disease in the USA to a common cause of fever in sub-Saharan Africa

Rickettsia felis is a spotted fever group rickettsia that has been definitely described in 2002. Within the last 20 years, there have been a growing number of reports implicating R. felis as a human pathogen, parallel to the fast-growing reports of the worldwide detection of R. felis in arthropod hosts, mainly the cat flea Ctenocephalides felis felis. R. felis is now known as the agent of the so-called flea-borne spotted fever, with more than 70 cases documented in the literature. Recently, two studies respectively conducted in Senegal and Kenya, have challenged the importance of R. felis infection in patients with unexplained fever in sub-Saharan Africa. We focus here on the epidemiological and clinical aspects of R. felis infection. More studies are needed, including the study of other arthropod vectors, but it can be speculated that R. felis infection might be an important neglected agent of fever in sub-Saharan Africa.

Intracellular Rickettsiales: Insights into manipulators of eukaryotic cells

The order Rickettsiales comprises obligate intracellular bacteria that are the ancestors of modern eukaryotes. These bacteria infect various vectors and hosts, with some species being pathogenic to man. Rickettsiales have small, degraded genomes and provide a paradigm for increased pathogenicity despite gene loss; significant levels of genetic exchange occur between bacteria that infect the same host and with the eukaryotic hosts themselves. Crosstalk between host and bacteria appears to be mediated by a Type IV secretion system and proteins containing eukaryotic-like repeat motifs. Rickettsiales also manipulate host reproduction and induce host resistance to viruses. Manipulation of its host by Rickettsiales has long been misunderstood because of technical difficulties, but recent advances in understanding bacterial-eukaryotes interactions have been made and are reviewed here.

Rickettsia felis, an emerging flea-transmitted human pathogen

Rickettsia felis was first recognised two decades ago and has now been described as endemic to all continents except Antarctica. The rickettsiosis caused by R. felis is known as flea-borne spotted fever or cat-flea typhus. The large number of arthropod species found to harbour R. felis and that may act as potential vectors support the view that it is a pan-global microbe. The main arthropod reservoir and vector is the cat flea, Ctenocephalides felis, yet more than 20 other species of fleas, ticks, and mites species have been reported to harbour R. felis. Few bacterial pathogens of humans have been found associated with such a diverse range of invertebrates. With the projected increase in global temperature over the next century, there is concern that changes to the ecology and distribution of R. felis vectors may adversely impact public health.

Isolation of a rickettsial pathogen from a non-hematophagous arthropod

Rickettsial diversity is intriguing in that some species are transmissible to vertebrates, while others appear exclusive to invertebrate hosts. Of particular interest is Rickettsia felis, identifiable in both stored product insect pests and hematophagous disease vectors. To understand rickettsial survival tactics in, and probable movement between, both insect systems will explicate the determinants of rickettsial pathogenicity. Towards this objective, a population of Liposcelis bostrychophila, common booklice, was successfully used for rickettsial isolation in ISE6 (tick-derived cells). Rickettsiae were also observed in L. bostrychophila by electron microscopy and in paraffin sections of booklice by immunofluorescence assay using anti-R. felis polyclonal antibody. The isolate, designated R. felis strain LSU-Lb, resembles typical rickettsiae when examined by microscopy. Sequence analysis of portions of the Rickettsia specific 17-kDa antigen gene, citrate synthase (gltA) gene, rickettsial outer membrane protein A (ompA) gene, and the presence of the R. felis plasmid in the cell culture isolate confirmed the isolate as R. felis. Variable nucleotide sequences from the isolate were obtained for R. felis-specific pRF-associated putative tldD/pmbA. Expression of rickettsial outer membrane protein B (OmpB) was verified in R. felis (LSU-Lb) using a monoclonal antibody. Additionally, a quantitative real-time PCR assay was used to identify a significantly greater median rickettsial load in the booklice, compared to cat flea hosts. With the potential to manipulate arthropod host biology and infect vertebrate hosts, the dual nature of R. felis provides an excellent model for the study of rickettsial pathogenesis and transmission. In addition, this study is the first isolation of a rickettsial pathogen from a non-hematophagous arthropod.

Urban focus of Rickettsia typhi and Rickettsia felis in Los Angeles, California

Classic murine typhus, caused by Rickettsia typhi, is endemic in the continental United States in areas of Texas and southern California. We conducted an environmental investigation in an urban area of Los Angeles identified as the probable exposure site for a case of murine typhus. Four Rattus norvegicus heavily infested with Xenopsylla cheopis (average 32.5 fleas per animal, range 20-42) were trapped, and fleas, blood, and tissues were collected. DNAs from all specimens were tested for R. typhi and Rickettsia felis using a TaqMan assay targeting the rickettsial citrate synthase gene. Although rickettsiemia was not detected, DNA of R. felis was detected in at least one tissue from each rat. Tissues from 3 rats were also positive for R. typhi DNA. R. typhi and R. felis DNAs were detected in fleas collected from each animal with average minimal infection rates of 10% and 32.3%, respectively. Although R. typhi still circulates in urban Los Angeles in the classic Oriental flea-rat cycle, R. felis is more prevalent, even in this association.

Human Infection with Rickettsia felis, Kenya

This flea-borne pathogen was detected in febrile patients in North Eastern Province, Kenya.

To determine the cause of acute febrile illnesses other than malaria in the North Eastern Province, Kenya, we investigated rickettsial infection among patients from Garissa Provincial Hospital for 23 months during 2006–2008. Nucleic acid preparations of serum from 6 (3.7%) of 163 patients were positive for rickettsial DNA as determined by a genus-specific quantitative real-time PCR and were subsequently confirmed by molecular sequencing to be positive for Rickettsia felis. The 6 febrile patients’ symptoms included headache; nausea; and muscle, back, and joint pain. None of the patients had a skin rash.

Isolation of Rickettsia parkeri and identification of a novel spotted fever group Rickettsia sp. from Gulf Coast ticks (Amblyomma maculatum) in the United States

Until recently, Amblyomma maculatum (the Gulf Coast tick) had garnered little attention compared to other species of human-biting ticks in the United States. A. maculatum is now recognized as the principal vector of Rickettsia parkeri, a pathogenic spotted fever group rickettsia (SFGR) that causes an eschar-associated illness in humans that resembles Rocky Mountain spotted fever. A novel SFGR, distinct from other recognized Rickettsia spp., has also been detected recently in A. maculatum specimens collected in several regions of the southeastern United States. In this study, 198 questing adult Gulf Coast ticks were collected at 4 locations in Florida and Mississippi; 28% of these ticks were infected with R. parkeri, and 2% of these were infected with a novel SFGR. Seventeen isolates of R. parkeri from individual specimens of A. maculatum were cultivated in Vero E6 cells; however, all attempts to isolate the novel SFGR were unsuccessful. Partial genetic characterization of the novel SFGR revealed identity with several recently described, incompletely characterized, and noncultivated SFGR, including "Candidatus Rickettsia andeanae" and Rickettsia sp. Argentina detected in several species of Neotropical ticks from Argentina and Peru. These findings suggest that each of these "novel" rickettsiae represent the same species. This study considerably expanded the number of low-passage, A. maculatum-derived isolates of R. parkeri and characterized a second, sympatric Rickettsia sp. found in Gulf Coast ticks.

Isolation of Rickettsia parkeri and identification of a novel spotted fever group Rickettsia sp. from Gulf Coast ticks (Amblyomma maculatum) in the United States

Until recently, Amblyomma maculatum (the Gulf Coast tick) had garnered little attention compared to other species of human-biting ticks in the United States. A. maculatum is now recognized as the principal vector of Rickettsia parkeri, a pathogenic spotted fever group rickettsia (SFGR) that causes an eschar-associated illness in humans that resembles Rocky Mountain spotted fever. A novel SFGR, distinct from other recognized Rickettsia spp., has also been detected recently in A. maculatum specimens collected in several regions of the southeastern United States. In this study, 198 questing adult Gulf Coast ticks were collected at 4 locations in Florida and Mississippi; 28% of these ticks were infected with R. parkeri, and 2% of these were infected with a novel SFGR. Seventeen isolates of R. parkeri from individual specimens of A. maculatum were cultivated in Vero E6 cells; however, all attempts to isolate the novel SFGR were unsuccessful. Partial genetic characterization of the novel SFGR revealed identity with several recently described, incompletely characterized, and noncultivated SFGR, including "Candidatus Rickettsia andeanae" and Rickettsia sp. Argentina detected in several species of Neotropical ticks from Argentina and Peru. These findings suggest that each of these "novel" rickettsiae represent the same species. This study considerably expanded the number of low-passage, A. maculatum-derived isolates of R. parkeri and characterized a second, sympatric Rickettsia sp. found in Gulf Coast ticks.

Rickettsia felis infection in a common household insect pest, Liposcelis bostrychophila (Psocoptera: Liposcelidae)

Many species of Rickettsia are well-known mammalian pathogens transmitted by blood-feeding arthropods. However, molecular surveys are continually uncovering novel Rickettsia species, often in unexpected hosts, including many arthropods that do not feed on blood. This study reports a systematic molecular characterization of a Rickettsia infecting the psocid Liposcelis bostrychophila (Psocoptera: Liposcelidae), a common and cosmopolitan household pest. Surprisingly, the psocid Rickettsia is shown to be Rickettsia felis, a human pathogen transmitted by fleas that causes serious morbidity and occasional mortality. The plasmid from the psocid R. felis was sequenced and was found to be virtually identical to the one in R. felis from fleas. As Liposcelis insects are often intimately associated with humans and other vertebrates, it is speculated that they acquired R. felis from fleas. Whether the R. felis in psocids causes disease in vertebrates is not known and warrants further study.

Ecology of Rickettsia felis: a review

It has been two decades since the first description of Rickettsia felis, and although a nearly cosmopolitan distribution is now apparent, much of the ecology of this unique microorganism remains unresolved. The cat flea, Ctenocephalides felis, is currently the only known biological vector of R. felis; however, molecular evidence of R. felis in other species of fleas as well as in ticks and mites suggests a variety of arthropod hosts. Studies examining the transmission of R. felis using colonized cat fleas have shown stable vertical transmission but not horizontal transmission. Likewise, serological and molecular tools have been used to detect R. felis in a number of vertebrate hosts, including humans, in the absence of a clear mechanism of horizontal transmission. Considered an emerging flea-borne rickettsiosis, clinical manifestation of R. felis infection in humans, including, fever, rash, and headache is similar to other rickettsial diseases. Recent advances toward further understanding the ecology of R. felis have been facilitated by stable R. felis-infected cat flea colonies, several primary flea isolates and sustained maintenance of R. felis in cell culture systems, and highly sensitive quantitative molecular assays. Here, we provide a synopsis of R. felis including the known distribution and arthropods infected; transmission mechanisms; current understanding of vertebrate infection and human disease; and the tools available to further examine R. felis.

Occurrence of Rickettsia felis in dog and cat fleas (Ctenocephalides felis) from Italy

Rickettsia felis is an obligate intracellular bacterium belonging to the spotted fever group, suspected to cause a murine typhus-like illness in humans, with a cosmopolitan distribution. This study was designed to estimate presence and occurrence of this pathogen in fleas collected from dogs and cats in different areas of Italy. Two species of fleas were identified, Ctenocephalides felis (80.3%) and Ctenocephalides canis (19.7%).

Overall, 320 fleas (257 C. felis and 63 C. canis) collected from 117 animals (73 dogs and 44 cats) were tested. Thirty-eight (11.9%) C. felis fleas, 13 from cats (17.6%) and 25 from dogs (10.2%) were positive for R. felis. No C. canis was positive. Fleas from cats showed a tendency to be more positive than fleas from dogs. Prevalence of R. felis among areas and within provinces of the same area was extremely variable, ranging from 0 to 35.3%. Overall, prevalence in north-eastern Italy (23.2%) was significantly higher than in south-western Italy (7.1%). This study confirmed the occurrence of R. felis in cat and dog fleas (C. felis) from Italy, similar to other European countries. The results also suggest that R. felis should be considered in the human differential diagnosis of any spotted-like fever in Italy, especially if the patient is known to have been exposed to flea bites.

Evolution and diversity of Rickettsia bacteria

Background

Rickettsia are intracellular symbionts of eukaryotes that are best known for infecting and causing serious diseases in humans and other mammals. All known vertebrate-associated Rickettsia are vectored by arthropods as part of their life-cycle, and many other Rickettsia are found exclusively in arthropods with no known secondary host. However, little is known about the biology of these latter strains. Here, we have identified 20 new strains of Rickettsia from arthropods, and constructed a multi-gene phylogeny of the entire genus which includes these new strains.

Results

We show that Rickettsia are primarily arthropod-associated bacteria, and identify several novel groups within the genus. Rickettsia do not co-speciate with their hosts but host shifts most often occur between related arthropods. Rickettsia have evolved adaptations including transmission through vertebrates and killing males in some arthropod hosts. We uncovered one case of horizontal gene transfer among Rickettsia, where a strain is a chimera from two distantly related groups, but multi-gene analysis indicates that different parts of the genome tend to share the same phylogeny.

Conclusion

Approximately 150 million years ago, Rickettsia split into two main clades, one of which primarily infects arthropods, and the other infects a diverse range of protists, other eukaryotes and arthropods. There was then a rapid radiation about 50 million years ago, which coincided with the evolution of life history adaptations in a few branches of the phylogeny. Even though Rickettsia are thought to be primarily transmitted vertically, host associations are short lived with frequent switching to new host lineages. Recombination throughout the genus is generally uncommon, although there is evidence of horizontal gene transfer. A better understanding of the evolution of Rickettsia will help in the future to elucidate the mechanisms of pathogenicity, transmission and virulence.