This study examined the ability of ticks to maintain multiple species of spotted fever group rickettsiae via transovarial transmission. Using a capillary feeding method, previously established Ricketisia montana- and Rickettsia rhipicephali-infected cohorts of Dermacentor variabilis (Say) were exposed to R. rhipicephali and R. montana, respectively, in two reciprocal challenge experiments. Eggs collected from individual females, for two successive generations, of each cohort were assessed for rickettsial infection by polymerase chain reaction for each challenge experiment. Assessment of the eggs from challenged ticks identified that both B. montana- and R. rhipicephali-infected ticks were refractory to their respective challenge rickettsiae. The prechallenged infection rate for both F1 and F2 generations (100%) of the B. montana-infected cohort was resistant to transovarial transmission of the second rickettsia species, and only R. montana was detected in the eggs of FL = (50%) and F2 = (74%) challenged females. The R. rhipicephali-infected cohort maintained a lower level of infection (20%) in the population and did not transovarially transmit the challenge species, however, detectable levels of infection were lost after the first generation. Second-generation ticks, no longer infected with B. rhipicephali, became susceptible to infection with R. montana and female ticks (approximately 4%) were able to transmit R. montana to their progeny. The resistance of the ovaries to co-infection and apparent host-specific nature of infection suggests that rickettsial infection of tick ovaries may alter the molecular expression of the oocytes so as to preclude secondary infection with other rickettsiae.

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.

Application of molecular diagnostic technology in the past 10 years has resulted in the discovery of several new species of pathogenic rickettsiae, including Rickettsia felis. As more sequence information for rickettsial genes has become available, the data have been used to reclassify rickettsial species and to develop new diagnostic tools for analysis of mixed rickettsial pathogens. R. felis has been associated with opossums and their fleas in Texas and California. Because R. felis can cause human illness, we investigated the distribution dynamics in the murine typhus–endemic areas of these two states. The geographic distribution of R. felis-infected opossum populations in two well-established endemic foci overlaps with that of the reported human cases of murine typhus. Descriptive epidemiologic analysis of 1998 human cases in Corpus Christi, Texas, identified disease patterns consistent with studies done in the 1980s. A close geographic association of seropositive opossums (22% R. felis; 8% R. typhi) with human murine typhus cases was also observed.

Rickettsia felis, the etiologic agent of spotted fever, is maintained in cat fleas by vertical transmission and resembles other tick-borne spotted fever group rickettsiae. In the present study, we utilized an Ixodes scapularisderived tick cell line, ISE6, to achieve isolation and propagation of R. felis. A cytopathic effect of increased vacuolization was commonly observed in R. felis-infected cells, while lysis of host cells was not evident despite large numbers of rickettsiae. Electron microscopy identified rickettsia-like organisms in ISE6 cells, and sequence analyses of portions of the citrate synthase (gltA), 16S rRNA, Rickettsia genus-specific 17-kDa antigen, and spotted fever group-specific outer membrane protein A (ompA) genes and, notably, R. felis conjugative plasmids indicate that this cultivatable strain (LSU) was R. felis. Establishment of R. felis (LSU) in a tick-derived cell line provides an alternative and promising system for the expansion of studies investigating the interactions between R. felis and arthropod hosts.Spotted fever group rickettsiae (SFGR) are obligately intracellular gram-negative bacteria that are typically associated with ticks, with one exception being the etiologic agent of flea-associated spotted fever, Rickettsia felis, described predominately in the cat flea, Ctenocephalides felis. There are a growing number of reports implicating R. felis as a human pathogen. Serological analysis and PCR amplification of rickettsial DNA in human samples coupled with clinical manifestations, including fever and maculopapular rash, characterize rickettsiosis (reviewed in reference 24). In colonized cat fleas, vertical transmission of R. felis is thought to be the primary route of maintenance (3, 43); however, the acquisition mechanism of R. felis by vertebrates and uninfected fleas in nature is unknown. Studies on the ecology of R. felis identified a role for opossums in the transmission cycle (6,32,45). Additionally, a role for companion animals, rodents, and, specifically, their fleas as the potential source of human exposure has been suggested (2,8,27).The relatively recent identification and characterization of R. felis yielded novel aspects for the genus Rickettsia. A rickettsia-like organism, first observed by electron microscopy in the midgut epithelial cells of colonized adult cat fleas, was designated the "ELB agent" after the source of the fleas, the Elward Laboratory (El Soquel, CA) flea colony (1). Amplification of rickettsial genes encoding citrate synthase (gltA) and the genus-specific 17-kDa antigen confirmed the presence of rickettsiae in the Elward Laboratory colony (3), and the name R. felis was proposed (10). Subsequent amplification of the genes encoding the 17-kDa antigen and the 190-kDa antigen (ompA) from a colony of fleas maintained at Louisiana State University (LSU) (Baton Rouge, LA) confirmed the molecular characterization of the bacteria and further classified R. felis as an SFGR (7). Although attempts were unable to produce a sustained culture of either the ELB ...

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.

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.

While typically a flea parasite and opportunistic human pathogen, the presence of Rickettsia felis (strain LSU-Lb) in the non-blood-feeding, parthenogenetically reproducing booklouse, Liposcelis bostrychophila, provides a system to ascertain factors governing not only host transitions but also obligate reproductive parasitism (RP). Analysis of plasmid pLbAR, unique to R. felis str. LSU-Lb, revealed a toxin–antitoxin module with similar features to prophage-encoded toxin–antitoxin modules utilized by parasitic Wolbachia strains to induce another form of RP, cytoplasmic incompatibility, in their arthropod hosts. Curiously, multiple deubiquitinase and nuclease domains of the large (3,841 aa) pLbAR toxin, as well the entire antitoxin, facilitated the detection of an assortment of related proteins from diverse intracellular bacteria, including other reproductive parasites. Our description of these remarkable components of the intracellular mobilome, including their presence in certain arthropod genomes, lends insight on the evolution of RP, while invigorating research on parasite-mediated biocontrol of arthropod-borne viral and bacterial pathogens.

To begin to explore the molecular dynamics of rickettsial tick interaction, subtractive hybridization was used to screen mRNAs in Rickettsia montanensis-infected and uninfected Dermacentor variabilis. We isolated 30 cDNA fragments, 22 of which were up-regulated and eight were down-regulated in response to rickettsial infection. Based on a putative identity of 11 cDNA fragments with similarity to known protein families, the tick genetic factors have been assigned into three groups including, the tick immune response factors (alpha-2 macroglobulin and IgE-dependent histamine release factor), the receptor/adhesion molecules (the signal transducer and activator of transcription-1/3 protein inhibitor, the clathrin adaptor protein and tetraspanin) and the stress response proteins (aldose reductase, glutathione-S transferase, ferritin, nucleosome assembly protein and cyclin A protein). Density analyses of semiquantitative RT-PCR amplified products demonstrated differential expression for 18 of the 23 tested genetic factors, apparently representing a 78% agreement with results obtained by subtractive hybridization. Additionally, mRNA transcripts of 17 of the 23 tested genetic factors were amplified from tick haemocytes/circulatory cells demonstrating that their expression is not restricted to the ovaries and suggesting a potential involvement in the immune response.