Wolbachia endosymbionts, Rickettsia felis and Bartonella species, in Ctenocephalides felis fleas in a tropical region

New records of pathogenic bacteria in different species of fleas collected from domestic and peridomestic animals in Spain. A potential zoonotic threat?

Climate change is causing many vectors of infectious diseases to expand their geographic distribution as well as the pathogens they transmit are also conditioned by temperature for their multiplication. Within this context, it is worth highlighting the significant role that fleas can play as vectors of important pathogenic bacteria. For this purpose, our efforts focused on detecting and identifying a total of 9 bacterial genera (Rickettsia sp.; Bartonella sp.; Yersinia sp.; Wolbachia sp., Mycobacterium sp., Leishmania sp., Borrelia sp., Francisella sp. and Coxiella sp.) within fleas isolated from domestic and peridomestic animals in the southwestern region of Spain (Andalusia). Over a 19-months period, we obtained flea samples from dogs, cats and hedgehogs. A total of 812 fleas was collected for this study. Five different species were morphologically identified, including C. felis, C. canis, S. cuniculi, P. irritans, and A. erinacei. Wolbachia sp. was detected in all five species identified in our study which a total prevalence of 86%. Within Rickettsia genus, two different species, R. felis and R. asembonensis were mainly identified in C. felis and A. erinacei, respectively. On the other hand, our results revealed a total of 131 fleas testing positive for the presence of Bartonella sp., representing a prevalence rate of 16% for this genus identifying two species B. henselae and B. clarridgeiae. Lastly, both Y. pestis and L. infantum were detected in DNA of P. irritans and C. felis, respectively isolated from dogs. With these data we update the list of bacterial zoonotic agents found in fleas in Spain, emphasizing the need to continue conducting future experimental studies to assess and confirm the potential vectorial role of certain synanthropic fleas.

Vector biology of the cat flea Ctenocephalides felis

Ctenocephalides felis, the cat flea, is among the most prevalent and widely dispersed vectors worldwide. Unfortunately, research on C. felis and associated pathogens (Bartonella and Rickettsia spp.) lags behind that of other vectors and vector-borne pathogens. Therefore, we aimed to review fundamental aspects of C. felis as a vector (behavior, epidemiology, phylogenetics, immunology, and microbiome composition) with an emphasis on key techniques and research avenues employed in other vector species. Future laboratory C. felis experimental infections with Bartonella, Rickettsia, and Wolbachia species/strains should examine the vector-pathogen interface utilizing contemporary visualization, transcriptomic, and gene-editing techniques. Further environmental sampling will inform the range and prevalence of C. felis and associated pathogens, improving the accuracy of vector and pathogen modeling to improve infection/infestation risk assessment and diagnostic recommendations.

Metagenomics of culture isolates and insect tissue illuminate the evolution of Wolbachia, Rickettsia and Bartonella symbionts in Ctenocephalides spp. fleas

While fleas are often perceived simply as a biting nuisance and a cause of allergic dermatitis, they represent important disease vectors worldwide, especially for bacterial zoonoses such as plague (transmitted by rodent fleas) and some of the rickettsioses and bartonelloses. The cosmopolitan cat (Ctenocephalides felis ) and dog (Ctenocephalides canis ) fleas, as well as Ctenocephalides orientis (restricted to tropical and subtropical Asia), breed in human dwellings and are vectors of cat-scratch fever (caused by Bartonella spp.) and Rickettsia spp., including Rickettsia felis (agent of flea-borne spotted fever) and Rickettsia asembonensis , a suspected pathogen. These Rickettsia spp. are members of a phylogenetic clade known as the ‘transitional group’, which includes both human pathogens and arthropod-specific endosymbionts. The relatively depauperate flea microbiome can also contain other endosymbionts, including a diverse range of Wolbachia strains. Here, we present circularized genome assemblies for two C. orientis -derived pathogens (Bartonella clarridgeiae and R. asembonensis ) from Malaysia, a novel Wolbachia strain (w Cori), and the C. orientis mitochondrion; all were obtained by direct metagenomic sequencing of flea tissues. Moreover, we isolated two Wolbachia strains from Malaysian C. felis into tick cell culture and recovered circularized genome assemblies for both, one of which (w CfeF) is newly sequenced. We demonstrate that the three Wolbachia strains are representatives of different major clades (‘supergroups’), two of which appear to be flea-specific. These Wolbachia genomes exhibit unique combinations of features associated with reproductive parasitism or mutualism, including prophage WO, cytoplasmic incompatibility factors and the biotin operon of obligate intracellular microbes. The first circularized assembly for R. asembonensis includes a plasmid with a markedly different structure and gene content compared to the published plasmid; moreover, this novel plasmid was also detected in cat flea metagenomes from the USA. Analysis of loci under positive selection in the transitional group revealed genes involved in host–pathogen interactions that may facilitate host switching. Finally, the first B. clarridgeiae genome from Asia exhibited large-scale genome stability compared to isolates from other continents, except for SNPs in regions predicted to mediate interactions with the vertebrate host. These findings highlight the paucity of data on the genomic diversity of Ctenocephalides -associated bacteria and raise questions regarding how interactions between members of the flea microbiome might influence vector competence.

New records of bacteria in different species of fleas from France and Spain

In this study, we assessed the presence of vector-borne microorganisms in different species of fleas collected from different hosts in diverse areas of South-Western Europe by molecular methods. A total of 319 fleas belonging to eight different species was tested for the presence of eight microorganisms. Wolbachia spp. endosymbionts were detected in Ctenocephalides felis, Pulex irritans, Archaeopsylla erinacei and Ctenophthalmus baeticus boisseauorum specimens. Rickettsia felis, an emerging pathogen, was detected in C. felis, A. erinacei and Ct. b. boisseauorum. Rickettsia typhi, the agent of murine typhus was detected for the first time in A. erinacei and Mycobacterium spp. were detected for the first time in fleas (C. felis, P. irritans and A. erinacei). Lastly, five different species of Bartonella were detected in fleas' DNA in this study, including a possible new bacterium belonging to this genus. With this study, we updated the knowledge of the flea-borne bacteria present in the South-West of Europe reinforcing the idea about the necessity to expand and increase the current knowledge on flea-borne pathogens.

Flea-borne pathogens in the cat flea Ctenocephalides felis and their association with mtDNA diversity of the flea host

Flea-borne pathogens were screened from 100 individual cat fleas using a PCR approach, of which 38 % were infected with at least one bacterium. Overall, 28 % of the flea samples were positive for Bartonella as inferred from ITS DNA region. Of these, 25 % (7/28) were identified as Bartonella clarridgeiae, 42.9 % (12/28) as Bartonella henselae consisted of two different strains, and 32.1 % (9/28) as Bartonella koehlerae, which was detected for the first time in Malaysia. Sequencing of gltA amplicons detected Rickettsia DNA in 14 % of cat flea samples, all of them identified as Rickettsia asembonensis (100 %). None of the flea samples were positive for Mycoplasma DNA in 16S rRNA gene detection. Four fleas were co-infected with Bartonella and Rickettsia DNAs. Statistical analyses reveal no significant association between bacterial infection and mtDNA diversity of the cat flea. Nevertheless, in all types of pathogen infections, infected populations demonstrated lower nucleotide and haplotype diversities compared to uninfected populations. Moreover, lower haplotype numbers were observed in infected populations.

Evolution of Wolbachia mutualism and reproductive parasitism: insight from two novel strains that co-infect cat fleas

Wolbachiae are obligate intracellular bacteria that infect arthropods and certain nematodes. Usually maternally inherited, they may provision nutrients to (mutualism) or alter sexual biology of (reproductive parasitism) their invertebrate hosts. We report the assembly of closed genomes for two novel wolbachiae, wCfeT and wCfeJ, found co-infecting cat fleas (Ctenocephalides felis) of the Elward Laboratory colony (Soquel, CA, USA). wCfeT is basal to nearly all described Wolbachia supergroups, while wCfeJ is related to supergroups C, D and F. Both genomes contain laterally transferred genes that inform on the evolution of Wolbachia host associations. wCfeT carries the Biotin synthesis Operon of Obligate intracellular Microbes (BOOM); our analyses reveal five independent acquisitions of BOOM across the Wolbachia tree, indicating parallel evolution towards mutualism. Alternately, wCfeJ harbors a toxin-antidote operon analogous to the wPip cinAB operon recently characterized as an inducer of cytoplasmic incompatibility (CI) in flies. wCfeJ cinB and three adjacent genes are collectively similar to large modular toxins encoded in CI-like operons of certain Wolbachia strains and Rickettsia species, signifying that CI toxins streamline by fission of large modular toxins. Remarkably, the C. felis genome itself contains two CI-like antidote genes, divergent from wCfeJ cinA, revealing episodic reproductive parasitism in cat fleas and evidencing mobility of CI loci independent of WO-phage. Additional screening revealed predominant co-infection (wCfeT/wCfeJ) amongst C. felis colonies, though fleas in wild populations mostly harbor wCfeT alone. Collectively, genomes of wCfeT, wCfeJ, and their cat flea host supply instances of lateral gene transfers that could drive transitions between parasitism and mutualism.

Wolbachia: A tool for livestock ectoparasite control

Ectoparasites and livestock-associated insects are a major concern throughout the world because of their economic and welfare impacts. Effective control is challenging and relies mainly on the use of chemical insecticides and acaricides. Wolbachia, an arthropod and nematode-infecting, maternally-transmitted endosymbiont is currently of widespread interest for use in novel strategies for the control of a range of arthropod-vectored human diseases and plant pests but to date has received only limited consideration for use in the control of diseases of veterinary concern. Here, we review the currently available information on Wolbachia in veterinary ectoparasites and disease vectors, consider the feasibility for use of Wolbachia in the control of livestock pests and diseases and highlight critical issues which need further investigation.

An Earliest Endosymbiont, Wolbachia massiliensis sp. nov., Strain PL13 from the Bed Bug (Cimex hemipterus), Type Strain of a New Supergroup T

The symbiotic Wolbachia are the most sophisticated mutualistic bacterium among all insect-associated microbiota. Wolbachia-insect relationship fluctuates from the simple facultative/parasitic to an obligate nutritional-mutualistic association as it was the case of the bedbug-Wolbachia from Cimexlectularius. Understanding this association may help in the control of associated arthropods. Genomic data have proven to be reliable tools in resolving some aspects of these symbiotic associations. Although, Wolbachia appear to be fastidious or uncultivated bacteria which strongly limited their study. Here we proposed Drosophila S2 cell line for the isolation and culture model to study Wolbachia strains. We therefore isolated and characterized a novel Wolbachia strain associated with the bedbug Cimexhemipterus, designated as wChem strain PL13, and proposed Wolbachiamassiliensis sp. nov. strain wChem-PL13 a type strain of this new species from new supergroup T. Phylogenetically, T-supergroup was close to F and S-supergroups from insects and D-supergroup from filarial nematodes. We determined the 1,291,339-bp genome of wChem-PL13, which was the smallest insect-associated Wolbachia genomes. Overall, the wChem genome shared 50% of protein coding genes with the other insect-associated facultative Wolbachia strains. These findings highlight the diversity of Wolbachia genotypes as well as the Wolbachia-host relationship among Cimicinae subfamily. The wChem provides folate and riboflavin vitamins on which the host depends, while the bacteria had a limited translation mechanism suggesting its strong dependence to its hosts. However, the clear-cut distinction between mutualism and parasitism of the wChem in C. hemipterus cannot be yet ruled out.

Isolation and Propagation of Laboratory Strains and a Novel Flea-Derived Field Strain of Wolbachia in Tick Cell Lines

Wolbachia are intracellular endosymbionts of several invertebrate taxa, including insects and nematodes. Although Wolbachia DNA has been detected in ticks, its presence is generally associated with parasitism by insects. To determine whether or not Wolbachia can infect and grow in tick cells, cell lines from three tick species, Ixodes scapularis, Ixodes ricinus and Rhipicephalus microplus, were inoculated with Wolbachia strains wStri and wAlbB isolated from mosquito cell lines. Homogenates prepared from fleas collected from cats in Malaysia were inoculated into an I. scapularis cell line. Bacterial growth and identity were monitored by microscopy and PCR amplification and sequencing of fragments of Wolbachia genes. The wStri strain infected Ixodes spp. cells and was maintained through 29 passages. The wAlbB strain successfully infected Ixodes spp. and R. microplus cells and was maintained through 2–5 passages. A novel strain of Wolbachia belonging to the supergroup F, designated wCfeF, was isolated in I. scapularis cells from a pool of Ctenocephalides sp. cat fleas and maintained in vitro through two passages over nine months. This is the first confirmed isolation of a Wolbachia strain from a flea and the first isolation of any Wolbachia strain outside the “pandemic” A and B supergroups. The study demonstrates that tick cells can host multiple Wolbachia strains, and can be added to panels of insect cell lines to improve success rates in isolation of field strains of Wolbachia.

An overview of rickettsiae in Southeast Asia: Vector-animal-human interface

Rickettsioses are emerging, and re-emerging diseases caused by obligate intracellular arthropod-borne bacteria that infect humans and animals worldwide. Various rickettsiae such as Orientia, Rickettsia, Anaplasma and Ehrlichia have been circulated in companion, domesticated and wild animals through bites of infected ticks, fleas, lice or mites. This review summarizes the infections of rickettsiae, including the newly discovered regional species Rickettsia thailandii, Candidatus Rickettsia sepangensis, Candidatus Rickettsia johorensis, Candidatus Rickettsia laoensis, Candidatus Rickettsia mahosotii, Candidatus Rickettsia khammouanensis, Candidatus Anaplasma pangolinii, and other novel genotypes in vectors, humans and animals in Southeast Asia. Issues on some unidentified rickettsiae that elicit immune responses and production of antibodies that are cross-reactive with the antigens used are discussed. Knowledge gaps which required attention are also identified in this review.

Bartonella rochalimae, B. grahamii, B. elizabethae, and Wolbachia spp. in Fleas from Wild Rodents near the China-Kazakhstan Border

The Alataw Pass, near the Ebinur Lake Wetland (northwest of China) and Taldykorgan (east of Kazakhstan), is a natural habitat for wild rodents. To date, little has been done on the surveillance of Bartonella spp. and Wolbachia spp. from fleas in the region. Here we molecularly detected Bartonella spp. and Wolbachia spp. in wild rodent fleas during January and October of 2016 along the Alataw Pass-Kazakhstan border. A total of 1,706 fleas belonging to 10 species were collected from 6 rodent species. Among the 10 flea species, 4 were found to be positive for Wolbachia, and 5 flea species were positive for Bartonella. Molecular analysis indicated that i) B. rochalimae was firstly identified in Xenopsylla gerbilli minax and X. conforms conforms, ii) B. grahamii was firstly identified in X. gerbilli minax, and iii) B. elizabethae was firstly detected in Coptopsylla lamellifer ardua, Paradoxopsyllus repandus, and Nosopsyllus laeviceps laeviceps. Additionally, 3 Wolbachia endosymbionts were firstly found in X. gerbilli minax, X. conforms conforms, P. repandus, and N. laeviceps laeviceps. BLASTn analysis indicated 3 Bartonella species showed genotypic variation. Phylogenetic analysis revealed 3 Wolbachia endosymbionts were clustered into the non-Siphonaptera Wolbachia group. These findings extend our knowledge of the geographical distribution and carriers of B. rochalimae, B. grahamii, B. elizabethae, and Wolbachia spp. In the future, there is a need for China-Kazakhstan cooperation to strengthen the surveillance of flea-borne pathogens in wildlife.

Molecular detection and identification of Wolbachia endosymbiont in fleas (Insecta: Siphonaptera)

The aim of this study was to determine the presence and prevalence of Wolbachia bacteria in natural population of fleas (Insecta: Siphonaptera) in Turkey, and to exhibit the molecular characterization and the phylogenetic reconstruction at the positive isolates with other species in GenBank, based on 16S rDNA sequences. One hundred twenty-four flea samples belonging to the species Ctenocephalides canis, C. felis, and Pulex irritans were collected from animal shelters in Kayseri between January and August 2017. All flea species were individually screened for the presence of Wolbachia spp. by polymerase chain reaction (PCR) targeting the 16S ribosomal RNA gene. According to PCR analyses, Wolbachia spp. were found prevalent in C. canis and P. irritans fleas, while it was not detected in the C. felis species. Totally, 20 isolates were purified from agarose gel and sequenced with the same primers for molecular characterization and phylogenetic analyses. The sequence analyses revealed 17 polymorphic sites and 2 genetically different Wolbachia isolates, representing two different haplotypes in two flea species. The distribution patterns, molecular characterization, and phylogenetic status of Wolbachia spp. of fleas in Turkey are presented for the first time with this study. Understanding of the role of Wolbachia in vector biology may provide information for developing Wolbachia-based biological control tools.

Wolbachia infections in mosquitoes and their predators inhabiting rice field communities in Thailand and China

Wolbachia are inherited, endocytoplasmic bacteria that infect a wide range of arthropods. Here is the first systematic report on the study of Wolbachia infection in mosquitoes and their predators from both Thailand and China. In Thailand, 632 mosquito specimens (20 spp.) and 424 insect predators (23 spp.) were collected from the rice agroecosystem, mostly from the Central region, followed by the Northeast, the North and the South and were inhabiting rice fields, wetlands and ditches. In China, 928 mosquitoes (15 spp.) and 149 insect predators (16 spp.) were collected from rice fields along the Weishan Lake in Shandong province. Specimens were classified in the orders Diptera, Coleoptera, Odonata and Hemiptera. Using wsp, ftsZ, 16S rRNA and groE gene amplifications, Wolbachia were detected in 12 mosquito spp. and 6 predator spp. from Thailand and 11 mosquito spp. and 5 predator spp. from China. The relative Wolbachia densities of these species were determined using quantitative real-time PCR. The mosquito, Aedes albopictus, and the predator, Agriocnemis femina, had the highest bacterial densities. These results imply that Wolbachia of supergroup B are distributed throughout these insects, probably via horizontal transmission in rice agroecosystems.

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.

Wolbachia pseudogenes and low prevalence infections in tropical but not temperate Australian tephritid fruit flies: manifestations of lateral gene transfer and endosymbiont spillover?

Background

Maternally inherited Wolbachia bacteria infect many insect species. They can also be transferred horizontally into uninfected host lineages. A Wolbachia spillover from an infected source population must occur prior to the establishment of heritable infections, but this spillover may be transient. In a previous study of tephritid fruit fly species of tropical Australia we detected a high incidence of identical Wolbachia strains in several species as well as Wolbachia pseudogenes. Here, we have investigated this further by analysing field specimens of 24 species collected along a 3,000 km climate gradient of eastern Australia.

Results

Wolbachia sequences were detected in individuals of nine of the 24 (37 %) species. Seven (29 %) species displayed four distinct Wolbachia strains based on characterisation of full multi locus sequencing (MLST) profiles; the strains occurred as single and double infections in a small number of individuals (2–17 %). For the two remaining species all individuals had incomplete MLST profiles and Wolbachia pseudogenes that may be indicative of lateral gene transfer into host genomes. The detection of Wolbachia was restricted to northern Australia, including in five species that only occur in the tropics. Within the more widely distributed Bactrocera tryoni and Bactrocera neohumeralis, Wolbachia also only occurred in the north, and was not linked to any particular mitochondrial haplotypes.

Conclusions

The presence of Wolbachia pseudogenes at high prevalence in two species in absence of complete MLST profiles may represent footprints of historic infections that have been lost. The detection of identical low prevalence strains in a small number of individuals of seven species may question their role as reproductive manipulator and their vertical inheritance. Instead, the findings may be indicative of transient infections that result from spillover events from a yet unknown source. These spillover events appear to be restricted to northern Australia, without proliferation in host lineages further south. Our study highlights that tropical fruit fly communities contain Wolbachia pseudogenes and may be exposed to frequent horizontal Wolbachia transfer. It also emphasises that global estimates of Wolbachia frequencies may need to consider lateral gene transfer and Wolbachia spillover that may be regionally restricted, transient and not inherited.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0474-2) contains supplementary material, which is available to authorized users.