ABSTRACT Ticks are obligate hematophagous arthropods and feed exclusively on blood. As blood is nutrient-poor, ticks rely on bacterial endosymbionts to synthesise nutrients, yet the diversity and functional roles of these symbionts in Australian ticks remain largely uncharacterised. This is critical to address as these ticks are of high medical importance in Australia. In this study, shotgun metagenomic sequencing was performed on Bothriocroton concolor , Bothriocroton hydrosauri , Haemaphysalis longicornis and Ixodes holocyclus , enabling the recovery of six complete or partial metagenome-assembled genomes (MAGs). These comprised Coxiella -like endosymbionts (CLE), a facultative Rickettsia symbiont, and two Midichloria mitochondrii strains (Ixholo1 and Ixholo2). Functional annotation of these taxon-specific symbionts revealed the absence of virulence factors and the presence of B-vitamin and/or heme biosynthesis genes, indicative of nutritional mutualism, which is essential for tick hematophagy. The CLEs additionally harbour genes of the shikimate pathway, which modulate blood feeding in ticks by regulating serotonin biosynthesis. Furthermore, functional annotation and pangenomic analysis of Midichloria spp. found evidence that the genus may encompass multiple species, as well as the retention of genes potentially associated with an intramitochondrial lifestyle in M. mitochondrii Ixholo2. Tick microbiomes are dominated by non-pathogenic microorganisms, which are often overshadowed by pathogens. These include the endosymbionts, which can influence host biology and pathogen transmission, and are fundamental for the development of diagnostic tools and taxon-specific tick biocontrols.

AbstractTolerance to infection should be selected when exposure is highly probable, when parasite virulence is relatively low and when the costs of resistance outweigh those of damage control. This strategy may be particularly favoured in slow-pace-of-life species that prioritize survival over short-term reproduction. Here, we investigate infection by Anaplasma amazonensis, a recently described tick-borne bacterium from the Amazon basin, in two sloth species: the pale-throated sloth (Bradypus tridactylus) and Linnaeus’s two-toed sloth (Choloepus didactylus), emblematic slow-pace-of-life species. Our screening of 175 wild individuals revealed a single A. amazonensis strain infecting both species, with high prevalence (48–63%) and evidence of sustained circulation for at least 25 years. Clinical assessments further indicated that infections were largely asymptomatic, with no significant effects on body condition, scaled mass index or reproduction, although rare instances of reduced haematocrit suggest transient erythrocyte loss during acute phases. The high prevalence of infection in apparently healthy wild sloths supports the hypothesis of a tolerance mechanism that mitigates overt clinical symptoms and favours the circulation of A. amazonensis.

Introduction:New species of Borrelia, Rickettsia, and Anaplasma have recently been detected in ticks in French Guiana. Additionally, a new species of Anaplasma was discovered in a human. Specific genovariants of Anaplasma and Ehrlichia have also been identified in the Amazonian forest of French Guiana, indicating a specific sylvatic cycle. Case presentation:We present the case of a patient who experienced fever, myalgia, rash, and inoculation eschars following dozens of tick bites while hiking in the Amazon rainforest. Laboratory results showed lymphopenia, neutrophilia, hepatic cytolysis and cholestasis, and a C-reactive protein level of 78 mg/L. Serology tests for HIV, hepatitis A, B, C, and E; cytomegalovirus; syphilis; and Q fever were negative. NS1 antigen, dengue, and leptospirosis PCR tests were also negative. A skin biopsy of an ulcerated papule on the leg revealed a polymorphic dermal inflammatory infiltrate with intramacrophagic Gram-negative bacilli. Blood PCR for Anaplasma phagocytophilum, Borrelia, Rickettsia, and Babesia were negative. Serology tests were negative for Rickettsia conorii and R. typhi, but positive for A. phagocytophilum, with weakly positive IgM and positive IgG. Serological results were confirmed by controlled sample. The patient improved with doxycycline treatment. Discussion and conclusion:Novel species of Anaplasma and Ehrlichia have been detected in the Amazon rainforest of French Guiana. These novel genovariants differ from other species in the Northern Hemisphere and are difficult to diagnose with molecular tools not designed to detect pathogens differing from known species. While not proven by molecular biology, this case may represent an anaplasmosis or another infection from the Rickettsiales group. This highlights that little is known about the potential for tick-borne diseases in French Guiana. In this context, recommendations for vector control should extend to ticks as well.

Bacteria of the Spiroplasma ixodetis clade are well characterized as reproductive parasites and defensive endosymbionts of arthropods. Nevertheless, clinical evidence indicates that they can also infect humans, causing neonatal ocular disease and acute febrile illness in adults. Using metagenomic assembly and phylogenomic analyses of Spiroplasma ixodetis-related human infections (SiRHIs), combined with a systematic meta-analysis of public datasets, we identified 25 human cases across ten European countries. Despite the frequent detection of multiple S. ixodetis strains in ticks, our data provide no evidence implicating tick-associated strains in human infections. Instead, SiRHI constitute a distinct monophyletic lineage within the S. ixodetis clade, consistent with a shared evolutionary origin with arthropod-associated relatives. Notably, SiRHI genomes harbor horizontally acquired chaperone genes absent from most arthropod-associated Spiro-plasma, while retaining conserved effector genes typical of endosymbionts, suggesting the preservation of ancestral symbiotic traits alongside newly acquired molecular adaptations.

Fleas (Insecta: Siphonaptera) are important vectors of zoonotic bacterial pathogens. However, the bacterial communities hosted by fleas remain poorly understood, especially regarding the interactions between nonpathogenic bacteria and pathogens, as well as the factors shaping these bacterial communities. This study aimed to explore the bacterial composition and diversity between female and male Polygenis (Polygenis) bohlsi bohlsi fleas collected from free-living Thrichomys fosteri (Rodentia: Echimyidae) in the Nhecolândia region of central-western Brazil. Here, bacterial profiling of fleas was performed using a Next-Generation Sequencing approach targeting the V3-V4 hypervariable region of the 16S rRNA gene to assess community structure. Bacterial diversity in P. (P.) bohlsi bohlsi fleas varied by sex, as indicated by significant differences in alpha diversity, while beta diversity showed a trend toward separation between sexes. Pseudomonadota was the predominant phylum in the flea microbiome. Ten bacterial genera showed differentially abundance between male and female fleas, including Bartonella spp., which was detected exclusively in males, and Wolbachia spp., which was more abundant in females than in males. Phylogenetic analysis positioned Bartonella gltA sequences detected in two fleas within the same clade as Bartonella harrusi. While the Wolbachia wsp and 16S rRNA sequences detected in P. (P.) bohlsi bohlsi grouped within the supergroups S (associated with pseudoscorpions), gatB sequences grouped with V (cat flea-related). These findings suggest sex-associated differences in the microbiome of P. (P.) bohlsi bohlsi fleas and highlight the need for further investigation into the potential role of Wolbachia spp. in shaping flea-associated microbial communities and their interactions with Bartonella spp.

Host-associated microbial communities play a central role in insect ecology and evolution, yet comparative studies across diverse taxa remain limited for many taxa and regions. Here, we conducted a multi-species survey of whole-bacterial communities associated with 17 Pentatomomorpha (Hemiptera) species from four families (Pentatomidae, Coreidae, Lygaeidae, Pyrrhocoridae) collected from natural populations in France, using 16S rRNA gene sequencing. Microbiota composition differed markedly among host families: mostbacterial communites hosted by Pentatomoidae species were dominated by members of the Pantoea/Erwinia genera, consistent with their putative role as obligate nutritional symbionts. In contrast, Pyrrhocoridae, Coreidae, and Lygaeidae harbored more diverse bacterial communities, including Sphingobium, Hungatella, and Gordonibacter. Serratia was frequently detected and often highly prevalent across species and sites, suggesting it may be a widespread facultative symbiont in Pentatomomorpha in Western Europe. Host taxonomy was a major determinant of microbiota diversity and composition, with a significant signal of phylosymbiosis, indicating that evolutionary history shapes microbiome assembly, while ecological factors such as diet, physiology, and environmental exposure further influence community structure. Network analyses revealed interspecific differences in bacterial community organization, with several low-abundance taxa occupying central positions within microbial interaction networks. By contrast, transovarially inherited endosymbionts, such as Wolbachia and Spiroplasma, were rare accross sample species. This study establishes a baseline for Pentatomomorpha microbiota, highlights the joint influence of host evolution and ecology on microbial communities, and provides a foundation for microbiome-based approaches to sustainable pest management.

Abstract Fleas (Insecta: Siphonaptera) are important vectors of zoonotic bacterial pathogens. However, the bacterial communities hosted by fleas remain poorly understood, especially regarding the interactions between nonpathogenic bacteria and pathogens, as well as the factors shaping these bacterial communities. This study aimed to explore the bacterial composition and diversity between female and male Polygenis ( Polygenis ) bohlsi bohlsi fleas collected from free‐living Thrichomys fosteri (Rodentia: Echimyidae) in the Nhecolândia region of central‐western Brazil. Here, bacterial profiling of fleas was performed using a Next‐Generation Sequencing approach targeting the V3–V4 hypervariable region of the 16S rRNA gene to assess community structure. Bacterial diversity in P. (P.) bohlsi bohlsi fleas varied by sex, as indicated by significant differences in alpha diversity, while beta diversity showed a trend toward separation between sexes. Pseudomonadota was the predominant phylum in the flea microbiome. Ten bacterial genera showed differentially abundance between male and female fleas, including Bartonella spp., which was detected exclusively in males, and Wolbachia spp., which was more abundant in females than in males. Phylogenetic analysis positioned Bartonella gltA sequences detected in two fleas within the same clade as Bartonella harrusi . While the Wolbachia wsp and 16S rRNA sequences detected in P. (P.) bohlsi bohlsi grouped within the supergroups S (associated with pseudoscorpions), gatB sequences grouped with V (cat flea‐related). These findings suggest sex‐associated differences in the microbiome of P. (P.) bohlsi bohlsi fleas and highlight the need for further investigation into the potential role of Wolbachia spp. in shaping flea‐associated microbial communities and their interactions with Bartonella spp.

Background: Q fever, caused by Coxiella burnetii, is a globally distributed zoonosis that remains largely underdiagnosed in Algeria. Since its first description in 1935, fragmented data have accumulated across human, animal, and vector hosts without comprehensive synthesis. This systematic review summarizes all available information on the occurrence of C. burnetii in Algeria from 1935 to 2025, across human, animal and ectoparasite compartments, and critically discusses potential reservoirs, vectors and knowledge gaps. Methods: Following PRISMA 2020 guidelines, we searched PubMed, Scopus, ScienceDirect and the Algerian national thesis repository. Studies reporting original data on C. burnetii in humans, livestock, pets, wildlife, ticks or lice were included. Data were extracted on study period, location, host species, diagnostic methods, prevalence and molecular typing. Results: Sixty-one studies met the inclusion criteria. Human cases consisted mainly of sporadic acute infections, endocarditis and obstetric complications, often diagnosed abroad, reflecting limited local testing. In livestock, serological and molecular surveys showed infection in cattle, sheep, goats and camels across 30 among 58 wilayas, with apparent seroprevalence ranging from 8 to 27% in cattle, 15–75% in small ruminants and up to 70% in camels. Seventeen studies investigated arthropods: C. burnetii or Coxiella-like endosymbiont DNA was found in 13 tick species from ruminants, camelids and companion or wild animals, though the significance of these detections remains uncertain. Four studies reported on the bacterium in human and animal lice. Molecular typing identified several genotypes, suggesting multiple transmission cycles. Conclusions: Available evidence supports the long-standing and widespread circulation of C. burnetii in Algeria, but the true burden remains underestimated due to limited diagnostic capacity and lack of integrated surveillance. Strengthening local serological and molecular tools, including strain typing, is essential to clarify reservoirs, transmission dynamics and human infection sources within a One Health framework.

Ticks are ectoparasites harboring complex microbial communities, typically dominated by nutritional symbionts that produce B vitamins and sometimes including pathogens affecting human and animal health. However, ticks also host a variety of commensal microbes whose diversity remains poorly documented. In this study, we isolated and identified culturable bacteria and fungi associated with various tick species from the genera Ixodes, Dermacentor, Amblyomma, and Ornithodoros, collected from their natural habitats or hosts in France and French Guiana. A total of 111 bacterial and 27 fungal isolates were obtained which were then identified using both molecular and morphological approaches. Substantial fungal diversity was observed in a few ticks, whereas culturable bacteria displayed a broader distribution and diversity across tick species. Interestingly, the diversity of culturable bacteria and fungi revealed a microbiome structure that reflected the ecological niches of the tick host, indicating habitat-specific microbial associations and a potential ecological role in tick biology. The isolation of common gut bacteria of other arthropods, as well as the isolation of a viable entomopathogenic fungus, underscores the potential influence of these microbes on tick biology.

Members of the Rickettsiella genus (order: Legionellales) are emerging as widespread bacteria associated with insects, arachnids, and crustaceans. While some Rickettsiella strains are highly virulent pathogens, others are maternally inherited endosymbionts that manipulate arthropod phenotypes, including the induction of defensive symbiosis and cytoplasmic incompatibility. However, the genomic diversity of Rickettsiella remains largely unexplored, and their genetic potential to induce complex phenotypes in arthropods is only partially understood. In this study, we sequenced five new Rickettsiella genomes isolated from three tick species. Through comparative genomics, we observed that Rickettsiella members share similar metabolic capabilities, and collectively lack virulence genes from pathogenic Legionellales. Additional analysis of Rickettsiella genomes revealed significant variability in metabolic properties related to endosymbiosis. Specifically, their capacity to biosynthesize certain B vitamins and heme varies, suggesting a functional role of some Rickettsiella strains in the nutrition of their arthropod hosts. Some Rickettsiella genomes harbour homologs of Wolbachia cif genes, the cause of Wolbachia-induced cytoplasmic incompatibility, suggesting that Rickettsiella may use a similar molecular mechanism to manipulate the reproduction of their arthropod hosts. Phylogenomics further revealed that tick-borne Rickettsiella exhibit distinct evolutionary origins within the genus, indicating that Rickettsiella have undergone repeated horizontal transfers between ticks and other arthropods.

Ticks are blood-feeding arthropods that can transmit a wide variety of microorganisms (bacteria, viruses, protozoa, and filarial nematodes) when they feed on various vertebrate hosts. In the recent years, high-throughput sequencing (HTS), also called next-generation sequencing, has become a key tool for detecting and characterizing microorganisms, whether they are pathogens or part of the tick's own microbiota. This narrative review summarizes current applications of HTS for the surveillance and diagnosis of tick-borne diseases within a One Health framework.From nucleic acids extracted from a tick sample, HTS enables the possibility of simultaneous detection of multiple microorganisms, and provides valuable information on potential reservoir hosts through blood meal analysis. It has revealed a vast diversity of bacterial, viral, protozoan agents and filarial nematodes in various tick species worldwide, including unexpected or novel pathogens. HTS has also improved our understanding of the tick microbiota and how it interacts with pathogens, which could have an impact on vector competence.In the field of microbiological diagnosis, HTS provides a complementary or alternative approach to traditional diagnostic tests, particularly in cases with non-specific symptoms or when the etiology is unknown. HTS has proven to be effective in detecting rare or novel pathogens, including some transmitted by ticks. It has also enabled the reconstruction of whole genomes of microorganisms from clinical samples or ticks, thereby improving our understanding of the molecular epidemiology of these agents.By bridging the vector, its pathogens, the reservoir host, and human or animal clinical outcomes, HTS represents a cornerstone technology for future integrated surveillance systems of TBDs within a One Health perspective.

Cytoplasmic Incompatibility (CI) causes embryonic lethality in arthropods, resulting in a significant reduction in reproductive success. In most cases, this reproductive failure is driven by Wolbachia endosymbionts through their cifA-cifB gene pair, whose products disrupts arthropod DNA replication during embryogenesis. While a cif pair has been considered a hallmark of Wolbachia, its presence and functional significance in other bacterial lineages remains poorly investigated. Here, we conducted a comprehensive survey of 762 genomes spanning non-Wolbachia endosymbionts and their close relatives, revealing that the cif pair is far more widespread than previously recognized. We identified cif loci in 8.4% of the surveyed genomes, with a striking incidence of 17.4% in facultative symbionts. Beyond Wolbachia, cif pair occurs across eight bacterial genera spanning α-Proteobacteria, γ-Proteobacteria, Mollicutes, and Bacteroidota. Notably, cif pair has been identified in several intracellular pathogens of mammals showing high rate of transovarial transmission in their arthropod hosts, suggesting a potential role of cif pair and CI in vector-borne disease dynamics. Structural analyses further reveal that the PD(D/E)-XK nucleases and AAA-ATPase-like motifs are consistently conserved across cif pairs in all bacterial taxa. Moreover, cif pairs are frequently integrated within diverse mobile genetic elements, from transposons to large intact WO prophages in Wolbachia and RAGEs in Rickettsiaceae. Phylogenetic analyses reveal recent and potentially ongoing horizontal transfers of cif pair between distantly related bacterial lineages, a process potentially facilitated by mobile genetic elements. Indeed, the PDDEXK2 transposase exhibits a phylogenetic pattern consistent with the co-transmission of cif genes, suggesting that it may facilitate horizontal transfers of cif across bacterial lineages. Furthermore, the detection of endosymbionts harboring cif pair in arthropod groups where Wolbachia is scarce, such as ticks, suggests that CI may be more widespread than previously known, with significant implications for arthropod symbiosis, reproductive manipulation, and future biocontrol strategies.

Ixodes frontalis, an ornithophilic tick species, is widely distributed all over Europe exhibiting two genetically diverging haplogroups based on differences in the cytochrome c oxidase subunit 1 mitochondrial gene. Despite its broad distribution, little is known about the presence of symbiotic bacteria in I. frontalis, while symbionts are generally widespread in ixodid ticks and responsible for important effects on host fitness. We collected I. frontalis from France and Italy (n = 277) and assessed that the most prevalent haplogroup was A (73%). We then investigated the presence of the symbionts, Midichloria mitochondrii and Spiroplasma ixodetis. They were both found at a high prevalence in adult ticks (66% and 77% respectively), while the number of positive immature ticks was significantly lower (18% for both). The experimental analysis of larvae hatched from egg clutches obtained from four females hints at vertical transmission of both symbionts. We obtained three genomes of Spiroplasma and one of Midichloria, and used them to perform comparative genomic analysis. Average nucleotide identity among available Spiroplasma or Midichloria genomes from I. frontalis are all extremely high, suggesting low genetic variability for both symbionts. Gene presence/absence analysis confirmed the presence of B vitamin synthesis genes in the genome of M. mitochondrii, and also showed the presence of the ETX/MTX2 gene, the RIP family and a partial Spaid-like gene in S. ixodetis. This gene repertoire indicates a nutritional role for Midichloria, while for S. ixodetis we hypothesize a role of this bacterium as a defensive symbiont or a manipulator of the host reproduction.

Symbiosis with bacteria is essential for the survival of animals with an obligate blood-feeding lifestyle. In ticks, two distinct bacterial lineages, Coxiella-like and Francisella-like endosymbionts, have independently evolved into nutritional symbionts, converging on a key biochemical function for the tick's survival and growth: the production of three B vitamins. In this study, we carried out comparative analyses across multiple tick species and characterised remarkable similarities in their tissue localisation, particularly in organs important for nutrient metabolism and maternal transmission to progeny. In these organs, both symbionts colonise similar intracellular niches, residing within membrane-bound, replicative vacuoles that occupy a substantial part of the cytoplasm of tick cells. Despite extensive genomic reduction, both symbionts have retained pathways for the biosynthesis of B vitamins and, in some cases, chorismate, a precursor used for the production of serotonin by ticks. However, differences exist: while Coxiella-like endosymbionts lack the ability to synthesise heme, Francisella-like endosymbionts possess a complete heme biosynthesis pathway and may potentially provide ticks with this essential cofactor. Overall, these phenotypic and genomic characteristics reveal a broad convergence among symbiotic interactions across major tick families, highlighting the essential role of symbiosis in tick nutrition, feeding behaviour, blood intake and subsequently in pathogen transmission.

Filarial nematodes of the Dipetalonema lineage include tick-borne filarioids that infect both domestic and wild vertebrate hosts, but they remain understudied in many cases. In this study, we conducted a molecular characterization of a Dipetalonema-like filarioid (DLF) recently identified in two tick species in French Guiana, South America. While the cox1 mitochondrial gene was the sole marker initially sequenced for describing DLF, its classification and phylogenetic relationship with other members of the Dipetalonema lineage were unclear. Therefore, we better characterized DLF through the sequencing of six additional gene markers and conducted phylogenetic analyses. Based on this multi-locus typing scheme, DLF exhibited significant divergence from known genera and species of filarioids, or other sequences available in public databases, suggesting its potential classification as a novel genus within the Dipetalonema lineage. Phylogenetic analyses further unveiled a close evolutionary relationship between DLF and all other filarioids associated with Acari (ticks and mites) within a robust monophyletic subclade in the Dipetalonema lineage. Overall, these findings confirm the existence of a specialized, Acari-borne group of filarioids and underscore the need for comprehensive investigations into their epidemiology and potential impact on animal health.

Tick-borne Apicomplexa encompass a group of parasites responsible for significant medical and veterinary diseases, including babesiosis, theileriosis, and hepatozoonosis. In this study, we investigated the presence and diversity of tick-borne Apicomplexa in wildlife and ticks inhabiting the Amazon rainforests of French Guiana. To this end, we conducted molecular screening and typing using 18S rRNA sequences on a collection of 1161 specimens belonging to 71 species, including 44 species of wild mammals, five species of passerines, and 22 species of ticks. We characterized eight genovariants of Babesia, Theileria, Hemolivia, and Hepatozoon parasites, some matching known species, while others suggested potential novel species. These parasites were detected in wild mammals, including opossums, sloths, armadillos, porcupines, margays, greater grisons, and ticks, but not in passerines. Finally, similarities with surveys conducted in Brazil highlight the specific sylvatic transmission cycles of South American tick-borne Apicomplexa.