Rickettsia beyond pathogenicity: endosymbiosis, evolution, and implications for One Health

Bacteria of the genus Rickettsia are widely recognized as human pathogens causing a range of mild to severe diseases. However, these bacteria are also endosymbionts of arthropod vectors, where their ecological and functional roles remain poorly understood. This perspective explores the underestimated diversity of the Rickettsia that are not classified as vertebrate pathogens, focusing on their persistence as arthropods’ (particularly, ticks) endosymbionts with different effects on host fitness and vector competence. We discuss the evolutionary and comparative genomic evidence supporting transitions along the symbiosis–pathogenicity continuum, in which genome length reduction seems to represent a key, but not an exclusive feature of pathogenic lineages. We emphasize that tick pathobiomes are dynamic and geographically biased in current data sets, potentially obscuring “silent” transmission cycles and overlooked symbiont–pathogen interactions. Finally, we highlight the importance of integrating genomic, ecological, and microbiome data to support the development of more sustainable, symbiont-based approaches to vector and pathogen control under a One Health perspective.