The chigger microbiome: big questions in a tiny world

In addition to agents of scrub typhus (Orientia spp.), several other bacterial and viral pathogens have been recorded from chiggers, some of which appear to be vertically transmitted.A number of arthropod symbionts have also been reported from trombiculid mites, and there is strong evidence for vertical transmission for approximately half of these taxa.Very few unbiased microbiome studies of chiggers have been conducted to date and have taken very different approaches (i.e., analysis of a mite colony or wild chiggers, whether fed or questing), rendering meaningful comparisons difficult; moreover, three of four studies were undertaken in the same country (Thailand).The vast majority of studies on pathogens or arthropod symbionts in chiggers have focused on bacteria, with no unbiased virome or mycobiome analyses performed to date. ‘Chiggers’ (trombiculid mite larvae) are best known as vectors of rickettsial pathogens, Orientia spp., which cause a zoonosis, scrub typhus. However, several other pathogens (e.g., Hantaan orthohantavirus, Dabie bandavirus, Anaplasma spp., Bartonella spp., Borrelia spp., and Rickettsia spp.) and bacterial symbionts (e.g., Cardinium, Rickettsiella, and Wolbachia) are being reported from chiggers with increasing frequency. Here, we explore the surprisingly diverse chigger microbiota and potential interactions within this microcosm. Key conclusions include a possible role for chiggers as vectors of viral diseases; the dominance in some chigger populations of unidentified symbionts in several bacterial families; and increasing evidence for vertical transmission of potential pathogens and symbiotic bacteria in chiggers, suggesting intimate interactions and not simply incidental acquisition of bacteria from the environment or host.