Authors

B Makepeace¹;  ¹ University of Liverpool, UK

Discussion

Trombiculid mites are highly diverse, globally distributed arachnids with parasitic larval stages (“chiggers”) that typically are only 0.25 mm in length. Chiggers feed primarily on terrestrial vertebrates, whereas subsequent stages in the trombiculid lifecycle are free-living predators in soil ecosystems. The main impact of chiggers is twofold: they cause irritation and potentially severe allergic reactions to their bites in humans and other animals (i.e., trombiculiasis); and a small number of species are vectors of a neglected zoonosis, scrub typhus, which is caused by Orientia spp. bacteria. Scrub typhus has a median mortality rate of 6% if untreated and is very challenging to diagnose, with major clinical impact across the Indian subcontinent, China, Southeast Asia, the Korean Peninsula, and Oceania. Endemic disease has also emerged in recent decades in the Middle East and South America, and is suspected to circulate in Africa. Here, I explain how research on chiggers has lagged behind that of ticks but recent data from chigger microbiome studies suggest overlapping features with their larger, distant ixodid cousins, as well as distinct characteristics that may reflect different modes of parasitism. Chiggers are frequently infected with what are considered to be tick or flea-borne pathogens, as well as with viruses that are believed to be transmitted directly without the involvement of a vector. However, overlooked data from China challenge these assumptions, and evidence for vertical transmission of multiple microorganisms (not only Orientia) has been obtained. Recent metagenomic studies on chiggers from the Arabian Peninsula have uncovered a unique, divergent clade of the Wolbachia symbiont and a rodent-associated spirochaete that has no known vector. Overall, the surprising complexity of the chigger microbiome suggests that the ecological impact of these vectors on wildlife hosts and in pathogen transmission has been underestimated, with implications for our understanding of zoonoses beyond scrub typhus.