Scrub typhus bacteria found in chiggers and ground birds

Every year, tens of thousands of people across the Asia-Pacific region fall ill with scrub typhus, a potentially fatal febrile disease caused by the obligate intracellular bacterium Orientia tsutsugamushi. For decades, the ecology of this pathogen has been understood through the lens of a classic zoonotic triangle: trombiculid mites, which harbor the bacterium as a symbiont; their parasitic larval stage, known as chiggers, which transmit the agent during blood meals; and small mammals, particularly rodents, which have long been regarded as the principal hosts that keep the transmission cycle churning in scrubby, secondary vegetation at the interface of wilderness and human settlement. Now, a team of researchers working across Malaysia, the United Kingdom and Thailand has turned its attention to a group of hosts that has been largely overlooked in this cycle: birds. Their findings, published in the open-access journal Parasites & Vectors, provide the first convincing molecular evidence that ground-dwelling birds can carry O. tsutsugamushi, and that at least one species of bird-associated chigger may harbor Orientia-related genetic material, opening an intriguing new chapter in the epidemiology of this neglected tropical disease.

The study, led by Praveena Rajasegaran of the Tropical Infectious Diseases Research & Education Centre at Universiti Malaya and Benjamin L. Makepeace of the University of Liverpool, was designed to answer a deceptively simple question: do birds feeding on or being fed upon by chiggers participate in the maintenance and dispersal of scrub typhus? Although ornithologists and parasitologists have documented that migratory and resident birds are frequently infested with chigger mites, remarkably little is known about whether these mites carry O. tsutsugamushi, or whether birds themselves can sustain infections. This gap matters because birds are mobile, wide-ranging animals capable of moving pathogens across landscapes, borders and even continents in ways that sedentary rodents cannot. If ground birds such as pheasants, rails and junglefowl serve as competent hosts, they could reshape our understanding of how scrub typhus persists in fragmented habitats and how new transmission foci emerge.

To investigate, the team conducted extensive field sampling across 14 sites in Malaysia, encompassing both Peninsular Malaysia and the East Malaysian state of Sarawak. Chigger mites were collected from a diverse array of bird species, pooled by mite species and host individual, and screened for O. tsutsugamushi using a nested polymerase chain reaction assay targeting the htrA gene, a well-established molecular marker for detecting this fastidious bacterium. Recognizing the limitations of targeted PCR alone, the researchers also subjected a subset of chigger pools to Illumina metagenomic sequencing, an unbiased approach that can reveal the full complement of microbial DNA within a sample. To validate this sequencing strategy before applying it to precious field material, the team used chiggers of the species Leptotrombidium chiangraiensis drawn from a laboratory colony in Thailand known to be infected with O. tsutsugamushi, providing a positive benchmark for what Orientia sequences should look like in high-throughput data.

The validation step worked beautifully. When the researchers sequenced pooled DNA from infected L. chiangraiensis chiggers, they recovered good coverage across almost the entire O. tsutsugamushi genome, demonstrating that metagenomic sequencing of chigger pools could, in principle, detect and even assemble substantial portions of the pathogen’s genome. This technical success lent confidence to the interpretation of the field samples, because it established a clear contrast between what a heavily infected chigger pool looks like and what the bird-associated mites actually contained.

The results from wild-caught birds and their chiggers were striking for how sparse they were, yet they were not entirely empty. Nested PCR failed to detect O. tsutsugamushi DNA in any of the pooled chigger samples collected from birds across the Malaysian sites, and metagenomic analysis likewise came up empty for most pools. There was one compelling exception: a pool of Blankaartia acuscutellaris chiggers collected from a wild slaty-breasted rail, Lewinia striata. Metagenomic sequencing of this pool revealed multiple contigs related to Rickettsiales amplified genetic elements of apparent Orientia origin. These are mobile genetic elements associated with the order Rickettsiales, to which O. tsutsugamushi belongs. However, the researchers could not assemble any core genes from the sample, which meant they were unable to confirm a genuine infection or to type any potential bacterial strain. The signal therefore suggests possible exposure or carriage of Orientia-related material by this chigger species, but falls short of definitive proof, underscoring the technical challenges of working with low-biomass, environmentally complex samples.

The bird tissue results were more convincing. The team screened organ and tissue samples from three ground-dwelling or ground-associated species: red junglefowl, the wild ancestor of the domestic chicken; domestic chickens; and house crows, an abundant commensal species across South and Southeast Asia. Nested PCR detected O. tsutsugamushi DNA in one red junglefowl and one domestic chicken. Phylogenetic analysis of these sequences revealed that they clustered with the Karp genotype, one of the classical antigenic variants of O. tsutsugamushi, which also ranks among the most widely distributed and clinically relevant strains. The presence of Karp-related sequences in galliform birds adds weight to the idea that these animals can become infected, even if the frequency of such infections appears low.

Taken together, the findings sketch a picture of rare but real involvement of birds in the ecology of O. tsutsugamushi. The authors are appropriately cautious: a single PCR-positive tissue sample from a junglefowl and a chicken, plus suggestive metagenomic signals in one chigger pool, do not amount to proof that birds are reservoir hosts. The bacterium may have been present transiently in the birds’ tissues after bites by infected chiggers, or the infections could represent dead-end events. Nevertheless, even transient infection in highly mobile hosts has epidemiological consequences. A chigger larva that feeds on a bacteremic bird could theoretically acquire the pathogen and establish a new infection focus after the bird flies elsewhere, a dispersal mechanism entirely absent from the rodent-centered model. This mechanism could help explain one of the enduring puzzles of scrub typhus epidemiology: the patchy, unpredictable distribution of transmission, with so-called tsutsugamushi islands of intense risk surrounded by apparently safe terrain.

The choice of bird species in the study also carries practical implications. Red junglefowl and domestic chickens are galliform birds that forage by scratching in soil and leaf litter, precisely the microhabitats where chiggers quest for hosts. Their close contact with the ground makes them far more likely than canopy species to accumulate chigger burdens, and their association with human settlements, villages and agricultural land places them squarely within zones of potential human exposure. House crows, meanwhile, thrive in urban environments, and although they are less obvious chigger hosts, their inclusion in the study reflects a broader effort to sample birds living alongside people. The detection of O. tsutsugamushi DNA in a domestic chicken, while a single finding, raises questions about backyard poultry as sentinels or even as bridging hosts in peri-domestic transmission.

Methodologically, the study is notable for its dual approach combining targeted nested PCR with unbiased metagenomic sequencing, and for its transparent validation using an infected chigger colony. The inability to recover core genes from the B. acuscutellaris pool illustrates a common limitation of metagenomics in vector research: when bacterial loads are low, host DNA and other microbial DNA swamp the signal, and genome assembly becomes fragmentary. The identification of Rickettsiales amplified genetic elements rather than core genes raises the possibility that the elements detected represent a related but distinct Orientia species or an ancestral symbiont of the mite itself. Indeed, growing evidence suggests that trombiculid mites harbor a diversity of Orientia species and strains, only some of which are pathogenic to humans. Future studies with deeper sequencing, perhaps using hybrid approaches that generate longer reads, could resolve whether B. acuscutellaris, a chigger species with a wide geographic range and known to bite humans, is a genuine vector in its own right.

The research received support from the Royal Society International Collaboration Award and the Higher Institution Centre of Excellence programme at Universiti Malaya, and it drew on positive control DNA derived from infected rodent tissues in Thailand, kindly provided by Dr. Ivo Elliott of the University of Oxford. The collaborative architecture of the study, spanning Malaysian field logistics, Thai laboratory resources and British genomic expertise, exemplifies the kind of partnership needed to tackle diseases that straddle the tropical belt.

What emerges from this work is less a definitive answer than a well-supported hypothesis with significant public health stakes. Scrub typhus is estimated to threaten more than a billion people, and its reported incidence has been climbing as diagnostic capacity improves across endemic countries. No licensed vaccine exists, and treatment relies on timely administration of antibiotics such as doxycycline, which makes understanding where and how transmission occurs a matter of direct clinical relevance. If birds contribute to pathogen dispersal, surveillance programs that currently focus on rodents and mites may need to incorporate avian sampling, particularly at sites where habitat fragmentation, poultry keeping and human encroachment converge. The authors themselves emphasize that the rarity of detections in their study argues for broader avian surveillance rather than complacency: only wider geographic coverage, larger sample sizes and longitudinal sampling can determine whether the infections observed in Malaysia’s junglefowl and chickens are isolated curiosities or the visible edge of a much larger pattern. For now, the sky above the scrub typhus belt has become a little more interesting, and a little more mysterious, to the scientists who track this ancient pathogen.