Researchers say public health campaigns should go one step further and stress the possibility of infection beyond agricultural work
Most human scrub typhus infections could occur inside villages rather than during agricultural work, suggests new research conducted with the help of communities living in Tamil Nadu.
The new findings are part of a collaboration between the London School of Hygiene & Tropical Medicine (LSHTM) and the Christian Medical College Vellore, India.
Scrub typhus is a potentially life-threatening infection caused by the bacterium Orientia tsutsugamushi. Infection spreads to humans through the bite of infected larval mites or chiggers, often found on grass, plant litter and bare soil across rural areas of Asia.
Groups including farmers and military personnel have been widely thought to be most at risk, due to their work in agricultural fields, forests and forest edges, where chiggers were thought to occur most commonly.
To assess how chigger numbers may influence which areas people are most likely to become infected in, the team caught over 800 small mammals in 25 villages across two districts of Tamil Nadu and the land surrounding them. Each animal was assessed for chiggers, usually attached to the ears or legs, and over 10,000 samples were tested to see if they were infected with scrub typhus-causing Orientia. The findings are published in Emerging Infectious Diseases.
Small mammals were four to five times more likely to be caught within human settlements than surrounding areas. The greater bandicoot rat, the black rat and the Asian house shrew accounted for most of the animals collected.
Chigger numbers on rats and shrews caught inside villages were far higher than on those caught in the land surrounding the villages. Of the chigger samples analysed, around 4% tested positive for scrub typhus-causing bacteria Orientia. Most worryingly, chiggers feeding on mammals collected within human settlements were more than twice as likely to be infected than those from mammals collected outside villages.
These results build upon an earlier study by the team, which surveyed and collected blood samples from over 2000 residents living in the same villages in Tamil Nadu over a two-year period. They found that agricultural activities, including taking animals for grazing and collecting firewood, did not increase the risk of scrub typhus infection. Further, people living on the village edge were not at a higher risk of infection than those living away from the fields in the village centre. The findings were published last year in Epidemiology and Infection.
Taken together, the results suggest that most human scrub typhus infections may occur inside villages rather than during agricultural work.
The researchers say that comparing chigger numbers in different environments is difficult, due to their small size of just 0.3 mm. Collecting chiggers from rats and shrews rather than directly collecting them from the soil is an important limitation of the study because rats and shrews show strong seasonal changes in behaviour, and may be roaming between fields and houses.
Wolf-Peter Schmidt, principal investigator of the study and Clinical Assistant Professor at LSHTM, said: "Over the past years, we've seen a strong increase in awareness for scrub typhus among medical professionals, and there are also efforts to encourage people living in affected regions to look out for possible symptoms and to seek medical assistance where needed.
"Our new findings suggest these public health campaigns may also need to go one step further, and stress that the possibility of infection may be as high or even higher close to homes than it is outside villages when working in fields."
Carol Devamani, lead author and MD in Community Medicine based at the Christian Medical College Vellore, said: "Our research wouldn't be possible without the continued support and collaboration of local communities affected by scrub typhus.
"While we collect this vital data to help inform public health interventions and local treatment centres, we also work with communities to help identify areas of rodent and mite infestation. Further efforts are needed to help spread awareness of the illness to improve its diagnosis and treatment."
Scrub typhus infections may affect up to 10% of rural populations in India annually and is a leading yet under-recognised cause of hospitalisations for fever. Symptoms such as fever, headache, body aches and rash usually begin around 10 days after infection. The tissue around chigger bites will also typically turn black, which can aid doctors with diagnosis.
If left untreated, severe illness from scrub typhus infection can lead to acute respiratory distress syndrome (ARDS), shock, meningitis and kidney failure, which can be fatal. Cases can be treated using the antibiotics doxycycline and azithromycin but there is currently no vaccine to prevent infection. The authors say that diagnostic tests need to be made available within communities to record and treat infections as soon as possible.
The researchers suggest that villages affected by scrub typhus may benefit from measures that reduce rodent infestation such as solid waste management and safe storage of food. Contact with soil and grass areas may also be reduced by paving courtyards of homes and pathways connecting houses.
LSHTM and CMC Vellore have a long history of collaborating on projects concerning common infectious diseases, such as scrub typhus and worm infection. For this series of studies, the researchers brought together a multidisciplinary team of clinicians, epidemiologists, entomologists and molecular biologists.
The study was supported with funding from the Medical Research Council, UK.
Publication
Devamani CS et al. Leptotrombidium imphalum Chiggers as Vector for Scrub Typhus in Human Settlements, India, 2022-2023. Emerging Infectious Diseases. 2026. DOI: 10.3201/eid3202.251170.
