A study of over 32,000 people living in Tamil Nadu, India suggests scrub typhus infection may affect up to 10% of rural populations annually and is a leading yet under-recognised cause of hospitalisations for fever across India.
The study, published in the New England Journal of Medicine, was conducted as 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, which belongs to the rickettsia family. It is spread to humans through the bite of infected larval mites or chiggers. Chiggers are found on grass, plant litter and bare soil across rural areas of Asia, and usually feed on small mammals such as rats and shrews.
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 study enrolled over 32,000 people living across 37 rural villages in Tamil Nadu, India. As scrub typhus is known to be endemic in the area, the sample represented a typical rural Indian population at risk of infection. The team visited households every six to eight weeks from August 2020 to July 2022, to collect blood samples and record any symptoms of illness they may have experienced. Blood samples taken from people who reported fever were then tested for scrub typhus infection.
The researchers found a high incidence of scrub typhus across the two-year study period, with almost 10% of the population infected annually. Most of these infections were asymptomatic, but of those who were infected, between 8% to 15% developed fever which often required hospitalisation and intensive care due to severe infection. A total of five people died from scrub typhus-associated complications during the study.
The researchers say that under-reporting of fever is an important limitation of the study and that it's likely more cases of scrub typhus may have occurred over the two-year period. The study was also conducted during the COVID-19 pandemic, where quarantine measures were in place in the area.
Wolf-Peter Schmidt, principal investigator of the study and Clinical Assistant Professor at the London School of Hygiene & Tropical Medicine (LSHTM), said: "India is a country particularly affected by scrub typhus, and infections typically occur between August and February.
"Over a period of two years, we found a high incidence of asymptomatic and symptomatic infection, with some people getting infected twice in as many years. It is not clear why some infections become severe or even life-threatening. While five cases in our study population died of scrub typhus, we did not record any deaths from malaria, dengue or typhoid fever which are usually thought of as the main causes of severe fever in India."
Carol Devamani, lead author and MD in Community Medicine based at the Christian Medical College Vellore, said: "After Covid, scrub typhus was the most important cause of fever in our study, accounting for almost 30% of fever hospitalisations.
"Despite cases being very common and treatable, scrub typhus is often overlooked as a possible cause when patients present with a fever. Diagnostic tests are available at major hospitals but not in the community.
"Our study highlights a need to inform communities about the possible dangers of scrub typhus infection, and to improve diagnostic capabilities in the community for timely treatment. Only by doing so will we reduce the number of unnecessary hospitalisations and fatal cases."
The study was supported with funding from the UK Medical Research Council.
