Scrub typhus behind fever-related hospitalisations?

New Delhi: Scrub typhus, a mite-borne bacterial infection, may be a major under-recognised cause of fever-related hospitalisations in India, an Indo-US team of researchers reported on Wednesday, after showing nearly 10 per cent of rural population in Tamil Nadu getting the infection every year.

This was the outcome of a two-year study carried out by medical scientists at Christian Medical College Vellore and London School of Hygiene and Tropical Medicine involving over 32,000 individuals from more than 7,600 houses spread across 37 villages.

“After Covid, scrub typhus was the most important cause of fever in our study, accounting for almost 30 per cent of fever hospitalisations," said Carol Devamani, lead author and a specialist in community medicine at CMC.

“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.”

A potentially life-threatening infection caused by a bacterium (Orientia tsutsugamushi), scrub typhus is spread to humans through the bite of infected larval mites or chiggers that 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 the infection can lead to acute respiratory distress syndrome shock, meningitis and kidney failure, which can be fatal.

"We estimate that at least one in ten cases may become serious if detected too late or left untreated. It is certainly as dangerous or even more dangerous than dengue, malaria or even Covid-19," Wolf-Peter Schmidt, principal investigator of the study and clinical assistant professor at the LSHTM told DH.

Cases can be treated using the antibiotics doxycycline and azithromycin but there is currently no vaccine to prevent infection.

The study enrolled over 32,279 people in 37 villages in Tamil Nadu where scrub typhus is known to be endemic.

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.

When blood samples taken from people who reported fever were tested for such infection, the researchers found a high incidence of scrub typhus across the two-year study period, with almost 10 per cent of the population infected annually.

“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,” said Schmidt.

“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.”

Most of these infections were asymptomatic, but of those who were infected, between 8-15 per cent developed fever which often required hospitalisation and intensive care due to severe infection. Five people died from scrub typhus-associated complications during the study.

“Karnataka certainly has a high prevalence of scrub typhus. Previously forest areas have been regarded as high risk for scrub typhus. However, in India scrub typhus seems to be most common in regions of intense agriculture such as low land Tamil Nadu, Punjab, Uttar Pradesh, Karnataka and Odisha. This puts hundreds of millions of people at risk,” he said.

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 conducted during the Covid-19 pandemic, where quarantine measures were in place in the area.

The study has appeared in the New England Journal of Medicine.