Cases in Bhutan illustrate the increasing rise of scrub typhus

A zoonotic disease associated with heavy rain, humidity and changing weather patterns, as well as the proliferation of rodents in close proximity to humans is expanding its reach
<p>Women harvesters in a cardamom plantation, in Kerala, South India. A doctor in Nepal noted that people working in cardamom fields complained of scrub typhus symptoms. (Image: Joana Kruse/Alamy)</p>

Women harvesters in a cardamom plantation, in Kerala, South India. A doctor in Nepal noted that people working in cardamom fields complained of scrub typhus symptoms. (Image: Joana Kruse/Alamy)

In October 2022, a 24-year-old patient arrived at Gedu Hospital in Bhutan’s Chukha district. She had been experiencing fever, body ache, cough and shortness of breath for 10 days.

Doctors in the primary health centre initially treated her for the ‘viral fever’ – an umbrella term used to describe a wide range of viral infections and treated symptomatically – but her condition did not improve.

“On examination, we found an eschar [dead tissue] on the left chest of the patient. It had turned into a blackened crust with an erythematous [inflamed] halo that resembles a cigarette burn,” says Tej Nath Nepal, the medical doctor who examined her at Gedu Hospital. “It’s almost a hallmark of chigger [microscopic mite] bites that cause scrub typhus.”

The diagnosis led to a change in her treatment, and she recovered within a week. Nepal and co-author Sonam Drukpa recounted the case in an article published in November 2023 in the journal SAGE Open Medical Case Reports.

Scrub typhus is a zoonotic illness caused by the bacterium Orientia tsutsugamushi, and traditionally restricted to the ‘tsutsugamushi triangle’ – a geographic area in the Asia-Pacific named after the disease. Approximately one million cases occur each year, and over one billion people are at risk of disease.

The scrub typhus pathogen is transmitted through the bite of larval mites of the Trombiculidae family, also called chiggers, which serve as both the vector and the reservoir of the virus. Chiggers often come into contact with humans in humid places like scrub vegetation and cardamom fields, and sometimes through rodents that carry these chiggers as ectoparasites.

Close up of a red chigger mite
A mite of the Trombiculidae family, known as chiggers or harvest mites. In humid and wet parts of south and southeast Asia, the mite can pass scrub typhus-causing bacteria to humans. (Image: Thomas Shahan / iNaturalist, CC-BY-NC)

However, it is relatively new to the mountainous country of Bhutan, where the humid conditions necessary for chigger breeding are largely missing. It was first detected in 2008, when there was a surge of patients with unknown fevers in Gedu – in the low-lying lands on the southern border with India.

According to Nepal, the hospitals initially struggled to diagnose the disease – many of them did not have the appropriate test kits, and the symptoms were similar with that of other viral fevers. They are more accurate now, he says, and Gedu hospital detected 185 cases from January 2017 to December 31, 2020.  

Nepal notes that the patient he treated in 2022 worked in cardamom fields. In fact, a study published in May 2023 identified harvesting cardamom as the major risk factor responsible for scrub typhus cases in Bhutan. Cardamom plantations offer favourable wet and humid conditions for mites responsible for scrub typhus to thrive, the study notes.

Tandin Zangpo, one of the authors of the study and medical epidemiologist at Thimphu-based Khesar Gyalpo University of Medical Sciences of Bhutan, told The Third Pole that a high number of scrub typhus cases have been reported in the past few years from Bongo, Darla and Phuentsholing gewogs (divisions) – areas known to have abundant scrub vegetation and cardamom fields.

Meteorological triggers

There is also a climate connection. A January 2024 study looked at 15 years of weather data in the south Indian state of Tamil Nadu, and found that a 1% rise in mean relative humidity led to a 7.6% increase in monthly scrub typhus cases. Additionally, for every millimetre increase in rainfall, the number of scrub typhus cases rose by 0.5-0.7%.

A 2023 study that analysed the impacts of meteorological factors on scrub typhus occurrence in China between 2006 to 2020, also associated “heavy rainfall to sharp increase of scrub typhus cases”. A first of its kind study published in July 2023, observed that extreme rainfall events were measured in all the country’s weather stations.

“Humid environments are known to be ideal for the reproduction of most chigger mites,” says Nepal, who has treated hundreds of cases of scrub typhus in the last few years from across Bhutan’s Chukha district. “This is why when there is a rise in relative humidity, we often see cases of scrub typhus rise.”

Bhutan has notified scrub typhus as a climate-sensitive disease in 2021 in its report on Assessment of Climate Risks on Health for National Adaptation Plan Formulation Process in Bhutan.

Climate change, mites and rodents

“As meteorological factors play a key role in creating a conducive environment for the chiggers to thrive,” Zangpo notes, “this also means climate change will have significant impacts on scrub typhus outbreaks.”

“If changes in the climate lead to a scenario where the rainfall and the humidity is at a level just ideal for mites to proliferate, we may see more frequent outbreaks,” he says.

Zangpo, however, cautions against generalisation and added that “impacts of microclimatic changes could be far more complex than we may think.”

For instance, while moderate amounts of precipitation may result in favourable humidity for chiggers, heavy precipitation may potentially damage chigger eggs, he adds.

There is another factor, that has more to do with maintenance of the environment and less to do with climate change, and that is the proliferation of rodents in close contact with humans.

The impacts of microclimatic changes could be far more complex than we may think
Tandin Zangpom, epidemiologist

“Synanthropic species [organisms that live in proximity to people and benefit from the environments we create, but are beyond our control] such as rodents and shrews often act as carriers of parasites, helping zoonotic diseases spread to humans,” says Philip Samuel Paulraj. Paulraj is a scientist at the Indian Council of Medical Research-Vector Control Research Centre (ICMR-VCRC) Field Station, Madurai, who studied the role of rodents as hosts of chiggers in the scrub typhus infected areas of Vellore in Tamil Nadu. “In case of scrub typhus, rodents are one of the primary intermediaries putting mites in contact with humans.”

In the aftermath of the deadly Nepal earthquake of 2015, the country witnessed outbreaks of scrub typhus. Epidemiologists pointed out that rodent infestation and increasing human-rodent encounters in temporary shelters after the devastation of the earthquake caused these scrub typhus outbreaks.

“As forests are cleared and agricultural fields expanded, rodents and humans increasingly share the same landscape and come into closer contact. This may have led to the rise in scrub typhus cases in many places,” says Paulraj.

The way forward: One Health

“To prevent zoonotic disease outbreaks,” epidemiologist Zangpo says, “we need an integrated One Health approach.” One Health is a concept that emerged in the 2000s, that uses the close, interdependent links among the health of people, animals and ecosystems to create new surveillance and disease control methods. The World Health Organization (WHO) has recognised One Health as critical to addressing zoonotic public health threats and environmental issues.

“The health of humans, domestic and wild animals, plants, and the wider environment are closely linked and interdependent. It’s like a cobweb where all dots are connected,” says Zangpo.