Tamil Nadu study links vector-borne scrub typhus cases with high rainfall, humidity

Every millimetre increase in rainfall could lead to a 0.5 to 0.7 per cent rise in monthly scrub typhus cases, according to a new study conducted in Tamil Nadu’s Vellore. The researchers looked at the association of meteorological factors like temperature, rainfall and humidity with scrub typhus using 15-year scrub typhus data from a tertiary care hospital in Vellore.

Monthly scrub typhus cases increased by 7.6 per cent for every per cent jump in mean relative humidity, the study published in journal Scientific Reports noted.

Read more: Scrub typhus in Odisha: Changing climate may have a role behind outbreak, says expert

Scrub typhus is an infectious disease caused by bacteria called Orientia tsutsugamushi and transmitted through infected mites. The symptoms typically include fever, headache, body ache and sometimes a rash. In severe cases, the infection can lead to respiratory distress, brain and lung inflammation, kidney failure and multi-organ failure, ultimately resulting in death.

Several factors, like vector abundance, climatic factors, exposures like farming and owning domestic animals, outdoor activities and sanitation, affect its prevalence. The disease tends to be seasonal in the endemic areas. “It has been noted that scrub typhus incidence is associated with relative humidity, temperature and rainfall,” read the paper.

The researchers assessed how meteorological factors like temperature, rainfall and humidity, along with socioeconomic factors, impact the prevalence of scrub typhus. They gathered 15 years of data from a tertiary care hospital.

Some 11,001 people were tested for scrub typhus during this period. Of them, 2,784 tested positive, taking the overall positivity rate to 25.3 per cent. The study authors noted most cases occurred from August to February, with cases peaking in October and November.

This agrees with previous data that found the disease is more prevalent in cooler months. For example, scrub typhus peaks in September and October in Jiangxi Province, China, June in Vientiane, Laos and June to November in Taiwan.

Read more: Why is Himachal seeing a spike in scrub typhus? Its weather trajectory holds the answer

In Northeast India, cases of scrub typhus occur from July to November in Manipur, July to October in Sikkim and September to November in Darjeeling, the study highlighted.

The researchers calculated that monthly cases decreased by 18.8 per cent for every degree Celsius increase in temperature. This is because mites prefer to lay eggs in a temperature range of 20°C-30°C.

“The maximum number of cases occurred in months with a mean temperature of 26.8°C, implying that a lower temperature is more favourable for scrub typhus to occur in Vellore, as the summer mean temperature is 31°C,” the paper read.

An increase in rainfall and humidity, on the other hand, led to a spike in cases, the study showed.

“This study suggests that healthcare professionals should be prepared for early diagnosis and treatment of scrub typhus during the rainy season and cooler months with increased humidity,” the researchers wrote in their study.

Read more: Fatal fever spreads to cities

Also, the data showed that the diseases were more prevalent in people in the third decade of life, meaning they were more common in those older than 60 years, with a prevalence rate of 21.1 per cent.

As for socioeconomic status, farmers, daily wage labourers, and housewives were more likely to contract the disease. 

Previous studies from Tamil Nadu and Darjeeling have shown that agricultural work is a risk factor for acquiring scrub typhus. Similar findings have been reported by researchers in Bhutan, China, Nepal, Thailand, South Korea and Japan.

Humans pick up the infection when they walk, sit, or lie down on the ground infested by mites.