Flooding increases leptospirosis risk across Thailand as cases exceed 3,300

Thailand’s health authorities are warning of increased leptospirosis risk as widespread flooding and waterlogged conditions expose people to contaminated water, soil and mud. By 23 September 2026, the country had recorded 3,303 cases and 46 deaths, corresponding to a case-fatality rate of 1.39%.

The Department of Disease Control reports that weekly case numbers have remained mostly above the five-year median since early July, with continued deaths in several areas. The highest incidence rates have been reported in Ranong, Nan, Trang, Mae Hong Son and Nakhon Si Thammarat.

People aged 60 years and older accounted for the highest number of cases, followed by those aged 50–59 and 40–49. Agricultural workers were the most affected occupational group because of their frequent contact with wet soil and standing water.

A zoonosis amplified by floodwater exposure

Leptospirosis is caused by Leptospira bacteria and is closely linked to animal reservoirs. The organisms can be shed in the urine of rats, dogs, cattle, buffaloes, pigs, goats and sheep, contaminating water and soil.

Humans can become infected when contaminated water, mud or soil comes into contact with cuts, water-softened skin or the mucous membranes of the eyes, nose or mouth. Flooding therefore creates particularly favourable conditions for exposure, especially during wading, clean-up work and prolonged contact with standing water.

End of the rainy season remains a high-risk period

Thai authorities are particularly concerned about the transition from the rainy season into the cooler months, when flooded ground can remain wet for prolonged periods.

Typical symptoms include high fever, headache, muscle pain (especially in the calves or thighs) nausea, vomiting, diarrhoea and red eyes. Health officials are urging anyone who develops these signs after floodwater exposure to seek medical attention quickly and report the exposure history. Delayed treatment was a common feature among fatal cases.

A clear One Health signal

The current situation illustrates the close interaction between animal reservoirs, environmental contamination and human exposure. Flooding increases the likelihood that bacteria shed by infected animals are dispersed through water and mud and brought into contact with people.

For prevention, authorities recommend avoiding floodwater where possible, wearing boots or other barriers when contact is unavoidable, washing thoroughly after exposure and reducing rodent populations around homes and farms.