Leptospirosis, a serious bacterial infection spread through animal urine in contaminated floodwater, often goes undiagnosed during monsoon seasons despite causing an estimated 59,000 global deaths annually, and while dengue and cholera dominate public health attention, leptospirosis cases, particularly prevalent in India during monsoon and post-monsoon months, can resemble common fevers, making early detection challenging, though it can lead to severe complications like kidney injury and liver damage, and with climate change increasing monsoon volatility, there's an urgent need to integrate climate forecasting with disease surveillance, urban planning, and community health education.

Leptospirosis, a serious bacterial infection spread through animal urine in contaminated floodwater, often goes undiagnosed during monsoon seasons despite causing an estimated 59,000 global deaths annually, and while dengue and cholera dominate public health attention, leptospirosis cases, particularly prevalent in India during monsoon and post-monsoon months, can resemble common fevers, making early detection challenging, though it can lead to severe complications like kidney injury and liver damage, and with climate change increasing monsoon volatility, there's an urgent need to integrate climate forecasting with disease surveillance, urban planning, and community health education.

Leptospirosis, a serious bacterial infection spread through animal urine in contaminated floodwater, often goes undiagnosed during monsoon seasons despite causing an estimated 59,000 global deaths annually, and while dengue and cholera dominate public health attention, leptospirosis cases, particularly prevalent in India during monsoon and post-monsoon months, can resemble common fevers, making early detection challenging, though it can lead to severe complications like kidney injury and liver damage, and with climate change increasing monsoon volatility, there's an urgent need to integrate climate forecasting with disease surveillance, urban planning, and community health education.

Monsoon brings a spike in waterborne diseases like dengue and cholera, but leptospirosis, spread through contaminated floodwater, often goes undiagnosed until complications develop.

Globally, a person dies of leptospirosis roughly every nine minutes. According to data, an estimated 59,000 deaths occur per year from the disease. It spreads through something as ordinary as walking through a waterlogged street, wading past a stalled car, or stepping into a flooded basement to salvage belongings. 

Most people who do this every monsoon have never heard its name. Yet during the monsoon, when the infection spreads the fastest, it barely gets a mention next to dengue and cholera, the two diseases that dominate headlines, hospital advisories, and public health campaigns every year when the rains arrive. 

Leptospirosis is a bacterial infection that spreads through floodwater contaminated with animal urine. A 2022 review puts the annual global case count at about one million, with a case fatality rate of 6.85%. The infection is caused by Leptospira, a spiral-shaped bacterium shed in the urine of rats and other mammals. 

"The rat is by far the most important carrier responsible for human leptospirosis" because rats live close to humans and continue to shed the bacteria in their urine for months after infection,” the review noted.

Humans usually get infected when broken skin or mucous membranes come into contact with water or soil contaminated with infected urine. This is precisely why monsoon floods create ideal conditions for outbreaks. 

How floods turn a fever into an outbreak 

A 2025 review published in Clinical Infection in Practice explained that leptospiral pathogens thrive in stagnant floodwater containing organic matter and can survive outside a host for weeks under these conditions. The same review pointed to state-wide surveillance from Maharashtra and Tamil Nadu, which found that over 85% of annual leptospirosis cases in India cluster within the monsoon and immediate post-monsoon months.

Mumbai has already lived through this. In 2005, the city recorded around 1,800 leptospirosis cases after a single day of 944 mm rainfall caused widespread waterlogging, with a case fatality rate of nearly 10%, according to data compiled in the 2025 review. 

Separate hospital data from Mumbai's municipal health department shows leptospirosis cases rose from 218 in 2018 to 281 in 2019, before dropping slightly to 240 in 2020, a period when citywide lockdowns reduced people's exposure to contaminated water, according to a 2021 study published in the Journal of Family Medicine and Primary Care.

The bigger problem is not just how the disease spreads. It is how often it goes unrecognised.

A fever that looks like everything else 

Leptospirosis begins like most monsoon fevers: high temperature, headache, muscle pain, chills. The 2022 review stated that "the clinical features of leptospirosis are similar to those seen in many other febrile illnesses" common in the tropics, including dengue, other haemorrhagic fevers, and bacterial sepsis. Around 90% of patients recover with a mild, self-limiting illness, meaning the infection often clears on its own and never gets tested or confirmed. It is the remaining 10% who go on to develop serious complications such as kidney injury, liver damage or pulmonary haemorrhage that make delayed diagnosis dangerous.

Diagnostic tools do exist. Blood culture, considered the gold standard, requires samples collected in the first week of illness and takes time and specialised laboratory conditions to process. Polymerase chain reaction (PCR) testing offers high sensitivity in the early stages of infection. Antibody-based tests, which are more widely available, only become reliable six to ten days after symptoms begin and can cross-react with other infections, which affects their accuracy. The 2022 review makes clear that in resource-poor settings, where the disease is most common, access to these tests is limited.

There is no vaccine for humans. The main line of defence is avoiding exposure to floodwater, and doxycycline is taken as a preventive antibiotic for those at high risk, such as sanitation workers, farmers, and people wading through contaminated water during floods.

The 2025 review noted that monsoon patterns showed "increasing monsoon volatility, including delayed onset, shortened duration, and extreme precipitation events" due to climate change. This shift was expected to intensify disease transmission by increasing vector breeding opportunities and expanding the period during which people were exposed to contaminated water. The review recommended combining climate forecasting with disease surveillance, urban planning, and community health education as a public health priority. 

How doctors spot it

Dr Aishwarya R., a consultant in infectious diseases at Aster Whitefield Hospital, said that diagnosis always began with a detailed patient history.

"The very first step is getting a detailed history, especially checking if the patient had recent contact with floodwater, stagnant water, sewage, or places where animal urine was present," she said.

“Leptospirosis starts to feel more plausible if fever shows up about 2 to 14 days after that kind of exposure,” she added. “Then there are clinical assessment and baseline blood tests; these can also offer key hints.” 

She further stated, “Dengue commonly links with low platelet counts, and malaria is usually confirmed using blood smears or rapid diagnostic tests, but leptospirosis can come with liver dysfunction, kidney impairment, or raised inflammatory markers.” 

Certain symptoms stood out more than others, she said. Intense pain in the calf muscles and lower back was "somewhat more typical for leptospirosis than for many other illnesses." Eye redness without much discharge was another sign, one less common in dengue or malaria.

“When the infection moves forward, some people can get jaundice, dark urine, lower urine output, breathlessness, bleeding issues, or confusion.”

Dr Aishwarya said leptospirosis had no symptoms unique to it in the early days. Patients are frequently treated as having generic viral fever. Many are tested only for dengue or malaria. "A lot of individuals simply don't bring up recent contact with floodwater unless someone specifically asks," she said.

Testing was complicated by timing, she said. "How well lab tests work depends on where the illness is in its timeline, or stage." Antibody tests were unreliable in the first week. Early PCR testing could still miss the infection. 

"If a clinician runs a test too early, you could easily end up with a false-negative result, even when disease is present," she said. Because of this, doctors often started antibiotics based on clinical suspicion and exposure history, rather than waiting for lab confirmation. This lowered the risk of severe complications while test results were awaited, she added.

This story is done in collaboration with First Check, which is the health journalism vertical of DataLEADS

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