What is actually in floodwater and why are doctors worried about it

What is actually in floodwater and why are doctors worried about it Legacy Thailand’s floods are back. Across the North, Northeast and parts of Central Thailand, floodwater is filling streets, homes and agricultural land. As of late September 2026, flash flood warnings were covering 24 provinces , and the images filling social media show people wading, swimming and, in some cases, jet-skiing through contaminated water without a second thought.

What is actually in that water is worth understanding before you step into it.

Assoc. Prof. Dr Maria Nina Chitasombat, Infectious Disease Specialist at MedPark Hospital’s Infectious Diseases Center , explains what the main risks are, who is most vulnerable and what symptoms should send someone straight to hospital rather than a pharmacy.

What is actually in floodwater

When a city floods, the water is not simply rainwater. It is a mixture of municipal sewage overflow, agricultural runoff, animal waste and whatever the storm drains were carrying before they backed up. Rodent urine, in particular, gets washed from burrows and gutters directly into the street water.

The result is a breeding environment for several categories of infection. Waterborne bacterial infections, including leptospirosis and melioidosis (soil-borne), enter through cuts or mucous membranes. Skin and soft tissue infections develop when bacteria like Staphylococcus or Streptococcus find their way into scratches or open wounds.

Fungal conditions, commonly called athlete’s foot or Hong Kong foot, follow from prolonged skin immersion. And as floodwater recedes and stagnates, mosquito populations spike, raising the risk of dengue fever in the weeks that follow.

What leptospirosis is and how people get it

Leptospirosis is a zoonotic disease caused by the bacterium Leptospira, found in the urine of infected animals, most commonly rats and dogs.

The animals themselves carry it asymptomatically; they shed the bacteria without getting sick. Floods wash that urine into street-level water, where the pathogen can survive in warm, wet conditions for weeks.

“When the city floods, this bacteria is washed directly into the street water, which is a toxic mix of overflowing municipal sewage,” Dr Chitasombat explains. “When people wade into the water, the bacteria can enter the body through skin cuts and scratches, or through normal skin that has been exposed to water for over three hours.”

Leptospirosis | Photo from Community Animal Hospitals Splashing contaminated water into the eyes, nose or mouth is another route of entry. The three-hour threshold for intact skin matters: waterlogged skin softens and begins to macerate, losing its normal barrier function and allowing bacteria to penetrate even without a visible wound.

Leptospirosis is most commonly seen in farmers doing rice cultivation in the North and Northeast of Thailand, where feet are soaked in flooded paddy fields for hours at a time. Urban flooding brings the same exposure into city streets.

Who is most at risk

Not everyone who wades through floodwater will develop an infection, and severe disease affects a minority of those who do. But certain groups carry significantly higher risk.

Older adults and anyone with a weakened immune system, including people with diabetes, liver disease, kidney disease or those on immunosuppressant medications, should avoid floodwater entirely if possible. Anyone with open cuts, scratches, eczema or athlete’s foot already has a compromised skin barrier that gives bacteria an easy entry point.

Rescue workers, sanitation crews and street vendors who spend extended time in the water without protective gear are also in an elevated risk category.

Early symptoms and what to watch for

The early presentation of leptospirosis can look like influenza or dengue: sudden onset of high fever, muscle aches and severe headache. Two features point more specifically toward leptospirosis. The first is intense calf pain, which tends to be more prominent than general muscle aches.

Photo by shayneppl from Getty Images The second is conjunctival suffusion: the whites of the eyes become deeply red and swollen, without the discharge associated with a typical eye infection.

If the infection is not caught early, it can progress in the second week to Weil’s Disease, the severe form of leptospirosis. This involves jaundice as liver function deteriorates, dark brown urine and reduced output as the kidneys fail, coughing up blood from pulmonary haemorrhage, and low blood pressure that can lead to shock or neurological complications such as meningitis . These outcomes affect a minority of leptospirosis patients, but they are life-threatening when they occur.

The diagnostic challenge

In the first three to five days of illness, leptospirosis, dengue and murine typhus, a flea-borne disease associated with rats, can present almost identically. All three cause sudden high fever, severe headache and generalised muscle pain.

Blood count results help differentiate them. Leptospirosis tends to produce an elevated white blood cell count with high neutrophils, pointing toward a bacterial infection. Dengue typically causes leukopenia, a low white blood cell count, along with low platelet counts. A battery of additional immunology and rapid tests can be used to narrow the diagnosis further, but a doctor needs to select the right tests for each patient’s specific picture.

Photo by someone25 from Getty Images Other flood-related infections to know about

Dengue is rising across Thailand this year , with 21,620 cases and 31 deaths recorded as of late August 2026. Flooding makes this worse: rushing water initially disrupts existing mosquito breeding grounds, but later, clean stagnant water creates ideal conditions for the Aedes mosquito to multiply, which can also bring Chikungunya and Zika, which follow similar patterns.

Gastrointestinal infections follow if contaminated water is ingested, either directly or via unwashed hands. Respiratory infections, including influenza and Covid-19 spike when people are crowded into shelters or relief centres. Fungal foot infections are the most common skin complaint; they open wounds that then become entry points for the bacterial infections described above.

Melioidosis, a serious bacterial infection from contact with flooded soil and mud in agricultural areas, is a further concern, particularly in rural settings.

What to do if you cannot avoid exposure

Waterproof boots that reach above the knee, secured at the top to prevent water from spilling inside, are the most important protection. Any existing cuts, scrapes or skin conditions should be sealed with waterproof dressing before entering the water. Time in the water matters: the longer the exposure, the greater the risk as skin softens.

When you exit, wash the legs and feet thoroughly with soap and clean water. Dry between the toes carefully to prevent fungal infection. Any new cuts sustained in the water should be flushed immediately with clean bottled water, treated with an antiseptic such as iodine and covered before returning to the water. All boots, gloves and protective clothing should be disinfected with bleach after use and kept out of clean living areas.

Photo by Karola G from Pexels Prophylactic doxycycline is sometimes discussed as a preventative measure for high-risk workers. Dr Chitasombat cautions that it is not routinely recommended and should only be used under medical supervision: it carries side effects, cannot be used in patients with liver disease, children or pregnant women, and does not eliminate leptospirosis risk, though it may reduce severity in some cases.

When to go straight to hospital

Several situations require immediate medical attention rather than a visit to a pharmacy.

Any puncture wound or sharp object injury sustained in floodwater warrants a hospital visit for appropriate wound dressing, a tetanus shot and antibiotics. A wound that starts as a cut but rapidly turns red across a widening area, develops painful blisters, begins tracking up the limb or produces foul-smelling discharge is developing into a serious soft tissue infection that requires intravenous antibiotics.

Go to hospital immediately for: sudden onset of high fever with severe calf pain; yellowing of the skin or whites of the eyes; dark or reduced urine output; coughing up blood; extreme dizziness, confusion or inability to stand; or a significant drop in blood pressure. These are signs of serious illness that cannot be managed at home.

No vaccine, but prevention is possible

There is currently no human leptospirosis vaccine commercially available or approved for use in Thailand. Vaccines are available for dogs and livestock, which can reduce one source of transmission, but people cannot be vaccinated directly.

Prevention depends on the measures above: barrier protection, minimising time in floodwater, thorough cleaning after exposure and prompt medical attention when symptoms appear.

Assoc. Prof. Dr Chitasombat sees patients at MedPark Hospital’s Infectious Diseases Center, 8th Floor, Counter C, and can be reached at 02-090-3129 . Appointments with Dr Chitasombat are available Tuesday to Saturday.

This article is based on an interview with Assoc. Prof. Dr Maria Nina Chitasombat, Infectious Disease Specialist, MedPark Hospital. It is for informational purposes only and does not replace individual medical advice from a qualified doctor.