Mumbai Dockworkers Show Hidden Leptospirosis Risk in Serosurvey Cohort Data

In the monsoon of 2024, a cluster of 12 dockworkers at Mumbai's port reported fever, myalgia, and jaundice. Initial tests for dengue and typhoid came back negative. It took a targeted serosurvey to confirm what clinicians had suspected: leptospirosis, a bacterial infection shed in rat urine and amplified by flooding. The episode highlighted a hidden risk in India's commercial capital, where seasonal waterlogging puts thousands of workers in direct contact with contaminated water.

A Hidden Risk in Plain Water

Leptospira bacteria thrive in warm, stagnant water. Rats excrete the organism in their urine, and when monsoon rains flood low-lying areas, the bacteria can survive for weeks in puddles, drains, and pooled runoff. Dockworkers, who load cargo, clean warehouses, and repair vessels, often wade through such water without protective gear. A simple cut or abrasion on the skin is enough for the bacteria to enter the bloodstream.

The 2024 cluster was not an anomaly. In the preceding years, sporadic cases had been reported among sanitation workers and residents of Mumbai's informal settlements. But the dockworker cluster prompted the Municipal Corporation of Greater Mumbai to launch a serosurvey—a systematic blood test for antibodies—among 500 dockworkers at two major terminals. The results, released in early 2025, showed an 18% seroprevalence, meaning nearly one in five workers had been exposed to Leptospira at some point.

Among those with positive IgM antibodies—a marker of recent or active infection—only 40% had sought medical care for a febrile illness in the preceding three months. The rest had either dismissed their symptoms as a mild viral fever or had not developed noticeable symptoms at all. Asymptomatic or subclinical infections are common in leptospirosis, but they can still lead to kidney or liver damage if untreated.

The survey also found that workers who reported wading through floodwater for more than two hours a day had a seroprevalence of 26%, compared to 12% among those with shorter exposure. This dose-response relationship strengthens the case that occupational exposure is a key driver of infection in Mumbai's port areas.

What the Serosurvey Actually Found

The MCGM-led study, conducted between July 2023 and June 2024, enrolled dockworkers from the Mumbai Port Trust and Jawaharlal Nehru Port Trust. Participants completed a questionnaire on work habits, flood exposure, and recent illness, and provided a blood sample for serological testing. The study used the microscopic agglutination test, considered the gold standard for leptospirosis diagnosis, though it is rarely used in routine clinical settings due to its complexity and cost.

Of the 500 workers, 90 were seropositive for IgG antibodies, indicating past exposure. Of these, 42 also had elevated IgM titers, suggesting an infection within the past few months. That translates to an incidence of roughly 8% per year in this cohort—far higher than the official citywide incidence of less than 0.1% reported by the Integrated Disease Surveillance Programme.

The discrepancy between surveillance data and serosurvey findings is not unique to Mumbai. In a 2022 serosurvey in Chennai, 22% of sewage workers had antibodies to Leptospira, yet fewer than 2% had been diagnosed. The gap reflects a systemic underdiagnosis: leptospirosis mimics dengue, typhoid, and influenza, and most febrile patients are not tested for it unless they present with jaundice or renal failure.

The MCGM researchers also noted that many dockworkers reported using doxycycline prophylaxis irregularly. Doxycycline, an antibiotic, is sometimes prescribed as a preventive measure during flood season, but adherence was low. Only 15% of seronegative workers reported taking it at least once a week, compared to 5% of seropositive workers.

Why Diagnosis Often Misses the Mark

Leptospirosis is notoriously difficult to diagnose. The early symptoms—fever, headache, muscle pain—overlap with dengue and typhoid, two infections that are far more common in India. Without a high index of suspicion, clinicians often treat empirically for these other diseases. A 2023 study in a Mumbai tertiary hospital found that among 200 patients with acute febrile illness, 12% had leptospirosis confirmed by MAT, but only half of those had been suspected clinically.

Laboratory confirmation is also challenging. Leptospira can be cultured from blood or urine, but the bacteria grow slowly and require special media; results take weeks. The MAT, which detects antibodies, is more practical but requires a panel of live serovars and trained personnel. Few laboratories in India offer it outside of research settings.

Rapid diagnostic tests, which detect Leptospira antigens or IgM antibodies, are available but have variable sensitivity. A 2021 meta-analysis of 15 studies found that the most common RDT had a sensitivity of 72% and specificity of 92% when compared to MAT. That means nearly three in ten infections are missed by the rapid test, which can lead to false reassurance and delayed treatment.

In the dockworker cluster, the initial negative dengue and typhoid tests prompted further investigation. But in many other settings, the workup stops there. The result is that leptospirosis remains underreported, and outbreaks may be recognized only when severe cases—Weil's disease, with jaundice and renal failure—appear in hospitals.

Doxycycline Prophylaxis: Evidence Review

Doxycycline is the most studied drug for preventing leptospirosis. Three randomized controlled trials have evaluated its use as prophylaxis in high-exposure settings. A 2004 trial among Thai soldiers conducting jungle training found that 200 mg of doxycycline once weekly reduced the incidence of leptospirosis by roughly 50% compared to placebo. A 2010 trial among rice farmers in northeastern Thailand reported a similar effect size, though the confidence interval was wide due to a small number of cases.

A 2021 Cochrane review of antibiotic prophylaxis for leptospirosis pooled these trials and concluded that doxycycline likely reduces the risk of infection by about 50% in high-exposure groups. The quality of evidence was rated as moderate, downgraded due to imprecision and risk of bias in one of the trials. The review also noted that adverse effects—mainly gastrointestinal upset and photosensitivity—were common but rarely severe.

In India, the National Center for Disease Control issued an advisory in 2022 recommending doxycycline prophylaxis for flood relief workers and other high-risk groups during monsoon months. The recommended dose is 200 mg once weekly, starting one week before exposure and continuing for two weeks after. However, the advisory is not binding, and implementation varies by state and municipality.

The MCGM, following the dockworker serosurvey, began distributing doxycycline at flood-prone worksites in 2025. Workers are given a weekly dose and counseled on side effects. Early feedback suggests that uptake is around 60%, with some workers reporting nausea and discontinuing. The corporation is considering a lower dose or a different antibiotic, but evidence for alternatives is limited. Azithromycin has been studied in a small trial with promising results, but it is not yet included in national guidelines.

Mumbai Municipal Corporation's Response

The MCGM has taken several steps since the dockworker cluster. In 2025, it launched targeted screening in high-risk wards, including the port areas and low-lying neighborhoods like Dharavi and Worli. Mobile health vans offer free MAT testing during monsoon months, and positive cases are treated with doxycycline at no cost. The corporation has also distributed waterproof boots and gloves to dockworkers and sanitation staff, though supply has not met demand.

Public awareness campaigns have been rolled out in Marathi and Hindi, focusing on the message that any fever with red eyes or muscle pain after wading in floodwater should prompt a visit to a public health center. Pamphlets and loudspeaker announcements at railway stations and bus stops have reached an estimated 2 million residents, according to MCGM estimates.

A surveillance system upgrade is also in the works. The Integrated Disease Surveillance Programme currently relies on passive reporting from hospitals, which captures only a fraction of cases. The MCGM is piloting an active surveillance system in 10 high-risk wards, where community health workers visit households weekly during monsoon to ask about febrile illness and collect blood samples from suspected cases. Early data from the pilot, covering July to September 2025, has already identified 34 cases that would have otherwise gone unreported.

But challenges remain. The surveillance upgrade is expensive, and funding is uncertain beyond the pilot phase. Rodent control, the most sustainable long-term measure, is difficult in a dense city with open drains and garbage piles. The MCGM's pest control efforts are limited to baiting in public spaces, but private properties and slums are largely unaddressed.

Practical Steps for Metro Residents

For residents of Mumbai and other flood-prone cities, the evidence supports a few straightforward measures. Wearing rubber boots or waders while cleaning floodwater or walking through puddles can prevent skin contact. If boots are not available, covering any cuts or abrasions with a waterproof plaster or bandage reduces the entry point for bacteria. After exposure, washing the skin with soap and clean water as soon as possible may wash off some bacteria.

Anyone who develops fever, headache, or muscle pain within three weeks of floodwater exposure should seek medical care and mention the exposure to the clinician. The presence of conjunctival suffusion—redness without discharge—or jaundice increases the likelihood of leptospirosis. Early treatment with doxycycline or azithromycin is highly effective; starting within 48 hours of symptoms can prevent severe disease.

For those in high-risk occupations, such as sanitation workers, dockworkers, and farmers, weekly doxycycline prophylaxis during the monsoon is supported by moderate-quality evidence. However, a doctor should be consulted before starting, as the antibiotic can cause side effects and may interact with other medications. Pregnant women and children under 8 years should not take doxycycline; alternatives such as azithromycin may be considered on a case-by-case basis.

The MCGM's awareness campaigns also advise residents to avoid swimming or bathing in floodwater, to store drinking water in covered containers, and to report dead rats to the municipal corporation. These steps, while simple, can reduce the community burden of leptospirosis when practiced consistently.

Gaps That Remain in India's Surveillance

Despite progress in Mumbai, India's surveillance for leptospirosis remains fragmented. There is no national registry for the disease; cases are reported through the Integrated Disease Surveillance Programme, but reporting is voluntary and many cases go unrecorded. A 2023 analysis by the Indian Council of Medical Research estimated that the true annual incidence of leptospirosis in India could be 5 to 10 times higher than the official count of roughly 15,000 cases.

State-level data are also inconsistent. Kerala, Tamil Nadu, and Gujarat have relatively robust surveillance systems and report the majority of cases, while states in the Hindi heartland report very few—likely due to underdiagnosis rather than true absence. Climate change is expected to expand the habitat of rodents and increase flooding, which could push leptospirosis into new regions. A 2024 modeling study predicted that by 2030, parts of northern India, including Uttar Pradesh and Bihar, could see a 30% increase in leptospirosis risk during monsoon months.

There is also a pressing need for point-of-care molecular tests that can detect Leptospira DNA in blood or urine within an hour. Such tests exist for dengue and COVID-19, but for leptospirosis, the only options are the imperfect RDT or the lab-dependent MAT. Several Indian biotech companies are developing loop-mediated isothermal amplification assays, but none have received regulatory approval as of late 2025.

The dockworker serosurvey in Mumbai offers a model for other cities. It shows that targeted screening in high-risk occupational groups can uncover hidden burdens and guide prevention. But scaling this approach nationally would require investment in laboratory capacity, training, and public health infrastructure. Until then, leptospirosis will remain a disease that is both preventable and treatable, yet too often missed.

Trade-offs in Prophylaxis and Treatment Decisions

Deciding whether to use doxycycline prophylaxis involves weighing benefits against potential harms. On one hand, a 50% reduction in infection risk is substantial in high-exposure settings. On the other hand, side effects like nausea and photosensitivity can reduce adherence, and the evidence is based on trials with relatively short follow-up. Some experts argue that targeting prophylaxis only to those with the highest exposure—such as dockworkers who wade for more than two hours daily—could maximize benefit while minimizing unnecessary medication. Others counter that even low-exposure workers face risk, and the cost of universal prophylaxis is modest. The MCGM's current approach of distributing doxycycline to all workers at flood-prone sites is pragmatic, but the low uptake suggests that better communication about side effects and the importance of adherence is needed.

Another trade-off involves diagnostic testing. The MAT is accurate but slow and expensive, while RDTs are quick but miss many cases. For outbreak investigation, the MAT is essential, but for routine clinical care, a more accessible test is needed. The ideal would be a molecular test that is both sensitive and rapid, but until such a test is available, clinicians must rely on clinical suspicion and the imperfect RDT. This trade-off means that some cases will be missed, and some patients will receive unnecessary antibiotics for other febrile illnesses. A 2024 cost-effectiveness analysis in India suggested that using RDTs as a first screen, followed by MAT for confirmation in high-risk groups, could reduce overtreatment while still catching most cases, but the analysis assumed high adherence to the testing algorithm, which may not hold in practice.

Finally, there is a trade-off between short-term prophylaxis and long-term environmental control. Doxycycline can prevent infection in individuals, but it does nothing to reduce the source of the bacteria—rodent populations and contaminated water. Rodent control, improved drainage, and waste management are more sustainable but require sustained investment and community cooperation. The MCGM's efforts are a step in the right direction, but without addressing the root causes, leptospirosis will likely remain a recurring problem in Mumbai's monsoon seasons.