Nipah in India: Why Kerala and West Bengal face the same virus but different outbreak risks

“To understand Nipah virus in India, it is important to look beyond virology and consider ecological factors and the strength of health systems in shaping outbreak outcomes,” said Dr Pragya Yadav, corresponding author of the report and Director-in-Charge of the National Institute of One Health, Nagpur.

According to Dr Yadav, Kerala experiences more frequent spillover events, but outbreaks are usually contained to single cases or small clusters because of strong surveillance and a One Health approach. In contrast, West Bengal has seen fewer spillovers, but these have historically been associated with healthcare-related transmission. Enhanced surveillance was critical in detecting the recent 2026 outbreak.

“The examples of Kerala and West Bengal show that the same virus, maintained in a common reservoir, can produce very different epidemiological outcomes depending on local ecological conditions, human behaviour and health-system preparedness,” authors Deepak Y. Patil, Rima R. Sahay, Sreelekshmy Mohandas and Dr Yadav have written in the report.

Recurrent Nipah spillovers in India

Nipah, a highly pathogenic virus carried primarily by Pteropus fruit bats, was first identified during a large outbreak in Malaysia in 1998-99. Since then, repeated spillover events have occurred across South and Southeast Asia, often with high fatality rates and no specific treatment available.

Although the bat reservoir is widely distributed across India, recurring human infections have been reported mainly in West Bengal and Kerala. These states differ markedly in ecology, population density, healthcare capacity and human-animal interactions, yet both continue to experience Nipah spillovers.

West Bengal reported outbreaks in 2001 and 2007, both epidemiologically linked to Bangladesh, and again in 2026. Kerala has recorded recurring spillovers since 2018, with outbreaks in 2018, 2019, 2021, 2023, 2024 and 2025. Most have been limited to isolated cases or small clusters that were rapidly contained.

How surveillance changed the response

India’s first Nipah outbreak occurred in Siliguri, West Bengal, in 2001 and remains a classic example of hospital-acquired transmission. The outbreak exposed major gaps in infection prevention, including limited awareness of the disease, inadequate use of personal protective equipment (PPE) and poor infection-control practices.

The outbreak resulted in 66 cases with a case fatality rate of 68.2%. Nearly 75% of infections occurred among healthcare workers and hospital visitors.

A smaller outbreak in Nadia district in 2007 caused five deaths and was linked to consumption of contaminated palm-derived products.

Before the 2026 outbreak, West Bengal had already established proactive surveillance for Acute Encephalitis Syndrome (AES) and Acute Respiratory Illness (ARI)/ARDS through a network of 10 Viral Research and Diagnostic Laboratories (VRDLs). According to the report, this surveillance system enabled rapid identification of the two infected healthcare workers — a significant improvement over the reactive response seen during the 2001 outbreak.

Following confirmation, authorities deployed a Mobile BSL-3 laboratory and conducted extensive contact tracing. All 196 identified contacts tested negative, helping prevent further spread.

Kerala, meanwhile, has invested heavily in Nipah preparedness since its first outbreak in 2018. Routine triage systems, rapid isolation protocols, intensive healthcare worker training on PPE use, and enhanced hospital surveillance during the COVID-19 pandemic strengthened infection-control practices.

“These measures have increased the capacity of healthcare facilities to identify suspected cases quickly, reduce exposure and prevent further transmission within hospitals,” the report noted.

As a result, apart from the large 2018 outbreak and the 2023 cluster, Kerala has reported virtually no hospital-acquired transmission during subsequent outbreaks. Following the 2024 spillovers, a 17-laboratory Virus Diagnostic and Research Laboratory network was established across Kerala and West Bengal for SARI/ARI surveillance, further strengthening early detection and outbreak containment.

Why ecology shapes outbreaks

Scientists say spillover events are strongly influenced by ecological and environmental factors. Human-driven changes such as deforestation, habitat loss and increasing intrusion into biodiversity-rich areas are major drivers of zoonotic outbreaks. These changes affect bat behaviour and increase opportunities for contact between bats and humans.

In Kerala, Nipah outbreaks have consistently occurred between April and September, when hot, humid conditions coincide with fruiting seasons. During this period, bats frequently feed near human settlements, increasing the likelihood of contamination of fruits and surfaces. Bat colonies are also commonly found within residential compounds across Kerala, creating opportunities for direct and indirect human exposure.

West Bengal presents a different risk profile. Bat roosts are typically located in peri-urban areas, and spillovers are more closely linked to the harvesting and consumption of fresh or fermented date palm sap. The risk peaks during the sap-harvesting season between November and April. The report cautions that attempts to reduce perceived risk by disturbing bat populations or removing roosting trees can sometimes worsen the problem by dispersing bats and expanding the geographic range of exposure.

These patterns suggest that Nipah spillovers are not random events but are driven by predictable ecological and human factors.

Why surveillance must move beyond outbreak response

The report argues that preparedness must extend beyond reacting to outbreaks after they occur. Continuous monitoring of both human populations and animal reservoirs can provide early warning signals. Surveillance for symptoms such as respiratory distress and encephalitis can help healthcare facilities identify suspected cases quickly, enabling timely isolation and rapid containment.

The authors also recommend targeted public-awareness campaigns in high-risk regions. In Kerala, messaging should focus on avoiding fallen fruits, especially those with bite marks or signs of partial consumption by animals.

In West Bengal’s date palm sap-producing districts, public health efforts should prioritise covering sap-collection pots with bamboo skirts — a low-cost intervention proven effective in Bangladesh — and discouraging the consumption of raw sap during winter months, when viral shedding by bats is highest.

While spillovers may not always be preventable, their consequences can be substantially reduced through strong surveillance, ecological awareness and public health preparedness.