Nipah Virus: When a Bat Virus Reaches the Brain

Some infectious diseases demonstrate the importance of One Health more clearly than any policy paper can, and Nipah virus is one of them. Its natural reservoir is a fruit bat; pigs have acted as amplifying hosts; human behaviour and food production create the opportunities for spillover; and once infection reaches people, transmission can continue from one person to the next. In its most severe form the virus attacks the nervous system, producing encephalitis, seizures, altered consciousness and, frequently, death.

Nipah is therefore not simply an infectious disease problem. It is simultaneously an animal health, human health, neurological, agricultural, environmental and biosecurity problem, and it cannot be understood properly from inside any one of those disciplines. That is precisely why it deserves attention, and why it makes such a useful case for thinking about how preparedness is organised.

From fruit bats to people

Nipah virus is a henipavirus in the Paramyxoviridae family, and fruit bats of the Pteropodidae family — often called flying foxes — are its natural host.¹ The bats themselves generally do not appear to become ill from infection, which is part of what makes the reservoir so difficult to manage: there is no sick animal to find.

The virus first came to international attention during an outbreak among pig farmers in Malaysia in 1998–99, and the epidemiological chain was striking.¹ Fruit bats were implicated in introducing the virus into pig populations, infection then spread efficiently through intensive pig production, and people working closely with infected pigs became infected in turn. The outbreak produced 265 cases of viral encephalitis and 109 deaths, and the control response destroyed roughly 1.1 million pigs — close to 40 per cent of the national herd.² WOAH, which lists Nipah as a notifiable animal disease, records that over a million pigs were destroyed to bring the outbreak under control.³

This was not an outbreak in humans that happened to involve animals. It was a disease event occurring across an interconnected system of wildlife, agriculture, livestock movement, occupational exposure and human health, and the agricultural structure was part of the cause rather than merely the setting. FAO notes that deforestation to support intensified pig farming, combined with fruit trees planted within pig farms, brought fruit bats into close proximity with pigs and facilitated the spillover.⁴

Not one route, but several

Subsequent outbreaks have shown that pigs are not required. In Bangladesh and India, transmission has frequently occurred directly or indirectly from bats to people, including through food contaminated with bat saliva, urine or excreta, and raw date palm sap has been a recognised and recurring route.¹ Nipah can also spread directly between people, particularly after close contact with an infected patient, which is how a food-borne introduction becomes a hospital problem.

That matters for prevention, because each route implies a different intervention and a different profession. A pig-mediated outbreak is addressed through farm siting, biosecurity and veterinary surveillance; a sap-mediated outbreak is addressed through food practice and simple physical barriers on collection vessels; a hospital cluster is addressed through infection prevention and control. A country that prepares for only one of these is not prepared.

When infection becomes neurological disease

Nipah infection can produce fever and respiratory illness, but it is the capacity to affect the brain that makes it so serious. Severe cases progress to encephalitis — inflammation of the brain — and patients develop headache, confusion, drowsiness and other neurological abnormalities that can deteriorate rapidly into seizures and profound impairment of consciousness.¹

The consequences are severe: WHO estimates the case-fatality rate at 40 to 75 per cent, although mortality varies considerably between outbreaks and with the availability of intensive clinical care, and there are currently no approved drugs or vaccines, although candidate countermeasures are in development.¹ Survival does not necessarily mark the end of the disease either: WHO reports long-term neurological conditions in approximately one in five people who recover.¹

That last figure raises a question which matters well beyond Nipah. Why do some patients recover neurologically while others are left with persistent impairment? The answer is unlikely to depend only on the presence of the virus, and probably involves the interaction between viral invasion, host immunity, inflammation, vascular injury, the blood–brain barrier and the neurological response to infection. That places Nipah squarely within the emerging intersection of infectious disease and neuroimmunology.

The unanswered neuroimmunology question

The immune system must respond to infection, but within the central nervous system that response has to be tightly controlled. An insufficient response may allow viral replication and dissemination, while an excessive or poorly regulated inflammatory response may itself contribute to tissue damage, and understanding where the balance lies is increasingly important across neuroinfectious disease generally.

For Nipah this produces questions that extend well beyond whether a patient tests positive. Why does one infected individual develop relatively limited disease while another develops catastrophic encephalitis? Can patterns in inflammatory or immunological markers predict neurological deterioration before it becomes clinically obvious? Are there identifiable immune profiles associated with survival, or with neurological recovery rather than merely survival? And could combinations of routine clinical data, virology, immunological measurement and neurological observation allow dangerous trajectories to be recognised earlier than they are now?

These are not only laboratory questions; they are increasingly health-data questions as well. Answering them requires longitudinal records that follow the same patients across virology, immunology, imaging and neurological assessment, held in a form where those observations can actually be related to one another. Nipah outbreaks are small, geographically scattered and separated by years, so the evidence that would answer these questions is distributed across institutions and countries by default — which is a problem of data architecture as much as of science.

Two geographies, one virus

The most instructive recent work on Nipah is not virological at all. In May 2026, researchers at India’s National Institute of Virology published a comparison of the two Indian states where human cases keep appearing, and their conclusion is uncomfortable in a productive way.⁵

Although the bat reservoir is widely distributed across India, human cases have been reported repeatedly in only two states: West Bengal in the east and Kerala on the south-west coast. The two are very different in ecology, demography, health-system capacity and the nature of the human–environment interface, yet both see recurrent spillover. West Bengal’s experience has been shaped by healthcare-associated transmission: the Siliguri outbreak of 2001 produced 66 cases with a case-fatality rate of 68.18 per cent, and around 75 per cent of those infections were in healthcare workers and hospital visitors. A smaller outbreak in Nadia in 2007 caused five deaths and was associated with consumption of contaminated palm-derived products. In January 2026, two healthcare workers at a private hospital in Barasat were confirmed infected; the probable primary case was a 55-year-old woman who had drunk raw date palm sap bought from a local vendor and who died of acute respiratory distress syndrome and multi-organ failure, and both healthcare workers had direct, unprotected contact with her — one during routine nursing care, the other during cardiopulmonary resuscitation.⁵

Kerala’s pattern has been markedly different, in that the state has seen recurring spillover since 2018, with events in 2019, 2021, 2023, 2024 and 2025, and these have frequently been limited to single cases or small clusters brought quickly under control. The authors argue that recurrent spillover in India is best understood not as a uniform virological phenomenon but as the outcome of distinct ecological interfaces interacting with reservoir dynamics, human exposure patterns and health-system preparedness. Their central finding is worth stating plainly: the same virus generates entirely different epidemiological narratives depending on the system it lands in, and large-scale transmission can often be prevented.⁵

That is a One Health argument supported by evidence rather than assertion. The difference between a single case and a hospital outbreak was not the pathogen. It was surveillance, clinical suspicion, infection control and the speed of the response around it.

A disease of systems, not species

Perhaps the most important lesson from Nipah is that focusing on the infected human arrives too late in the chain, because the opportunity for prevention usually exists much earlier. Fruit bats forage across large landscapes, and agricultural expansion, changes in land use, intensive livestock production and food-production practices all alter the points at which wildlife, farm animals and people come into contact. FAO identifies land-use change, agricultural intensification, habitat disruption and food-production practices as factors capable of increasing the opportunities for spillover.⁴

None of that means bats are the problem, and the distinction is not a sentimental one. Bats perform important ecological functions including pollination and seed dispersal, and FAO advises specifically against killing or capturing them as a disease-control response, on the grounds that such measures are largely ineffective and can be actively counterproductive: disturbing colonies causes stress that may increase viral shedding.⁴ A control measure that increases the hazard it was meant to reduce is worse than doing nothing, and it is the kind of error that follows from treating a systems problem as a species problem.

Prevention instead means reducing the hazardous interfaces between wildlife, livestock, food and people. The objective is not to remove wildlife from an ecosystem but to understand the system well enough to narrow the openings through which pathogens move between species — which is the same argument we make about the wildlife–livestock interface more generally.

Surveillance before the hospital

Traditional surveillance often begins when a seriously ill patient reaches a hospital, and for an emerging zoonosis that is far too late. A One Health surveillance system for a pathogen like Nipah can in principle draw on information from several points along the chain: wildlife ecology, animal infection, agricultural conditions, food exposure, human cases, neurological presentation and laboratory confirmation. Each of those is observed by someone, but rarely by the same organisation.

That makes veterinary observation directly relevant to human epidemic intelligence. Changes in wildlife behaviour may matter; unusual disease in livestock may matter; and a pattern of human encephalitis without an established cause may matter a great deal. WOAH lists infection with Nipah virus within its international animal-health reporting framework, so the veterinary signal has a formal route,³ while FAO emphasises coordinated surveillance, joint risk assessment and early-warning systems spanning animal health, agriculture, public health and the environment.⁶

The value, though, lies less in collecting more data than in connecting it. A veterinary report and a cluster of unexplained encephalitis cases are far more informative together than apart, and the ability to put them side by side quickly is the thing that distinguishes a system that detects from one that merely records. We set that argument out in full in Surveillance Before the Outbreak, and the question of who is legally obliged to pass a signal on is the subject of Who Has to Tell Whom?

Nipah has not disappeared

Nipah is sometimes discussed as though it were an exotic historical outbreak, and it is not. On 11 June 2026, the Kerala State Health Department confirmed a laboratory-confirmed case in Kozhikode district. The patient, an adult male, developed symptoms on 30 May and was hospitalised on 10 June; he presented with neurological manifestations and was receiving ventilatory support in intensive care at the time of WHO’s report. By 18 June, 104 contacts had been identified and placed under monitoring, including health and care workers, and no secondary infections had been identified.⁷

The event illustrates both halves of emerging-disease preparedness at once. The pathogen remains fully capable of crossing into humans, and nothing about that has changed. But surveillance, laboratory capacity, isolation, contact tracing and a coordinated response can substantially alter what happens next — which is precisely the contrast the Indian comparison draws out. Two confirmed cases in two Indian states within six months of each other, with very different trajectories, is not a coincidence of virology.

The One Health lesson

Nipah gives an unusually clear view of how an emerging infectious disease develops. A pathogen circulates silently within wildlife; changes in the interfaces between wildlife, agriculture and people create an opportunity for spillover; livestock sometimes amplify the threat; food practices open another transmission pathway; human-to-human transmission then moves the problem into households and hospitals; and finally a clinician finds themselves treating an individual with life-threatening encephalitis.

Organised conventionally, every one of those stages belongs to a different profession — ecology, veterinary medicine, agriculture, food safety, virology, public health, infection control, immunology, neurology and intensive care. The pathogen does not recognise those boundaries, and surveillance and preparedness systems cannot afford to recognise them too rigidly either.

Nipah is therefore more than an important emerging virus; it is an argument for One Health itself. If we want to prevent the next patient from developing encephalitis, the most important signal may not appear first in a neurology ward. It may appear in a bat population, a pig farm, a food-production system or a small cluster of unexplained animal disease. The challenge is ensuring that somebody is listening, and that the evidence can travel across those boundaries quickly enough to matter.

This is the first article in our Zoonotic Neuroinfectious Disease pathway, which follows pathogens that cross from animals to people and then attack the nervous system.

Questions & Answers

References

1. World Health Organization, Nipah virus, fact sheet. Geneva: WHO.
2. Suit-B Y, Hassan L, Krauss SE, Ooi PT, Ramanoon SZ, Yasmin AR and Epstein JH (2021) Mental model of Malaysian pig farmers in implementing disease prevention and control practices. Frontiers in Veterinary Science 8:695702, which records the 1998–99 outbreak totals of 265 encephalitis cases, 109 deaths and 1.1 million pigs culled, together with USD 171 million spent on eradication and USD 446 million of wider losses.
3. World Organisation for Animal Health, Nipah virus, listed disease. Paris: WOAH.
4. Food and Agriculture Organization of the United Nations, Nipah virus, animal diseases. Rome: FAO.
5. Patil DY, Sahay RR, et al. and Yadav PD (2026) Two geographies, one virus: what recurrent Nipah spillover in India reveals. npj Viruses 4, 25. Published 13 May 2026.
6. Food and Agriculture Organization of the United Nations, Nipah virus disease: risk management and emergency preparedness in agrifood systems. Rome: FAO.
7. World Health Organization, Nipah virus disease – India, Disease Outbreak News, 2026-DON609, 25 June 2026. Geneva: WHO.