A sleepy morning in late February 1999 in the Malaysian state of Negeri Sembilan would, within a few weeks, become the start of one of the country’s most frightening and consequential infectious‑disease emergencies.
Farmers who raised pigs, veterinarians who tended the animals, and clinicians who treated the ill were all soon to learn that something more sinister than Japanese encephalitis — the working diagnosis at the time — was spreading between animals and people.
The discovery of what came to be called Nipah virus was the result of a small team’s persistence, a clash of opinion inside a university laboratory, and rapid international collaboration.
This is the story, told from the perspective of the virologist who first recognised the new virus in culture.
The early signs were confusing. In parts of northern Peninsular Malaysia in 1998 and into 1999 there were clusters of febrile encephalitis in adults. Simultaneously, pig farms were reporting unusual respiratory disease and an increase in pig deaths.
Official accounts later placed the start of the human outbreak in September 1998, but stored patient samples from as early as 1997 showed that something had likely crossed from wildlife into pigs and then into humans even earlier.
That detail — anti‑Nipah antibodies in sera from encephalitis patients admitted in 1997 — would only become clear later, but even then clinicians and public‑health officials were slow to move away from the conventional explanation.
Japanese encephalitis (JE) was the comfortable default. For decades JE had been known to cause encephalitis in Malaysia and across Asia, and public‑health responses had relied heavily on mosquito control, insecticide fogging and mass vaccination in communities considered at risk.
When the new cluster of encephalitis cases appeared, authorities and many senior virologists initially assumed JE. Mosquito fogging was intensified around affected pig farms; thousands of doses of JE vaccine were rushed in from Japan; and villagers and farm workers were reassured they were protected.
Those responses were grounded in expertise, but in retrospect they fitted the wrong causal model. The outbreak’s demographic pattern — predominantly adults, many of them pig‑farm workers — and the existence of a concurrent pig disease should have prompted a reconsideration of the diagnosis.
Some physicians raised that point early on, but their concerns were reported in the national press as a different trend in JE rather than the sign of a distinct pathogen. In the laboratory of the University of Malaya’s Medical Microbiology Department, serological testing for JE revealed many positives, and a postdoctoral scientist began running JE RT‑PCRs on samples.
Those results too suggested JE.
But there were cautionary signs. JE is endemic in the region and dengue and other flaviviruses are hyperendemic, producing serological cross‑reactivity and complicating interpretation. Not all the clinicians were convinced.
On 27 February 1999, Dr Kaw Bing Chua joined the outbreak investigation. A patient from Bukit Pelandok, a pig‑farming area, had arrived at the University of Malaya Medical Centre with acute encephalitis.
By the time Chua received samples he knew the patient’s serum had tested JE‑IgM positive and that JE nucleic acid had been reportedly detected by RT‑PCR. But the epidemiology nagged at him. An adult pig‑farm worker with encephalitis, and a swine respiratory disease unfolding in the same locality?.
If this really was JE it could be an unusual genotype, but the sequence of events argued for a different agent. He decided viral culture was essential.
Permission for viral culture was not easily secured. The head of department was convinced the cause was JE and saw no need for broader isolation attempts. Chua quietly proceeded anyway, inoculating cerebrospinal fluid and serum from the patient into an array of continuous cell lines — Vero (African green monkey kidney), MDCK (Madin–Darby canine kidney), Hep2 (human carcinoma), PS (pig spleen) and MRC5 (human fibroblast). The samples were incubated and carefully observed.
On 1 March material from five patients admitted to Seremban General Hospital arrived at the laboratory. Within days a syncytial cytopathic effect — fused giant cells — appeared in Vero cell cultures inoculated with several cerebrospinal‑fluid samples.
What made the finding more puzzling was that JE is not characteristically associated with the type of cytopathic effect seen. Colleagues who looked at the same flasks dismissed the appearance as possible contamination.
Lab hierarchy and defensiveness, amplified by the pressure of a major outbreak, contributed to scepticism.
Chua pressed on still. He scaled up infected Vero cell cultures and prepared antigens and fixed infected cells for indirect immunofluorescence testing. His question was simple. If the virus in culture was the cause of illness, patient sera should show antibodies to infected cells.
Using available monoclonal antibodies against known syncytial viruses — measles, mumps, respiratory syncytial virus and herpes simplex virus — he tried typing the agent, but all were negative.
He then tested sera and cerebrospinal fluid from patients in the outbreak area against infected cell‑antigen slides. By the early evening of 6 March the results came back. There were “bags of green fluorescence” where the syncytial infected cells lit up with antibodies from patient samples.
That fluorescence was the turning point. Chua called the head of department at home and asked him to come to the laboratory. The appearance under the ultraviolet microscope visibly changed the senior scientist’s expression; yet, despite the positive serology against the cultured agent, scepticism remained.
Some colleagues still urged caution, suggesting cross‑reactivity. Chua did not give up. He persuaded clinicians involved in the outbreak, including a leading neurologist, to consider the possibility of a new pathogen and to attend a Ministry of Health meeting on 9 March. The head of department finally conceded at the meeting that a new virus might likely be responsible.
Even with mounting laboratory evidence, Chua knew definitive identification demanded electron microscopy and international reference work. Local electron microscopes were either unavailable or poorly maintained.
He refrained from hasty negative staining — where surface detail is seen — because the virus appeared dangerous and there was a lack of high‑containment facilities. Instead he prepared thin‑section specimens of infected Vero cells and sought help from overseas.
The first electron micrographs prepared within Malaysia were not clear; the institute positive for electron microscopy had technical limitations. Determined, Chua arranged to take the fixed infected cell material to the Division of Arbovirus‑borne Diseases at the Centers for Disease Control and Prevention (CDC) in Fort Collins, Colorado, United States — a branch focused on arthropod and zoonotic viruses.
Getting there required a fast turnaround. He was given an expedited US visa via intervention at ministerial level, packed carefully and flew out on 12 March.
At Fort Collins the response was immediate and collaborative. Scientists placed sections in an electron microscope and, for the first time, Chua saw the ultrastructure that would change everything. Under the scope the infected cells displayed the tell‑tale “concrete ring‑like” nucleocapsid structures in cross section and an envelope budding pattern characteristic of paramyxoviruses — a family that includes measles, mumps and several respiratory pathogens.
The morphological evidence made clear what the epidemiology already hinted at. This was not JE — a flavivirus transmitted by mosquitoes — but a paramyxovirus likely spread by close contact and respiratory droplets. That explained why mosquito control and JE vaccination had been ineffective.
The discovery carried urgent implications. If the agent spread by respiratory routes and involved pigs as an amplification host, the outbreak control measures required immediate adjustment such as close contact precautions, changes in how pig houses were handled, restrictions on animal movement and urgent testing of animals and workers.
Chua phoned Malaysian public‑health officials from Fort Collins and urged them to stop JE‑centric interventions and to request international assistance. He also arranged for the electron micrographs and virus isolates to be forwarded to CDC Atlanta where further characterisation could proceed.
In Atlanta a team rapidly confirmed the organism as a novel paramyxovirus. The agent would be formally named Nipah virus — after a village near where some of the early human cases were identified — and it was shown to cause severe encephalitis in humans along with respiratory disease in pigs.
The discovery also triggered broader investigations into disease ecology: the likely natural reservoirs, the circumstances under which spillover to pigs and then to humans occurred, and the agricultural practices that may have allowed an animal virus to jump species.
Subsequent research implicated fruit bats of the Pteropus genus as the reservoir hosts; deforestation and changing land use that brought bats into closer contact with pig farms probably facilitated viral spillover.
The early misclassification as JE had real human cost. Some pig‑farm workers — and even people living near pig farms — believed they had been vaccinated against JE and were therefore protected; they returned to work and, in some cases, became infected and died.
The episode highlighted how routine laboratory tests and targeted molecular assays, while powerful, can mislead when the clinician’s and epidemiologist’s lens is narrow. JE serology and RT‑PCR had produced a pattern that, taken in isolation, seemed to point to familiar territory. It was the combination of epidemiological reasoning (adult cases, pig involvement), classical virology (culture and cytopathic effect), immunofluorescence with patient sera, and electron microscopy that revealed the true nature of the agent.
The Malaysian response ultimately shifted. Once Nipah virus was recognised, control measures were recalibrated. Authorities culled tens of thousands of pigs to halt transmission; markets and farms were closed, and tighter biosecurity rules were implemented.
International teams assisted in surveillance, diagnostics and public‑health measures. The outbreak prompted intense scientific study and policy debate. It also left deep social and economic scars: entire pig‑farming businesses were devastated, and families and communities that depended on swine rearing were thrown into hardship.
The Nipah outbreak also changed how emerging zoonoses are seen in the region. It highlighted the fragile boundaries between wildlife, domestic animals and human populations — boundaries that can be eroded by deforestation, changes in farming practices and increasing human encroachment into wildlife habitats.
Anthropogenic environmental changes, including land clearing and the timing of fruiting seasons, were later linked to bat movements and to the ecology that permitted the virus to move from bats to pigs. The incident became an instructive case for public health and veterinary services worldwide, illustrating how integrated human, animal and environmental health surveillance — a One Health approach — can be crucial in recognising and controlling novel threats.
For the scientists involved, the outbreak was an intense lesson in professional humility and the value of diverse methods. Molecular diagnostics and serology are indispensable in modern outbreaks, but they are not foolproof, especially when the pathogen is novel and the clinical picture atypical.
Viral culture, careful observation of cytopathic effects, immunofluorescent testing with patient sera, and the classical interpretive skill of electron microscopy were decisive in identifying Nipah virus.
The story also shows the importance of open communication. Without the eventual willingness of the department head to allow the broader investigative work, and without rapid international collaboration, identification and response would have been delayed further.
The human toll was heavy. Nipah virus causes severe febrile encephalitis in humans with significant fatality rates, and the outbreak in Malaysia led to numerous deaths among pig‑farm workers and close contacts.
The outbreak in 1998–99 also prompted heightened surveillance for encephalitis of unknown origin across Asia and eventually to recognition of other Nipah outbreaks in Bangladesh and India where person‑to‑person transmission has been documented.
The discovery of Nipah virus is a reminder that outbreaks often teach in painful ways. In Malaysia the early focus on the familiar — JE — delayed recognition of a new danger that was spreading by very different routes and required different interventions.
The persistence of a scientist who trusted clinical intuition and classic virological methods, the cooperation of clinicians and laboratory technicians who pursued uncomfortable evidence, and the support of international reference laboratories combined to save lives thereafter by steering the response in a new direction.
The episode changed public‑health practice in the region, advanced scientific understanding of paramyxoviruses, and contributed to the global conversation about how human activity, environmental change and animal health intersect to produce new infectious threats.
In the years since, Nipah virus has remained a pathogen of significant concern. Its potential for severe disease and, in some outbreaks, human‑to‑human spread has put it on lists of priority pathogens for research and countermeasure development. Lessons learned from the 1998–99 Malaysian outbreak — rapid recognition of unusual epidemiology, the need for broad laboratory testing beyond routine assays, the value of virus isolation and morphology, and the critical role of timely international collaboration — continue to inform outbreak responses around the world.
The crisis also left a quieter legacy: the people who raised their voices, took apparently risky steps in the laboratory, and argued with authority for a different interpretation of the data.
Without that persistence, the identification of Nipah virus could have been delayed still further.
The story is sometimes told as a triumph of modern science, but it is also a testament to the enduring value of practical judgement, a readiness to question assumptions, and the importance of connecting on‑the‑ground clinical observation with laboratory inquiry.
Those are the tools that continue to defend societies against the next unknown microbe.
This piece is adapted from The discovery of Nipah virus: A personal account, History of Neurology, Neurology Asia 2004;9:59 by Dr Kaw Bing CHUA
