Nipah Virus

The Nipah virus (NiV) is a highly contagious and often fatal zoonotic pathogen belonging to the family Paramyxoviridae, genus Henipavirus. It is recognised as an emerging infectious disease of global concern due to its capacity to cause severe respiratory and neurological illnesses in humans, as well as significant economic losses in the livestock sector. The virus was first identified in Malaysia in 1998 and has since caused multiple outbreaks in South and Southeast Asia, particularly in Bangladesh and India.

Background and Discovery

The Nipah virus was first detected during an outbreak of severe respiratory and neurological disease among pig farmers in Kampung Sungai Nipah, Malaysia, from which the virus derives its name. The outbreak led to over 100 human deaths and the culling of more than one million pigs to contain its spread. Initially, the disease was thought to be caused by Japanese encephalitis; however, laboratory investigations revealed a novel paramyxovirus closely related to the Hendra virus, previously identified in Australia.
Subsequent research confirmed that fruit bats of the genus Pteropus, also known as flying foxes, serve as the natural reservoir hosts. The discovery established a new category of emerging zoonoses—viruses capable of crossing from bats to intermediate animal hosts and humans.

Transmission and Reservoir Hosts

The transmission cycle of Nipah virus involves both animal and human hosts. The primary reservoir is the Pteropus bat, which excretes the virus in saliva, urine, and partially eaten fruits. Human infections have been linked to several routes of transmission:

• Animal-to-human transmission: Through direct contact with infected pigs, horses, or other animals.
• Human-to-human transmission: Through contact with bodily fluids of infected individuals, a pattern particularly noted in outbreaks in Bangladesh and India.
• Food-borne transmission: Consumption of raw date palm sap contaminated by bat secretions has been identified as a significant route in South Asian outbreaks.

Unlike the Malaysian outbreak, which primarily involved pigs as intermediate hosts, later outbreaks in Bangladesh and India involved direct bat-to-human transmission and limited human-to-human spread.

Symptoms and Clinical Manifestations

Nipah virus infection in humans presents with a broad spectrum of symptoms, ranging from mild illness to fatal encephalitis. The incubation period generally ranges from 4 to 14 days, although longer periods up to 45 days have been reported.
Typical clinical manifestations include:

• Fever, headache, and muscle pain (myalgia)
• Drowsiness, dizziness, and vomiting
• Respiratory distress and cough in some cases
• Rapid progression to encephalitis, leading to disorientation, seizures, and coma

The case fatality rate (CFR) varies between 40% and 75%, depending on the outbreak and quality of medical response. Survivors may experience long-term neurological complications, including personality changes and recurrent convulsions.

Diagnosis and Laboratory Testing

Accurate and early diagnosis of Nipah virus infection is crucial for outbreak control. Diagnostic methods include:

• Real-time polymerase chain reaction (RT-PCR) for detecting viral RNA in bodily fluids such as throat swabs, urine, or cerebrospinal fluid.
• Enzyme-linked immunosorbent assay (ELISA) to identify IgM and IgG antibodies.
• Virus isolation and neutralisation tests, conducted only in high-containment biosafety laboratories (BSL-4).

Differential diagnosis is essential, as symptoms may mimic other encephalitic or respiratory diseases such as Japanese encephalitis, rabies, or influenza.

Outbreaks and Geographic Distribution

Since its emergence, the Nipah virus has caused sporadic outbreaks in several Asian countries:

• Malaysia and Singapore (1998–1999): The first outbreak affected pig farmers and abattoir workers, resulting in 265 human cases and 105 deaths.
• Bangladesh (from 2001 onwards): Annual outbreaks have occurred almost every year, with human-to-human transmission being a key feature.
• India (notably Kerala, 2018, 2019, and 2023): Limited outbreaks have been recorded with high fatality rates and effective containment through early isolation and contact tracing.

The virus is now recognised as endemic in South and Southeast Asia, where bat habitats overlap with human populations.

Prevention and Control Measures

There is currently no specific antiviral treatment or licensed vaccine for Nipah virus infection. Therefore, control measures focus on prevention and containment through public health interventions, including:

• Avoiding consumption of raw date palm sap and other foods possibly contaminated by bats.
• Improving farm biosecurity to prevent spillover from bats to domestic animals.
• Using protective gear (masks, gloves, gowns) when caring for infected patients or animals.
• Quarantine and contact tracing to prevent secondary transmission.
• Public awareness campaigns in high-risk regions, particularly during date palm sap collection seasons.

Ongoing research is being conducted on monoclonal antibodies, mRNA-based vaccines, and viral vector vaccines under the World Health Organization’s R&D Blueprint for priority pathogens.

Pathogenesis and Viral Structure

The Nipah virus is an enveloped, negative-sense, single-stranded RNA virus. It encodes six major structural proteins—nucleocapsid (N), phosphoprotein (P), matrix (M), fusion (F), glycoprotein (G), and large polymerase (L)—that contribute to replication and host infection mechanisms. The G and F glycoproteins facilitate viral attachment and fusion with host cells, enabling systemic infection of the respiratory tract and central nervous system.
Pathological studies have shown vasculitis, neuronal necrosis, and microinfarctions in infected tissues, particularly in the brain and lungs, explaining the neurological and respiratory manifestations observed clinically.

Economic and Public Health Impact

Outbreaks of Nipah virus infection have considerable economic, social, and health consequences. The 1998–1999 Malaysian outbreak led to the closure of pig farms, mass culling, and export bans, causing economic losses exceeding US$350 million. Beyond economic impacts, the virus poses a severe threat to healthcare workers and rural communities with limited medical infrastructure.
Its classification as a Biosafety Level-4 pathogen underscores the high containment and specialised laboratory facilities required for handling the virus. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) classify Nipah virus as a priority pathogen with pandemic potential.

Research and Global Surveillance

Global efforts to combat the Nipah virus involve collaborative surveillance systems and cross-border cooperation under the One Health approach, which integrates human, animal, and environmental health. Key initiatives include:

• The WHO Nipah Virus Research Consortium coordinating vaccine and therapeutic development.
• The Coalition for Epidemic Preparedness Innovations (CEPI) supporting vaccine candidates entering early-phase clinical trials.
• EcoHealth Alliance and related agencies conducting ecological surveillance in bat populations.

Molecular epidemiology and genomic sequencing have improved understanding of viral evolution, revealing distinct strains such as NiV-Malaysia and NiV-Bangladesh, which differ slightly in genetic makeup and transmission potential.