What is the Nipah virus and is it spreading in southern India?

Al Jazeera

7 days ago

On July 12, a new deadly Nipah virus infection was confirmed in a 52-year-old man in the Palakkad district of Kerala, marking the tenth instance of Nipah virus spillover (transmission of the pathogen from animals to humans) in the southern Indian state since 2018.
This year alone, Kerala has reported four Nipah cases, including two deaths, all within a 50km (30-mile) radius, on the border of the Malappuram and Palakkad districts.
The state remains on high alert, with 675 people under surveillance across five districts.
Here is what we know about the Nipah virus, its symptoms and how authorities are containing it.
What is the Nipah virus?
Nipah virus (NiV) is a highly pathogenic zoonotic virus (a virus that can be transmitted from animals to humans) which causes death in 40 to 75 percent of human infected cases. Alongside the Hendra virus, which is found in Australia, NiV is one of the most notable viruses from the henipavirus genus of the paramyxovirus family, responsible for a range of neurological – often respiratory – diseases in humans and animals.
Fruit bats of the Pteropodidae family, ubiquitous across Oceania, South and Southeast Asia, and sub-Saharan Africa, are natural reservoirs of the virus, meaning the virus naturally lives and reproduces in these mammals without causing them any harm.
The spillover of the virus to humans can happen directly or via intermediate hosts such as pigs or horses, which come into contact with humans.
What are the symptoms of the Nipah virus?
According to the World Health Organization (WHO), human NiV infections range from asymptomatic infection to acute respiratory infection, seizures and fatal encephalitis (inflammation of the brain).
The clinical presentation of NiV infection is neurological, affecting the central nervous system and resulting in acute encephalitis syndrome (AES), characterised by seizures, confusion and loss of consciousness. When the disease advances, it can cause damage to the lungs and can lead to life-threatening acute respiratory distress syndrome (ARDS).
The incubation period of the virus is believed to be four to 14 days.
Where have there been Nipah virus outbreaks before?
The first human NiV infection was recorded in 1998, when pig farmers and butchers from Malaysia and Singapore contracted the virus from infected pigs. The outbreak affected more than 250 individuals and caused more than 100 deaths.
There have been subsequent, almost annual outbreaks in Bangladesh since 2001, with human infection traced to the consumption of date palm sap contaminated with urine or saliva from infected fruit bats.
In 2014, NiV infections in the Philippines were associated with the slaughter of horses and consumption of infected horse meat.
India has reported two outbreaks in West Bengal in 2001 and 2007. In 2018, South India reported its first NiV outbreak in Kerala, when 19 confirmed cases led to 17 deaths. Since then, Kerala has reported NiV spillovers almost every year.
What has happened in Kerala?
Although Kerala has recorded 10 instances of NiV spillover since 2018, only two of them turned into outbreaks with instances of human-to-human spread.
'We are now reporting single cases of Nipah infections rather than a cluster or outbreak like in 2018,' Thekkumkara Surendran Anish, nodal officer at the Kerala One Health Centre for Nipah Research and Resilience, said.
The last six NiV infections in the state have been single-case spillovers with no human-to-human transmission. The recent uptick in recorded cases is down to the strengthened surveillance system in the state, according to Anish.
'Thousands of people die in India every year due to AES or ARDS, [where] we don't know the cause. NiV, in fact, is not a common cause for respiratory and encephalitis syndromes,' he said. 'But in Kerala, because the entire health system is prioritising Nipah, we are detecting more Nipah infections.'
At the same time, it is concerning that all four NiV infections in 2025 were reported in a small timeframe within a small area, Anish added. 'Four independent spillover events within a couple of months in a 50km radius suggest a very high presence of infected bats and the virus in the area.
'What it tells us is that in certain geospatial locations in Kerala, there is a high likelihood of Nipah spillovers, primarily because the bats in those localities seem to be highly infectious for a short period during the year.'
What is causing the spread of the Nipah virus in Kerala?
Unlike in Bangladesh, where there was a dedicated channel for the virus to spill over to humans (contaminated date palm sap), there is no obvious source in Kerala – or, at least, the 'spillover mechanism' so far remains unclear.
'We don't know the exact spillover mechanism, but it seems to be highly sporadic in Kerala. For example, you unknowingly come in contact with an infected bat or its droppings,' Anish said.
The commonly accepted cause is spillover from the human consumption of fruits contaminated by bat saliva or urine. However, virological examination of bat-bitten fruits has yielded negative results so far.
A new paper by the Indian Council of Medical Research suggests the virus may be airborne.
'Disease-causing microbes have different routes of transmission to reach and infect human hosts,' Thekkekara Jacob John, one of the authors of the paper, and an emeritus professor at the Christian Medical College, Vellore, said. 'One of them is airborne transmission, like in Tuberculosis, whereby microbes float in the air for longer distances and are inhaled far away from the source.'
The authors of the paper believe their hypothesis 'accommodates the rare but recurrent spillovers of NiV in Kerala', where there are no mechanical vectors for the virus, such as in Bangladesh or Malaysia.
What are the authorities doing to contain the Nipah virus in Kerala?
So far, Kerala has been successful in containing the spread of the virus using trace-and-test procedures. Across 10 'spillover events', the case count stands at just 37 infected individuals.
The key to this is a strong surveillance system, said Anish. 'When a Nipah case is established, all primary contacts of the patient are immediately traced and monitored under house quarantine. If they test positive for the virus, we immediately start antiviral treatment,' he said.
The prophylactic treatment of 'primary contacts' with broad-spectrum antiviral drugs has helped reduce mortality. 'In our experience, if we can treat Nipah early, it can definitely be cured,' Anish said.
'Syndromic surveillance is another crucial aspect, whereby, in hospitals across the state, any patient presenting with AES or ARDS is tested for Nipah,' Anish said.
Kerala has been steadily improving its health system. The state now has four labs capable of conducting RT-PCR (Reverse Transcription Polymerase Chain Reaction) tests for detecting active NiV infections.
'It all comes down to political commitment,' Anish said. 'The state ensures that efforts of the entire health system remain focused on Nipah outbreaks.'
What is the treatment for Nipah virus? Is there a vaccine in development?
The WHO has identified Nipah as a priority disease for its Research and Development Blueprint – a global strategy and preparedness plan for epidemics.
The University of Oxford's NiV vaccine, which began in-human trials in January, was granted support from the Priority Medicines (PRIME) scheme of the European Medicines Agency (EMA) in June.
However, there are no drugs yet available that specifically target NiV infection.
Since there are no approved treatment protocols for NiV, and due to the high risk of mortality, doctors have used broad-spectrum antivirals. Ribavirin is the antiviral of choice, as it has been found to be effective against NiV infections in humans on several occasions.
During a 2023 outbreak in Kerala, early application of the antiviral Remdesivir resulted in an improved case fatality rate. Monoclonal antibodies (copies of antibodies, created in a laboratory) have also been used to prevent severe disease manifestations in high-risk individuals.
How can we avoid zoonotic viruses like the Nipah virus?
According to Anish, NiV is a model case study for a 'one health' approach to combating high-threat pathogens. The one health approach recognises the fact that human health is interlinked with the health of animals and the environment.
'[One health] is a combination of three things – human health, animal health and environment health,' Anish said. 'You have to tackle all these things to reduce the chances of zoonotic spillovers such as Nipah.'
Approximately 60 percent of emerging human pathogens are zoonoses, transmitted from animals to humans. Ecological disruption and the spread of human populations into wildlife habitats are primary causes for the emergence of these zoonotic viruses.
Climate change is another growing concern, with rising temperatures attributed to influencing infections, viral load and human-animal interactions.