Nipah virus, a deadly zoonotic virus, was first reported in Malaysia in 1998. The country documented a total of 265 human cases, including 105 deaths (nearly 40 percent), largely attributed to severe neurological complications. In contrast, when the virus was first identified in India and Bangladesh in 2001, the reported case fatality rate reached nearly 70 percent. Several subsequent outbreaks in these regions have recorded case fatality rates as high as 100 percent. This marked difference in fatality rates between Malaysia and India/Bangladesh raises an important question: Is the Indian variant of the Nipah virus more severe than the Malaysian variant?

Recently, two laboratory-confirmed cases of Nipah virus infection have been reported in the state of West Bengal, India, igniting concerns about the potential spread of the virus to neighbouring Nepal and the risk of international spread. According to the World Health Organisation (WHO), 190 individuals who had contact with the patients were tested, and all results were negative. Despite this, the incident has reignited serious debate about the risk of cross-border transmission, particularly given the virus’s known ability to spread from human to human.

Although India has experienced several Nipah virus outbreaks in the past, the current situation appears to have heightened awareness among healthcare workers, leading to increased vigilance and proactive efforts to rule out Nipah virus infection in patients with compatible symptoms and travel history. Recently, I received a phone call regarding a patient who initially presented with flu-like symptoms and had a recent travel history to India, whose condition subsequently deteriorated, necessitating admission to the intensive care unit of a Kathmandu Valley-based hospital. Neuroimaging revealed abnormal findings on a brain CT scan, prompting a query regarding appropriate referral pathways for clinical samples to rule out Nipah virus infection. A similar consultation was received for another patient with a similar travel history who presented with flu-like symptoms and pneumonia. In light of the resurgence of Nipah virus cases in West Bengal, clinicians expressed concern and sought to exclude Nipah virus infection based on clinical presentation and epidemiological risk.

Nipah virus is a highly pathogenic virus transmitted from animals to humans (close contact), through contaminated food sources (for example, saliva, urine), and via direct human-to-human contact through exposure to bodily fluids. The initial clinical presentation typically resembles a flu-like illness, characterised by fever, headache, cough, sore throat, vomiting and fatigue, followed by severe neurological manifestations, including acute encephalitis. In India and Bangladesh, delayed health-seeking behaviour may contribute to poorer outcomes, as early symptoms are often perceived as a self-limiting influenza-like illness, leading individuals to remain at home rather than seek medical care. Such delays can result in late diagnosis and rapid disease progression, with patients frequently deteriorating to coma within 24 to 48 hours. This may partially explain the higher case fatality rates observed in the Indian subcontinent compared with Malaysia; however, further research is required to substantiate this hypothesis.

To date, Nipah virus infection has not been officially confirmed in Nepal; however, the risk of its introduction cannot be dismissed. Pig farming is a common agricultural practice in Nepal, and notably, the first Nipah virus outbreak was identified among pig populations in Malaysia in 1998. Given the periodic fatal outbreaks of infectious diseases reported in pigs in Nepal, the Nipah virus should not be overlooked in the differential diagnosis of such events. Although the exact mechanism by which the Nipah virus was introduced into pigs in Malaysia remains unclear, fruit bats have been suggested as the most likely reservoir and source of spillover.

Fruit bats (Pteropus species) are recognised as the primary reservoir responsible for Nipah virus outbreaks in India and Bangladesh. Evidence from these countries indicates that the human infections frequently occurred following the consumption of raw date palm sap (known as “TADI” in Nepali), which may have been contaminated by fruit bats. Raw palm sap is also widely available, popular and consumed in the Western Tarai region of Nepal. Although no confirmed cases of Nipah virus infection associated with palm sap consumption have been reported in Nepal to date, relevant government and public health authorities should proactively disseminate information on the potential risk of contamination. Public awareness campaigns should emphasise avoiding the consumption of leftover or uncovered raw palm sap that may be contaminated with fruit bat saliva, urine or other bodily fluids. Scientific reports indicate that fruit bats are widely distributed throughout Nepal, including urban areas such as Kathmandu and Bhaktapur. There is therefore a need for systematic surveillance and virological investigations in fruit bat populations to determine the presence of Nipah virus. Such efforts would enhance early detection, support preparedness and enable rapid containment should an outbreak occur.

One of the most striking features of Nipah virus infection is its markedly variable case-fatality rate across geographic regions. During the Malaysian outbreak, the fatality rate was nearly 40 percent, whereas outbreaks in India and Bangladesh have reported an average fatality rate of around 70 percent, with some outbreaks reaching up to 100 percent. Similar variability in disease severity has been observed with other infections; for example, during the Covid-19 pandemic, the Delta variant was associated with more severe disease, while the Omicron variant demonstrated higher transmissibility but lower severity.

Molecular studies have shown distinct genetic mutations in the Nipah virus variant circulating in India and Bangladesh compared with the Malaysian variant. These genetic differences may represent a plausible explanation for the increased severity observed in the Indian subcontinent. In Malaysia, a large proportion of cases (nearly 70 percent) occurred among individuals directly involved in pig farming, whereas outbreaks in India and Bangladesh have been characterised by more frequent human-to-human transmission. This epidemiological difference may reflect differences in variants and their associated severity.

To sum up, recent reports of highly pathogenic Nipah virus cases in neighbouring India serve as a wake-up call for Nepal. At present, the Indian variant appears to be more severe than the Malaysian variant but less transmissible; however, increased infectivity cannot be ruled out, as the virus has the capacity to undergo further natural mutations, raising concerns about its potential to cause wider outbreaks in the future. Although the virus has not yet been confirmed in Nepal, the risk of introduction remains high due to recurrent outbreaks in India, the presence of fruit bat reservoirs and pig farming practices within the country. These factors underscore the need for continued vigilance, strengthened surveillance and preparedness to detect and respond promptly to any potential emergence of Nipah virus in Nepal.