Explained | Nipah virus and its history in India

The deadly Nipah virus has struck Kerala again – for the fourth time in six years – causing five cases so far and several suspects. The state has introduced containment measures in 45 wards of Kozhikode district and nearly 800 people who came in contact with the infected individuals have been told to remain in isolation. 

Here's an overview of the disease and its history in India.

What is Nipah?

It is the name of a virus, first reported in Malaysia in 1998 during an outbreak among pig farmers. Nipah is a zoonotic virus (transmitted from animals to humans) but can also be transmitted through contaminated food or directly between people if they are in close contact. The virus is highly pathogenic and is among the ten priority pathogens identified by the World Health Organisation.

Is this the first Nipah outbreak in India?

No, there are at least five known occurrences, including two major outbreaks. The first one was in Siliguri in 2001 in which 66 persons were identified as having the infection with a case fatality rate of 74 per cent. The samples were not tested for Nipah at that time, but a retrospective analysis carried out a few years later suggested that the mystery infection was indeed caused by the Nipah virus. In 2007, it happened in a border village in Nadia district of West Bengal killing five individuals with a case fatality rate of 100 per cent. Neighbouring Bangladesh has witnessed multiple Nipah outbreaks over the past two decades. More than ten years after the Nadia outbreak, the Nipah virus surfaced in Kozhikode in May 2018 with a case fatality rate of 91 per cent (21 deaths out of 23 cases). In 2019, there was a single case with full recovery in Ernakulam district, but in 2021, a 12-year-old boy died of Nipah infection in Kozhikode while the administration was able to prevent the occurrence of a larger outbreak.

What are the symptoms?

Human infections range from asymptomatic infection to acute respiratory infection (mild, severe), and fatal encephalitis. Infected people initially develop symptoms including fever, headaches, myalgia (muscle pain), vomiting and sore throat. This can be followed by dizziness, drowsiness, altered consciousness, and neurological signs that indicate acute encephalitis. Some people can also experience atypical pneumonia and severe respiratory problems, including acute respiratory distress. Encephalitis and seizures occur in severe cases, progressing to coma within 24 to 48 hours. The incubation period (interval from infection to the onset of symptoms) can range from 4 to 14 days, even though an incubation period as long as 45 days has also been reported in a rare case. According to the WHO, the case fatality rate is estimated at 40 per cent to 75 per cent, but this can vary depending on local capabilities for epidemiological surveillance and clinical management.

How is the disease transmitted?

During the first recognised outbreak in Malaysia, which also affected Singapore, most human infections resulted from direct contact with sick pigs or their contaminated tissues. Transmission is thought to have occurred via unprotected exposure to secretions from the pigs, or unprotected contact with the tissue of a sick animal.

In subsequent outbreaks in Bangladesh and India, consumption of fruits or fruit products (such as raw date palm juice) contaminated with urine or saliva from infected fruit bats was the most likely source of infection. Human-to-human transmission of Nipah virus has also been reported among family and caregivers of infected patients.

Why is the Nipah outbreak recurring in Kozhikode district?

No one has been able to pinpoint a specific reason, barring the fact that there are large populations of fruit bats in the area. An analysis by the National Institute of Virology, Pune showed a clear distinction of the Kerala samples from Malaysia and Bangladesh ones, suggesting the presence of a new genotype independently circulating in southern India. Four outbreaks in Kerala which is far distant from the known “Nipah belt” with no identified intermediate animal host or confirmed mode of entry into human population warrants a heightened need of constant surveillance of the virus in bats, animals and humans, says the NIV.