Staying vigilant on Nipah

The outbreak has ended, but health officials need to be on guard

What are the latest Nipah-related developments?

A swift and unexpected outbreak of the Nipah virus in Kerala has finally ended, with the government declaring Kozhikode and Malappuram Nipah-free on July 1. Also, last week, the National Institute of Virology found the Nipah virus in Kozhikode’s fruit bats, suggesting that the virus had travelled to humans from bats. But the outbreak ought to put healthcare surveillance officials across India on guard. It confirms that the virus could be circulating in regions of India without our knowledge.

Will such outbreaks happen again?

Even though West Bengal and Kerala are the only States that have seen Nipah outbreaks, the virus and antibodies have been detected in bats in Assam and Haryana too. So, yes, it could. Bats in other places have not been tested, which means we do not know if the virus is circulating in these locations.

If the virus was always in Kerala’s bats, why didn’t an outbreak happen earlier?

The transmission of a virus to humans, or zoonotic spillover, is a rare event for Nipah. The virus was found in countries which never saw an outbreak, such as Thailand, and the only known outbreaks so far have been in Malaysia, Bangladesh and India. A number of factors must come together for spillover to occur. First, the bats must carry a lot of the virus (whereas the viral load is frequently low in these mammals). Second, bats must come in close contact with humans. This is happening more frequently with deforestation and urbanisation. Yet, the presence of a bat colony near a human dwelling isn’t reason to worry. Though the mammals excrete the virus, the viral load in bat stool is frequently too low for infection. Two modes of spillover have been identified so far. In Malaysia, the virus moved to humans from pigs that had consumed bat-bitten fruit. In Bangladesh, patients drank palm sap in which bats had urinated.

Why the global interest in a local outbreak?

Nipah virus can potentially trigger global pandemics. At present, the virus transmits inefficiently, through respiratory droplets, to people within a metre of sick patients. If the microbe mutates to become airborne, or evolves to transmit via patients who are asymptomatic, it will be hard to control. Sadly, every outbreak gives the virus the chance to mutate. With a fatality rate of over 80%, the virus could become a big killer.