Ever since its first encounter with Nipah virus (NiV) in 2018, when 23 cases were identified in the outbreak, Kerala has reported several rare and random NiV spillover events almost every year.
This year, the State reported four cases between May and July and unusually, all of these were primary cases. Of this, three cases were reported within a 30-50 km radius in Palakkad-Malappuram districts within a span of 15 days, but each were random spillover events, with no secondary transmission of infection.
The exact bat to human NiV spillover mechanism in Kerala continues to remain elusive, even though, the latest hypothesis proposed by eminent microbiologist and public health expert, T. Jacob John, suggests that the recurrence of rare, randomly distributed spillover events in the State raises the rare possibility of air-borne transmission of NiV.
Air-borne transmission possibility
Dr. John points out that bats’ urination from roosting sites may produce aerosols, especially when droplets of bat excreta mix with dust particles (Viral RNA has been detected in bat urine in many previous studies) Air-borne transmission of NiV is theoretically possible if humans are exposed to aerosols containing viable virus or free virus particles. He suggests exploring air sampling methods for NiV around trees with bat roosts, apart from lab studies.
The fact that three of this year’s NiV-affected persons were living in houses surrounded by bat roosts with fairly high bat density, also aligns with the possibility that they could have been exposed to bat secretions – directly or as aerosols.
Health officials say that there was clear evidence that some disturbing events had preceded the three random Nipah human infections.
Dispersion risk
“The local people said that the bat roosts in the affected localities had been there since ages. But recently, there had been some attempts by local people to drive away the bats through the bursting of crackers. Disturbing bat roosts disrupts the ecosystem and increases the spillover risk by causing stress-induced viral shedding in bats and leading to the wider dispersion of infected bats,” an official says.
At a recent online interaction with public health experts and scientists in Kerala, Jonathan Epstein, an epidemiologist at EcoHealth Alliance and one who has done extensive research in NiV, reiterated that human behaviour is the primary risk for NiV spillover and that viral shedding and NiV outbreaks amongst bats do not follow any tight annual or seasonal patterns.
Spillovers could result when there is an alignment of increased viral shedding in bats (because of stressful events like threat to habitat, food scarcity, mating or birthing times) and coincidental high-risk human behaviour.
“Bats have a vital role in the environment and rather than trying to remove them from the landscape, it is human behaviour that has to be modified so that there are minimal human-bat interactions. Going forward, the most plausible, evidence-based way to prevent random Nipah spillovers is by “learning to live safely with bats”, says R. Aravind, Head of Infectious Diseases, Government Medical College, Thiruvananthapuram.
Monitoring bat colonies
This involves protecting natural habitats, reducing high-risk human behaviours through public education, regular bat colony monitoring, strengthening surveillance and rapid outbreak response capabilities, all of which had been encapsulated in the Zero Nipah campaign that the State had launched in 2019.
“We have already initiated a risk stratification of districts, by mapping bat roosts and studying the bat density in each. Increased bat density, periodic viral shedding, and waning immunity to NiV in bats over time can increase the spillover risk,” a health official said.
“Educating the public about bat ecology and convincing them that it is their behaviour that has to changed to prevent Nipah, is not going to be an easy task. We are in the process of drawing up IEC messages – which have to be nuanced and focussed – so that people understand how they can reduce their vulnerability through behaviour modification,“ the official said.
Kerala has already strengthened AES (Acute Encephalitis Syndrome) through syndromic surveillance, so that early detection and management of Nipah is possible. Tighter infection control practices are being advocated in hospitals, including proper education to healthcare workers that universal precautions such as masking are to be maintained at all times inside hospitals.
Published – August 02, 2025 08:08 pm IST
