Nipah virus (NiV) has surfaced in Kerala yet again, making this the eighth appearance in as many years. Whether virus spillovers (single index case, with no secondary cases in the community), or outbreaks (spillover with subsequent cases), Kerala has primed its response to the virus, seamlessly managing early detection, limiting the transmission and successfully saving lives.
However, in all these encounters with NiV, researchers have identified several missing links or knowledge gaps. These pertain to the exact virus spillover mechanism from bats ( the fruit bats of the Pteropus species are the proven natural reservoir hosts of NiV) to humans; the recurrence of the spillover events/outbreaks in a specific geographic belt in northern Kerala and how exactly human behaviour, land use and bat ecology interactions are facilitating the virus spillovers.
“We have toyed with several hypotheses regarding the spillover mechanism. Are direct spillovers possible in areas near bat colonies , such as through droplet transmission from bat secretions or poop? Can individuals be indulging in high-risk behaviours, such as consuming bat meat (evidence of NiV in internal organs of bats has been reported)?,” said T.S. Anish, a public health expert who heads the Kerala One Health Centre for Nipah Research and Resilience, Kozhikode.
The possibility that NiV transmission to humans could be through fruits bitten by bats has remained conjecture at best because NiV has never been isolated in any fruit samples collected and tested during outbreak investigations in Kerala. NiV RNA has been detected in date palm sap in Bangladesh but live virus has never been isolated from any field samples. But the link between consumption of raw date palm sap in Bangladesh and human NiV infections is strongly supported by epidemiological and experimental evidence.
Also, as NiV is an enveloped virus, its ability to remain infectious on a fruit in an outdoor environment is extremely fragile, said Dr. Anish
Intermediate hosts?
“Our current thinking about NiV spillovers strongly leans to the fact that there is an indirect transmission route to humans, involving an indirect host. Ephrin B2, the main cell receptor used by NiV for infecting host cells is highly conserved in all mammals, which means that all animals like dogs or cattle or pigs could harbour the virus, as has been demonstrated in Malaysia and Bangladesh,” he pointed out.
None of the animal samples sent so far to the National Institute of High Security Animal Diseases in Bhopal have so far turned out to be NiV positive. However, it is also a fact that NiV research is relatively nascent and that the research institutions in the country, including the National Institute of Virology, are still in the process of developing antibody markers and testing assays specific to NiV.
PVNT platform
Thus, while Kerala’s public health response against NiV infections has been effective, the development of a novel pseudovirus neutralization test (PVNT) platform, based on a Vesicular Stomatitis Virus (VSV) system at the State’s Institute of Advanced Virology (IAV), Thonnakkal has opened up new research possibilities on Nipah.
IAV has developed a novel way of generating non-infectious Nipah virus-like particles (VLPs) as well as pseudovirions in the laboratory, which mimic the wild type NiV. These engineered “ghost viruses” carry most of the characteristics of the virus, including the NiV structural proteins G, F, and M, except their ability to replicate (because it lacks the viral genome). VLPs and pseudovirions have long been recognised as effective platforms for studying cell binding and entry kinetics of the virus.
These VLPs and pseudovirions mimic viral entry but can only undergo a single round of infection and cannot replicate and reproduce infectious viruses, making them safe to handle in standard Biosafety Level 2 (BSL-2) facilities.
PVNT-based serological survey
“We are proposing a pseudovirus neutralisation assay-based serological survey among potential intermediate animal hosts and high-risk human populations in areas of previous NiV spillovers, with the collaboration of various stakeholders, including the departments of Health, Animal Husbandry, Forests and wildlife. The proposal is to collect healthy domestic animal blood samples (cattle, pigs, goats, dogs and cats) and human samples from within a few km radius of Nipah hotspots,” E. Sreekumar, the Director of IAV, told The Hindu.
Serum samples will be tested using the IAV’s VSV-based PVNT assays. The assay measures the ability of IgG antibodies in human/animal serum to neutralise these pseudovirions, thereby preventing infection of target cells. Infectivity and neutralisation, is quantified by measuring the expression of a reporter gene incorporated into the pseudovirus genome
Positive PVNT results will indicate the potential of those animal species as intermediary hosts and such samples will be further tested against live Nipah virus in a BSL-4 facility with the help of NIV, Pune.
This study is expected to provide robust data on NiV seroprevalence in high-risk human and animal populations in Kerala and open the doors to the mystery of NiV disease spillover and transmission pattern in the State.
Futuristically, the successful validation of serological data and its integration with geo-spatial analysis will enhance Kerala’s diagnostic and surveillance capabilities for NiV, it is hoped
Published – July 05, 2025 07:24 pm IST
