Kozhikode: An ongoing study by the National Institute of Virology (NIV) in Pune found the footprint of Nipah virus in bats in 10 of the 16 states and Union Territories surveyed in India.

The study found Nipah viral antibodies in Kerala, Tamil Nadu, Karnataka, Goa, Maharashtra, Bihar, West Bengal, Assam, Meghalaya and Puducherry. 

Yet, only Kerala is seeing repeated Nipah viral infection among humans, almost annually. The 2018 outbreak killed 17 of the 18 persons who were confirmed to have been infected by the viral disease in Kozhikode. In 2019, one case was reported in Ernakulam. In 2021, a 12-year-old boy died of encephalitis, a symptom of Nipah viral infection, in Kozhikode. This year, two persons succumbed to the viral infection, and five others are battling for their lives.

Dr Pragya D Yadav, Group Leader of the Maximum Containment Laboratory at NIV who heads the survey, said there are two reasons for the repeated human infection in Kerala.

"Not just Kerala, in states along the Western Ghats, human habitations are either close to the forest or within the forests," she told Onmanorama.com, implying that proximity to forest areas could increase human exposure to infected bats and other host animals. In the North, forested areas are far from human habitations, she said.

But that is not the only reason. Kerala could be reporting Nipah virus infections because of better surveillance, said Dr Yadav, a senior scientist at the national lab run by the Indian Council of Medical Research (ICMR).

Without adequate surveillance, human infections can go unreported. "We don't know if it is causing infection in other states. A certain level of monitoring and surveillance is required to pick up every case," she said, and added: "Hospital-based surveillance should be increased in other states as well".

Prof Rajendra Pilankatta, an expert in the core committee of the Institute of Advanced Virology, Thiruvananthapuram, went one step ahead and called for a retrospective review of all fever deaths in similar zones along the Western Ghats. "Fever is a symptom, not the cause. Analysis of these unexplained fever deaths can be the start for better surveillance," said Pilankatta, professor at the Central University of Kerala's Department of Biochemistry and Molecular Biology.

The other states where the survey was done but did not find antibodies in bats were Telangana, Gujarat, Punjab, Himachal Pradesh, Odisha, and the Union Territory of Chandigarh.

Kerala strain is different from Bangladesh, but no comparative study done

Dr Yadav said the Nipah virus strain found in Kerala was different from the genotypes found in Bangladesh (B) and Malaysia (M), the two major strains with common descent. She called the one found in Kerala the 'I' genotype.

When the first Nipah outbreak happened in India in Siliguri district in 2001 and then in Nadia district in 2007, the strains were more like the Bangladesh strain, she said.

"But the sequence we received from bats and humans in Kerala has some difference," she said. The dissimilarities ranged between 14.86 percentage points for the Malaysia genotype and 3.85 percentage points for the Bangladesh genotype, said her study published in 'Emerging Infectious Diseases' journal. Though the dissimilarities with the Bangladesh strain are less, the Kerala Nipah virus strain forms a separate cluster, it said.

In the same journal, researchers from Australia published their findings on why the Bangladesh strain was more lethal than the strain found in Malaysia.

Nipah virus spreads from person to person in Bangladesh but not in Malaysia, according to the study led by Glenn Marsh, Principal Research Scientist in the Dangerous Pathogens Team at the Australian government's Commonwealth Scientific and Industrial Research Organisation (CSIRO).

When they infected ferrets, they found higher concentrations of the Bangladesh strain in secretions from the mouth. "Increased oral excretion of the Bangladesh strain in humans might explain why person-to-person transmission of Nipah virus occurs in that region," the study said.

No such comparative study has been done with the Indian genotype, said Dr Yadav."We don't have the virus 'isolate' from Bangladesh or Malaysia. So we cannot compare," she said. (An isolate is the virus isolated from an infected host and propagated in culture.)

Without the comparative study, scientists cannot tell if there is any significant difference in symptoms and pathogenicity, that is the ability of an organism to harm its host or cause disease. "We need to know the differences and how the differences impact therapeutic goods," she said.

Dr Yadav said such pieces of information were needed for making vaccines and monoclonal antibodies, which are made-in-lab proteins meant to stimulate the immune system.