New Delhi: The death of four big cats in a Nagpur zoo due to a deadly bird flu has raised fresh alarm bells amid growing concerns that avian flu virus is fast evolving with a potential to trigger a global pandemic.
The concerns grew further as reports suggested that the disease had killed a 65-year-old man with a history of exposure to personal flock of poultry and wild birds in the US Tuesday.
Three tigers and a leopard had died at the Balasaheb Thackeray Gorewada Zoological Park in late December but the reason behind their deaths was confirmed last week. They died due to infection from the Highly Pathogenic Avian Influenza (HPAI) virus, also known as Influenza A virus subtype H5N1, which can cause severe illness in birds and other animals, and is associated with a high mortality rate.
Several Indian states—including Jharkhand, Maharashtra, Andhra Pradesh and Kerala—had reported an outbreak of bird flu in poultry caused by H5N1, which has been decimating wildlife around the world for the past few years.
But it is the spread of the virus in mammals that has scientists and health authorities worried. This is because the spread of the virus to mammals would suggest that it is evolving to cross the species barrier, scientists say.
“Every mammalian infection provides an opportunity for the virus to adapt to mammalian transmission,” biologist and genome scientist Dr Vinod Scaria, who is associated with Vishwanath Cancer Care Foundation and Indian Institute of Technology (IIT) Kanpur, told ThePrint.
Infectious disease specialist Dr Ishwar Gilada told ThePrint that going by the pattern of evolution of viruses behind diseases such as Ebola, Zika, Nipah and SARS-CoV-2 in recent times, it appears clear that they are fast getting adapted to first animal-to-human transmission and then human-to-human transmission.
“Viruses could mutate for easier transmission and hence, there is an urgent need for monitoring and controlling outbreaks in the poultry population in case of H5N1,” Dr Gilada said.
The Centre last week announced that it was raising vigilance in zoos across the country.
But experts demanded that the government needs to do more, such as looking at bird flu in poultry, in non-human mammals potentially in contact with infected birds, and humans with influenza symptoms who may be in contact with them.
Symptoms of bird flu in humans include mild conjunctivitis, fever, cough, sore throat, runny nose and headache, and can even lead to death. The problem, according to experts, is that the identification and reporting of different types of influenza is limited in India.
Speaking to ThePrint, Dr Gautam Menon, professor of physics and biology at Ashoka University, said that H5N1 killing zoo animals is worrying.
While H5N1 is normally limited to birds, Dr Menon said it can move across species, infecting animals that have no natural immunity to it, as in the case of the big cats in Nagpur.
“The worry is whether it can spread among humans. Sporadic infections in humans have been reported, but almost exclusively in those in close contact with poultry. Careful surveillance is called for at this stage, looking at bird flu in poultry, in non-human mammals potentially in contact with infected birds, and humans with influenza symptoms who may be in contact with them,” Dr Menon said.
Dr Menon believes that the risk of human-to-human transmission is small, and while there is no need to panic even if humans are infected, vigilance is called for, including genome sequencing of the virus to establish its properties.
Dr Scaria underlined that genetic epidemiology of the virus through genome sequencing is necessary to ascertain the virus’s source and lineage, and to take corrective action appropriately.
Dr Gilada emphasised the need for pandemic preparedness at global, national, state and local levels all around the world.
Growing concerns
According to the US Centers for Disease Control and Prevention (CDC), avian influenza or bird flu refers to the disease caused by infection with avian influenza type A viruses whose various other lineages also cause common flu in humans.
Apart from avian flu, influenza type A viruses also cause swine flu, equine flu, canine flu and bat flu, among others. Different influenza virus genomes encode different haemagglutinin (H) and neuraminidase (N)—two distinct types of proteins on the virus.
H5N1 is an influenza A subtype that has one H protein of type 5 and one N protein of type 1. With 18 known types of H and 11 known types of N, theoretically, 198 different combinations of these proteins or subtypes of influenza A virus may be possible, Dr Gilada explained.
Avian influenza A (H5)—a type of virus that can infect birds, cows and, in rare cases, humans—has been found to be circulating among wild and domestic bird populations since 1996, when it was first detected in China.
The virus may spread to humans who are in close contact with birds, in which case it can be fatal in over 50 percent of the instances.
H5N1 2.3.4.4b is mainly carried by migratory birds such as American crows and wild birds in Egypt. An emerging lineage of the virus, it has been spreading across the globe since late 2020.
Scientists say that the transmission of bird flu viruses to humans has been inefficient so far due to the biology of these viruses, and the types of cells that line the upper and lower respiratory pathways of humans.
But, over the past three years, the frequency of transmission of the virus among birds and mammals has grown.
According to the World Health Organization (WHO), 67 countries across five continents reported H5N1 outbreaks in poultry and wild birds in 2022, with more than 131 million domestic poultry lost to death or culling in affected farms and villages.
In 2023, a total of 14 countries reported bird flu outbreaks, mainly in the Americas. Several mass death events, caused by H5N1 2.3.4.4b, have been reported in wild birds.
Between 2003 and March 2024, the WHO reported a total of 888 confirmed human cases of H5N1 infection. Of these, 463 people died.
In a bulletin issued last month on avian influenza, the WHO said that between August and November 2024, an additional 49 human cases of infection with bird flu viruses were reported.
What is even more surprising is that human cases are now also being reported in people with no history of interaction with poultry or wildlife.
According to the WHO’s December bulletin, 45 cases of H5N1 infection in humans were reported in the US last month. Of these, 28 people were exposed to H5N1-infected dairy cattle in California, and 15 were involved in depopulation of H5N1-infected commercial poultry farms.
There has been no confirmation of human-to-human transmission of H5N1 in India so far, but that is where the biggest fear lies, experts say.
(Edited by Radifah Kabir)
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