Scientists have discovered that a gene present in humans is preventing most avian flu viruses moving from birds to people. This gene can be found on all human beings and is located in the upper respiratory tract and lungs, which are where influenza viruses reproduce. Scientists knew about the antiviral properties of this gene, but it was a recent discovery.
A six-year investigative study led by the MRC-University of Glasgow Centre for Virus Research found that the BTN3A3 gene is a powerful barrier against most avian flu viruses.
Although relatively rare, some avian flu virus strains have periodically spilled over into humans. Two of the most recent H5N1 cases were reported in UK poultry farm workers in May this year. Human cases of H5N1 bird flu were first found in Hong Kong in 1997. Globally since 2003, 873 human H5N1 infections have been reported to the World Health Organization. Of those, 458 people died.
The study found that some bird and swine flu viruses have a genetic mutation that allows them to escape the blocking effects of the BTN3A3 gene and infect people. By tracking the history of human influenza pandemics and linking resistance to the gene with key virus types, the researchers concluded that all human influenza pandemics, including the 1918 Spanish flu and the 2009 swine flu pandemics, were a result of BTN3A3-resistant strains.
The findings suggest that resistance to the gene could help determine whether flu strains have human pandemic potential or not. That may lead to testing of wild birds, poultry and other animals susceptible to flu viruses such as pigs, for BTN3A3-resistant viruses.
“The antiviral functions of the [BTN3A3] gene appeared 40m years ago in primates,” said Prof Massimo Palmarini who led the study and is director of Glasgow’s virology research centre. Understanding the barriers to avian influenza in humans will allow for better-targeted solutions, and control measures that prevent spillovers.
He said that if a gene-resistant virus was identified, “we can direct preventative measures to those viruses, sooner, to prevent the spillover [into humans]”.
Dr Rute Maria Pinto, the study’s lead author, said: “It’s pretty amazing, isn’t it? We are all fairly proud about the outcome. This gene had been identified before, and other functions were attributed to it, but we found that the gene is antiviral against avian flu. No one had found that before.”
She said the discovery was expected to have immediate practical applications. “Now, when we find cases of bird flu, we can basically swab sick birds, carcasses or faeces and find out whether the virus can overcome the BTN3A3 gene, simply by looking at its sequence and determining if this virus is more or less likely to jump into humans. If the virus can in fact overcome BTN3A3 then stricter measurements should be put in place to prevent spillovers.”
Pinto added: “As a scientist we are expected to find solutions to problems, to give back to the taxpayers that fund this [research], and to find something that can be used straight away is really impactful.”
The study was also able to retrospectively identify increases in gene-resistant viruses ahead of previous human spillovers.
Before the H7N9 bird flu virus outbreak, which first infected humans in China in 2013, the study found “an increased frequency of the BTN3A3-resistant genotype [in birds] in 2011-2012”. To date there have been almost 1,570 cases of H7N9 and at least 616 deaths.
“I guess what these observations provide is a snapshot of what the frequency of the BTN3A3 resistant gene was at different period of times in the birds,” Palmarini said. “Because birds haven’t got BTN3A3, there is no evolutionary pressure to keep, or not, those traits.”
He said certain mutations “might make the virus more or less fitter in the birds” but this had not yet been studied.
Similarly, before the 2009 H1N1 swine flu pandemic, the study found H1N1 gene resistance in pigs occurring between 2002 and 2006. The swine flu pandemic is estimated to have killed between 150,000 and 575,400 people during the first year the virus circulated in humans.
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