Bird flu outbreaks are quite common, and pose a significant threat to wild birds, poultry, and – if the avian influenza virus acquires sufficient mutations – possibly even humans. Currently, the highly pathogenic H1N1 subtype is spreading at unprecedented rates in both North America and Europe.
To better understand the dynamics of bird flu outbreaks, a team of scientists led by ETH Zürich has recently investigated the epidemic caused by another subtype of avian influenza (H7N9) in China from 2013 to 2017, which not only decimated a vast number of poultry, but also killed 616 people who had close contact with infected poultry.
The experts analyzed published genetic sequences of H7N9 viruses isolated from infected poultry and humans from the metropolitan regions of Shanghai and Guangdong to build phylogenetic trees that could reveal how the disease spread at Chinese poultry markets, which play a key role in transmission between animals, as well as from animals to humans.
The analyses provided evidence that the H7N9 virus was likely circulating in poultry for several months before it was first detected in poultry markets and humans, suggesting that, between 2013 and 2016 – when the virus caused relatively few symptoms in poultry – it actually affected more markets than previously thought. As the virus mutated and started to cause more severe symptoms in poultry from 2016, the affected animals were easier to identify.
“Our findings highlight the importance of not waiting until bird flu cases are discovered, because then the virus has probably already been circulating for quite some time,” said study senior author Tanja Stadler, a professor of Biostatistics and Computational Evolution at ETH Zürich. “Instead, it would be wise to continuously monitor the health of the animals in their stalls and at the live poultry markets.”
Although some scientists hypothesized that the virus was introduced repeatedly between regions by transporting infected birds, the phylogenetic trees the researchers reconstructed indicate no such pattern, suggesting that it rather circulated silently in various poultry markets in urban regions before it was first detected.
Despite the fact that the H7N9 epidemic was restricted to China due to improved hygiene measures in poultry markets, along with a poultry mass vaccination campaign that started in 2017 – initiatives able to significantly mitigate the epidemic in animals and greatly decrease rates of transmission to humans – isolated outbreaks still occur (with the last human fatality recorded in 2019), highlighting the insidious nature of bird flu epidemics.
Moreover, since virus genomes mutate constantly, the risk of H7N9 and other influenza strains becoming once more a threat to both birds and mammals (including humans) remains. These findings stress the need for more efficient and coordinated efforts of early detection of flu infections, in order to avoid massive outbreaks that could potentially lead to a new pandemic.
The study is published in the journal Proceedings of the National Academy of Sciences.