The largest ever recorded outbreak of avian influenza is ravaging poultry and wild bird populations around the world. There is a growing apprehension that it may also pose a threat to humans. On 8 February, Tedros Adhanom Ghebreyesus, head of the World Health Organization, warned that the world had to prepare for a possible bird flu pandemic, How worried should mankind be?
Tedros’ warning was inspired by the recent spread of H5N1, a subtype of influenza virus, from birds to mammals. In recent weeks, Peru has reported 585 sea lion deaths from H5N1. In October an outbreak was detected at a mink farm in Spain. In Britain the virus has been found in foxes and otters.
However, these cases do not provide proof that the virus is transmissible to humans. H5N1 is not well adapted to infect the upper respiratory tract of mammals; People contract it only after handling birds. Nevertheless, the wide range of species that can carry the disease suggests that the threat is increasing. Dr Tedros said the risk to humans is low, but added that it cannot stay that way.
Prior to COVID-19, it was widely believed that the next human pandemic would be caused by an influenza virus. The 1918 influenza pandemic, now known to have a gene of avian origin, killed around 50m people. Flu pandemics also occurred in 1957–58, 1968 and 2009. Although cases of avian-flu are rare in humans, their mortality rate can be devastating. In 1997, after a poultry outbreak in Asia, half of the 860 people infected with avian flu died.
H5N1 has not been shown to spread directly between humans. But because the virus is constantly mutating, its ability to jump from birds to humans has been evolving since the outbreak in 1997. The extent of its current spread, especially in the US, is worrying. As the virus encounters new bird and animal populations, unexpected variants may emerge.
A particular concern would be mammal-to-mammal spread. Experts have yet to confirm whether sea lions passed the virus between each other, rather than catching it from a dead, infected seabird. Mink-to-mink infection appears to have occurred at a Spanish fur farm, but the animals were culled and mammal-to-mammal transmission appears to be extremely rare.
The bird-flu vaccines that exist do not induce a strong immune response. Most are produced using an older method that involves injecting vaccines into chicken eggs. It takes time. Dr Tedros’ warning should spur interest in modernizing the manufacture of flu vaccines as well as storing antiviral drugs. It may also prompt greater surveillance of the current outbreak to provide early warning of any new signs of H5N1’s ability to spread among mammals. Or it may have evolved the ability to infect humans.
Culling of infected animals, and even vaccination of flocks of some domestic birds, can reduce outbreaks. The evolution of viruses, be it influenza or covid, is difficult to predict. But the signs of bird flu are so worrying that there is a need to plan for the worst, even if the risk appears low.
© 2023, The Economist Newspaper Limited. All rights reserved.
From The Economist, published under license. Original content can be found at www.economist.com
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