Bird flu poses a threat that is “unique and new in our lifetime” because it has become a “‘panzootic” that can kill huge numbers across multiple species, experts warn. For months, highly pathogenic bird flu, or H5N1, has been circulating in dairy farms, with dozens of human infections reported among farm workers. It has now jumped into more than 48 species of mammals, from bears to dairy cows, causing mass die-offs in sea lions and elephant seal pups. Last week, the first person in the US died of the infection.
This ability to infect, spread between, and kill such a wide range of creatures has prompted some scientists to call H5N1 a “panzootic”: an epidemic that leaps species barriers and can devastate diverse animal populations, posing a threat to humans too. As shrinking habitats, biodiversity loss and intensified farming create perfect incubators for infectious diseases to jump from one species to another, some scientists say panzootics could become one of the era’s defining threats to human health and security.
Panzootic means “all” and “animals”. “Panzootic is almost a new thing, and we don’t know what sort of threat it is,” says Prof Janet Daly from the University of Nottingham. “We have some viruses that can infect multiple species, and we have some viruses that can cause massive outbreaks, but we haven’t tended to have the combination – that’s something of a new phenomenon … That’s where H5N1 is going, and it just makes it so unpredictable. [It’s] unique and new in our lifetime and memory.”
Ed Hutchinson from the MRC-University of Glasgow Centre for Virus Research says: “It is really hard for infectious diseases to effectively stop being specialists and move over into a new species. So when that happens, it is striking and concerning.”
The impacts on biodiversity can be huge. Bird flu has led to “catastrophic” declines in seabird populations, with millions of wild birds killed. More than 20,000 South American sea lions have died in Chile and Peru and an estimated 17,000 southern elephant seal pups have died in Argentina – equivalent to 96% of all pups born in the country in 2023.
The risk of bird flu spreading among humans is an “enormous concern”, UN health authorities have warned. Most recently, a man in Louisiana died after being exposed to a combination of a non-commercial back-yard flock and wild birds. Since March last year, 66 confirmed bird flu infections in humans have been reported in the US, but previous cases have been mild. So far, there is no evidence it is spreading between humans, and that is what experts are keeping a close eye on.
Three-quarters of emerging diseases can be passed between animals and humans. This matters in terms of mapping them and protecting people, researchers say.
Some researchers argue that Covid-19 could be an example of a panzootic because it has infected more than 58 non-human species, including deer, mink and even snow leopards (although unlike bird flu, it does not effectively spread between them and kill them, so does not fit the traditional definition).
“We are being overwhelmed by the number of animal species which are susceptible to [Covid-19] infection,” researchers have said.
Identifying the virus that causes Covid-19 as a potential panzootic could have resulted in active surveillance in animals, researchers say, and the earlier development of vaccines.
Scientists also warn that cross-species pandemics are on the rise. “There is a fair body of work now demonstrating that most human viruses are zoonotic (ie of animal origin),” says Michelle Wille, a senior research fellow at the Centre for Pathogen Genomics at the University of Melbourne.
Mark Honigsbaum, a medical historian and author of Pandemic Century, says: “Why are they becoming more frequent? Well, the simple answer is, it’s because of the way we humans settle and colonise larger and larger areas of the planet.”
Biodiversity loss is the leading driver of infectious disease outbreaks, as habitat decline and industrialised farming place people in close proximity with other species. Humans have already transformed or occupied more than 70% of the world’s land. Since the 20th century, the most significant driver of this transformation of the way we use land has been the “livestock revolution”. The number of food animals and the amount they produce has increased rapidly to feed growing populations.
Deforestation and climate breakdown also force humans and animals into close contact, as wildlife is pushed into smaller spaces. In addition, as the climate warms, mosquitoes, midges and ticks are expanding their geographical ranges.
The more pathogens there are in animal populations, the greater risk there is of humans being exposed to them. Honigsbaum says: “We are going to see more of these outbreaks, and it’s only just a matter of time before one of them causes another pandemic.”
“I don’t see those risks going away,” says Hutchinson, but adds that the risks could be reduced. “We increasingly have an understanding – if not a fantastic, practical setup – of what we could do to start reducing some of those risks. That’s my attempt at a moment of hope.”
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