A highly contagious and deadly strain of bird flu virus has infected tens of millions of poultry in Europe, Asia, Africa and North America. But scientists are particularly concerned about the unprecedented spread in wild birds – outbreaks pose a significant risk to vulnerable species, are difficult to contain and increase the chances of the virus spreading to humans.
Since October, the H5N1 strain has caused nearly 3,000 outbreaks in poultry in dozens of countries. More than 77 million birds have been culled to curb the spread of the virus, which almost always causes serious illness or death in chickens. Another 400,000 birds other than poultry, such as wild birds, also died in 2,600 outbreaks, double the number reported during the last major wave in 2016-2017.
Researchers say the virus appears to be spreading more easily than ever among wild birds, making outbreaks particularly difficult to contain. Wild birds help carry the virus around the world, with their migration patterns determining when and where it will spread next. Regions of Asia and Europe will likely continue to see large outbreaks, and infections could spread to currently unaffected continents such as South America and Australia.
Although people can catch the virus, infections are rare. Only two cases have been reported since October, one in the UK and one in the US. But scientists fear that high levels of the virus circulating in bird populations mean there is more opportunity for it to spread to humans. Bird flu viruses evolve slowly over time, but the right mutation could make them more transmissible to humans and other species, says Ian Barr, deputy director of the influenza center working with the World Health Organization ( WHO) at the Doherty Institute in Melbourne, Australia. “These viruses are like ticking time bombs,” he says. “Casual infections aren’t a problem – it’s the gradual gain in function of these viruses” that’s the real concern, he says.
Origin of the virus
The highly pathogenic H5N1 strain emerged in commercial geese in Asia around 1996 and spread to poultry throughout Europe and Africa in the early 2000s. In 2005, the strain was causing mass deaths in birds wild, first in East Asia, then in Europe. Since then, the strain has repeatedly infected wild birds in many parts of the world, says Andy Ramey, a wildlife research geneticist at the US Geological Survey Alaska Science Center in Anchorage. Through repeated fallout, Ramey says, H5N1 appears to have become more suited to wild birds. It has “now become an emerging wildlife disease,” he says.
In 2014, a new, highly pathogenic H5 lineage – called 22.214.171.124 – emerged and began infecting wild birds without always killing them. This created opportunities for the virus to spread to North America for the first time. The lineage has since dominated outbreaks around the world, including current outbreaks.
The virus affects some wild bird species more severely than others. For example, some infected mallards (Anas platyrhynchos) show no signs of disease, while the virus has killed about 10% of the breeding population of barnacle geese (Branta leucopsis) in the Norwegian archipelago of Svalbard late last year and hundreds of Dalmatian pelicans (Pelecanus crispus) in Greece earlier this year. Wildlife researchers are trying to figure out why the virus affects species differently. They are particularly concerned about the impact of the virus on susceptible bird species with smaller populations or restricted geographic ranges, and on species particularly susceptible to infection, such as whooping cranes (American crane) and emperor geese (Anser canagicus), says Ramey.
Ramey adds that only a fraction of cases in wild birds are diagnosed and reported. Increased surveillance could reveal the true extent of wild bird mortality, he says.
Control the spread
According to Keith Hamilton, head of the preparedness and resilience department of the World Organization for Animal Health.
Tracking disease in wild birds is resource-intensive and difficult due to the sheer size of their populations, Hamilton says. It suggests targeted surveillance in areas most likely to encounter the virus, such as popular migration routes or breeding grounds.
An effective vaccine for poultry could help stem the spread, as well as dwindling bird numbers in production facilities, says Michelle Wille, a wild bird virologist at the University of Sydney in Australia. The poultry industry can also continue to improve biosecurity by limiting access to facilities, protecting their water sources and reducing contact between poultry and wild birds.
Although poultry populations can be culled to stop the spread of highly pathogenic avian influenza, researchers emphasize that wild birds should not be harmed to mitigate outbreaks. Killing wild birds to prevent new infections wouldn’t work because of the huge size and wide range of their populations, says Lina Awada, a veterinary epidemiologist at the World Organization for Animal Health. It could even make the situation worse, as it would disrupt the movements and behaviors of wild birds, helping the virus to spread further, she says.
“In the same way that we shouldn’t shoot bats because of the coronavirus, the solution to that is not to try to kill wild birds,” Wille says.
Researchers say what is needed is a holistic approach that considers how bird flu spreads through wild birds, poultry and humans. Collaboration between public health researchers and animal health groups is essential to detect events that affect people. “If we control that in poultry, we control that in humans, and it’s likely that we control that in wild birds as well,” Wille says.