The worst ever outbreak of bird flu is sweeping the world.
The highly contagious H5N1 strain is responsible for the deaths of hundreds of thousands of wild birds and millions of domestic birds. It is also found in mammals.
What is bird flu and how deadly is it?
Bird flu is a contagious disease of poultry and wild birds that has been around for a century. It tends to rise in autumn before disappearing in spring and summer.
“It came among ducks in Europe and Asia, and spread to other birds,” says Paul Digard, professor of virology at the Roslin Institute at the University of Edinburgh.
The H5N1 virus, which is now the most widespread strain, was first reported in China in 1996.
It can spread through entire flocks of poultry within a few days, through bird droppings and saliva, or through contaminated feed and water.
The World Organization for Animal Health (WOAH) has recorded nearly 42 million individual cases in domestic and wild birds since the outbreak began in October 2021.
Nearly 15 million domestic birds, including poultry, have died from the disease, and more than 193 million have been killed.
In some European countries there is a shortage of eggs in shops as a result.
What is so unusual about this outbreak?
This outbreak has killed more wild birds than ever before – and seabirds have been hit hard.
The current virus has affected 80 different species of birds,” says Professor Munir Iqbal, head of the Avian Influenza Virus (AIV) group at the Pirbright Institute.
More than 40% of the skua population in Scotland, and thousands of Dalmatian pelicans in Greece have died.
Scientists are not sure why this outbreak is so much worse than others. It is possible that the virus has mutated to be able to spread more easily from bird to bird, or to stay longer in the environment.
Dr Nancy Beerens, a bird flu expert at Wageningen Bioveterinary Research in the Netherlands, who analyzes suspected bird flu samples, says the virus may now be ubiquitous in wild birds.
“Since the virus has now infected many species of wild birds, it is unlikely that it will disappear from the bird population again,” she says.
What is being done to combat the outbreak?
China is vaccinating its poultry flocks.
However, other countries avoid this because it is difficult to judge which birds are immunized and which are not – so meat and eggs from vaccinated flocks cannot be sold abroad.
“Export controls are strict when a country decides to vaccinate,” says Dr. Maurice Pitesky from the School of Veterinary Medicine, University of California, Davis.
Instead, governments in EU countries and North America in general have told their farmers to cull all the poultry in any flock where bird flu has broken out.
Farmers in the UK and France have also been told to bring flocks of free-range poultry indoors, to stop them being infected by wild birds.
Despite the commercial disadvantages of vaccinating poultry, governments in France and the Netherlands have begun vaccine trials to try to bring the bird flu epidemic under control.
How does bird flu get to mammals?
WOAH has counted 119 outbreaks of the virus among mammals, although it says this is undoubtedly an underestimate.
Species affected include dolphins and seals, foxes and otters in the UK, grizzly bears in the US and mink in Spain.
It is believed that the infected mammals fed on dead or sick wild birds that had the virus.
There is still no evidence that the virus has spread between mammals.
The virus may also have mutated to infect mammals more easily.
Is bird flu a danger to humans?
The World Health Organization (WHO) says that 870 people have been infected with bird flu in the last 20 years, and 457 have died.
These cases occurred when people came into close contact with infected birds.
The World Health Organization says that the further spread of the H5N1 virus must be closely monitored to see if it is transforming into a form that could spread among humans.
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