The world is going through its worst outbreak of bird flu.
The highly infectious 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?
Avian influenza is an infectious disease of poultry and wild birds that has existed for a century. It usually sprouts in the fall before disappearing in the spring and summer.
“It originated among ducks in Europe and Asia, and spread to other birds,” says Paul Digard, a professor of virology at the Roslin Institute at the University of Edinburgh.
The H5N1 virus, which is the most prevalent strain now, was first reported in China in 1996.
It can spread through entire flocks of domestic birds in a matter of days, through bird droppings and saliva, or through contaminated food and water.
The World Organization for Animal Health (WOAH) has recorded almost 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 culled.
In some European countries it has caused egg shortages in stores.
What is unusual about this outbreak?
This outbreak has killed more wild birds than ever before, with seabirds particularly affected.
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 aren’t sure why this outbreak is so much worse than others. It may be that the virus has mutated to allow it to spread more easily from bird to bird, or to stay longer in the environment.
Dr. Nancy Beerens, an avian flu expert at Wageningen Bioveterinary Research in the Netherlands, who analyzes samples suspected of avian flu, says the virus may now be ubiquitous in wild birds.
“As the virus has now infected many wild bird species, it is unlikely that it will disappear from the bird population again,” he says.
What is being done to address the outbreak?
China has been vaccinating its domestic bird flocks.
However, other countries avoid this because it is difficult to judge which birds have become immune and which have not, so meat and eggs from vaccinated flocks cannot be sold abroad.
“There are strict export controls when a country decides to vaccinate,” says Dr. Maurice Pitesky of the University of California, Davis, School of Veterinary Medicine.
Instead, governments in EU and North American countries have generally told their farmers to cull all 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 prevent them from becoming infected with wild birds.
Despite the commercial drawbacks of vaccinating poultry, the governments of France and the Netherlands have started vaccine trials to try to control the bird flu epidemic.
How is bird flu transmitted to mammals?
WOAH has counted 119 outbreaks of the virus among mammals, though it says this is certainly an underestimate.
Affected species include dolphins and seals, foxes and otters in the UK, brown bears in the US and mink in Spain.
Infected mammals are believed to have fed on dead or diseased wild birds that carried the virus.
There has not yet been any evidence that the virus spreads between mammals.
The virus may also have mutated to infect mammals more easily.
Is bird flu a risk to humans?
The World Health Organization (WHO) says that 870 humans have been infected with bird flu in the last 20 years and 457 have died.
These cases occurred when humans came into close contact with infected birds.
The World Health Organization says the further spread of the H5N1 virus will need to be closely monitored to see if it is mutating into a form that can spread between humans.
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