CHICAGO (Reuters) - The strain of bird flu that infected a chicken farm in Tennessee in recent days shares the same name as a form of the virus that has killed humans in China, but is genetically distinct from it, U.S. authorities said on Tuesday.
The U.S. Department of Agriculture identified the strain in Tennessee as H7N9, following a full genome sequencing of samples from the farm. It said all eight gene segments of the virus had North American wild bird lineage.
On Sunday, the USDA confirmed the farm in Tennessee was infected with highly pathogenic bird flu, making it the first case in a commercial U.S. operation in more than a year.
In China, at least 112 people have died from H7N9 bird flu this winter, Xinhua news agency said on Friday.
However, that virus has Eurasian lineage, U.S. flu experts said.
"Even though the numbers and the letters are the same, if you look at the genetic fingerprint of that virus, it is different," said Dan Jernigan, director of the influenza division at the National Center for Immunization and Respiratory Diseases at U.S. Centers for Disease Control and Prevention.
Jernigan said the risk to humans from the virus found in Tennessee is low. Genome sequencing shows the H7N9 virus did not have genetic features present in the virus in China that make it easier for humans to become infected, he said.
The virus found in Tennessee likely mutated to become highly pathogenic from a less dangerous, low pathogenic form, he said.
Disease experts fear a deadly strain of bird flu could mutate into a form that could be passed easily between people and become a pandemic.
Multiple outbreaks of the virus have been reported in poultry farms and wild flocks across Europe, Africa and Asia in the past six months. Most involved strains that were low risks for human health, but the sheer number of different types, and their simultaneous presence in so many parts of the world, has increased the risk of viruses mixing and mutating - and possibly jumping to people, according to disease experts.
China's Center for Disease Control and Prevention has said the majority of people infected by H7N9 in China reported exposure to poultry, especially at live markets.
Identifying the viruses in Tennessee and China both as H7N9 is similar to having two cars from different states with the same license plate number, said Carol Cardona, avian flu expert at the University of Minnesota.
The strain in Tennessee "is NOT the same as the China H7N9 virus that has impacted poultry and infected humans in Asia," the USDA emphasized in a statement.
"While the subtype is the same as the China H7N9 lineage that emerged in 2013, this is a different virus and is genetically distinct from the China H7N9 lineage," the USDA added.
U.S. officials are working to determine how the Tennessee farm, which was a supplier to Tyson Foods Inc, became infected. All 73,500 birds there were killed by the disease or suffocated with foam to prevent its spread.
Tyson, the world's biggest chicken company, is "hopeful this is an isolated incident," spokesman Worth Sparkman said.
Authorities have not identified the name of the farm or the town in Lincoln County, Tennessee, where it is located.
Editing by Matthew Lewis and Bernard Orr