A Louisiana woman with the first severe case of bird flu in the US had a mutated form of the disease that may make it more infectious to humans.
Scientists believe the virus, which has spread across bird and cattle farms in the US and caused more than 60 mild human infections, could jump more easily to the public in future.
California has declared a state of emergency over the virus, which has been found in more than 600 dairy farms there over the last four months.
