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.

Only one severe case in the US has been found, in a woman from Louisiana who needed hospital treatment for the disease. A severe case of bird flu was also reported in a teenager in British Columbia, Canada.

A genetic analysis of the woman’s disease found that the bird flu virus – known officially as H5N1 – had mutated inside her body.