The World Health Organization says there have been increasing reports of deadly outbreaks among mammals caused by influenza viruses, including H5N1.

Dozens of tigers and lions in captivity died in the past month in southern Vietnam with tests showing they were positive for bird flu, health ministry and state media said on Thursday.

U.S. health officials have run into obstacles in their efforts to determine whether a Missouri person infected with H5N1 bird flu passed the virus on to others.

Missouri officials are under growing pressure from national public health experts to pinpoint the source of the state’s first human bird flu infection.

Chances are low that human-to-human spread of bird flu occurred in a group of people who were exposed to a patient infected with the virus in Missouri, US health authorities said.