Dozens of tigers and lions in captivity died last month in the south Vietnam and tests showed they were positive for bird flu, the Ministry of Health and state media announced today.

Two samples from dead tigers at the Mango Garden Resort in Dong Nai province tested positive for the H5N1 strain of bird flu, the health ministry said in a statement.

Twenty tigers have died in this shelter since the beginning of last month.

The spread of highly pathogenic avian influenza to mammals, including cows, dogs, cats and even dolphins, around the world has raised concerns about possible human-to-human spread of the virus.

In neighboring Long An province, 27 tigers and three lions died of bird flu at My Quynh Safari Park from September 6 to 18, state media reported, citing the province’s agricultural authorities.

The tigers at Mango Garden Resort they had been fed chicken before they died, Fan Van Fook, an official at the Dong Nai Provincial Center for Disease Control, said in a health ministry statement.

“It is possible that the tigers were infected by a sick chicken, and authorities are tracing the source of the chicken to determine the cause,” Fan said, according to the statement.

On its website, the World Health Organization warns against eating raw or undercooked meat and eggs from regions where there have been bird flu outbreaks because of the high risk of infection.