Thailand ordered a cull of 40 tigers suspected of having bird flu Wednesday after seven more of the big cats died, bringing the tiger death toll at an infected zoo to 30.

“We need to cull 40 sick tigers with a potassium chloride injection,” Thawat Suntrajarn, head of the health ministry’s Department of Disease Control, told Reuters. “Otherwise they will become a reservoir for the disease.”

The cull will take place Wednesday evening to prevent the risk of further infection. “They all died after three days of showing symptoms so there is no need to keep them,” he said, referring to the first 23 tigers to die of flu-like symptoms after eating raw chicken at the Sri Racha Tiger Zoo, 50 miles east of Bangkok.

Health experts fear bird flu, which has killed 31 people in southeast Asia this year as well as domestic cats, a leopard and tigers in Thailand, may mutate to become highly infectious in humans and unleash a global flu pandemic.

Tests on the 23 dead tigers, aged between eight months and two years, showed they were infected with the H5N1 virus, Charal Trinwuthipong, an adviser to the ministry’s bird flu committee told reporters Tuesday.

Seven more died with similar symptoms Tuesday evening, he said. The zoo, which has some 400 tigers, was closed on Tuesday after tigers started dying a week ago.

As many as 20 million people are believed to have died in the 1918-1919 Spanish flu outbreak which swept through a global population of humans who had little to no immunity.

However, the World Health Organization said the deaths of the tigers had no implications for humans as tigers were not known to host the human influenza virus and thus be able to serve as a genetic mixing vessel.