Bird flu has killed 20 wild cats after it infected more than half of the animals living at a wild cat sanctuary in Washington state.

"We've lost 20 cats due to this virus or symptoms thereof," said Mark Mathews, director of the Wild Felid Advocacy Center. "We think hopefully it's a bad dream, but it's not."

The cats, including a number of cougars, bobcats, Eurasian servals and an Amur/Bengal tiger, died in November and December, according to a statement from the center.

The facility is now quarantined and closed to the public while the staff works with health officials and veterinarians to protect the remaining cats.

The Washington Department of Fish and Wildlife warned earlier in December that it had detected bird flu infections in wild birds and other animals. At the time, it noted that there was no evidence that the virus was being transmitted from mammal to mammal. Instead, it's likely that cases appeared after mammals scavenged on infected birds.

At this time it's still not clear how the sanctuary cats contracted the virus.

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Concern over bird flu infections is growing nationwide after a house cat in Oregon died after eating cat food found to be contaminated with bird flu. Nearly 900 cattle herds across the U.S. have also tested positive for bird flu.

Iowa health officials reported the state's first human case in December. As of Dec. 23, the CDC has calculated 65 confirmed cases of bird flu in humans in the U.S. The agency maintains the public health risk is low.