Bird flu has been on the rise in Washington state and one sanctuary was hit hard: 20 big cats – more than half of the facility’s population – died over the course of weeks.

Twenty big cats, including a half-Bengal tiger and four cougars, died between late November and mid-December at a sanctuary in Washington State after becoming infected with bird flu, according to the facility’s director.

The ongoing spread of bird flu in the United States has alarmed experts—not just because of human cases causing severe illness, but also due to troubling new instances of infections in cats.