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Two gorillas at the San Diego Zoo Safari Park have tested positive for COVID-19, and it's believed they caught the virus from an asymptomatic staff member.

This is the first known instance of COVID-19 being naturally transmitted to apes, Fox 5 San Diego reports. In a press release, San Diego Zoo Global said the two gorillas were tested last Wednesday after they started coughing, and preliminary tests came back positive. The results were confirmed on Monday by the U.S. Department of Agriculture National Veterinary Services Laboratories. A third gorilla is now symptomatic.

The asymptomatic staffer was wearing personal protective equipment while working with the gorillas, as well as following other safety precautions, officials said. San Diego Zoo Safari Park Executive Director Lisa Peterson said aside from "some congestion and coughing, the gorillas are doing well. The troop remains quarantined together and are eating and drinking. We are hopeful for a full recovery."