Several gorillas at the San Diego Zoo Safari Park in California, have tested positive for Covid-19, in the first known instance of natural transmission to the species, officials said.

The US Department of Agriculture National Veterinary Services Laboratories confirmed the positive results on Monday. The test results confirmed the presence of SARS-CoV-2, the virus that causes Covid-19, in some of the gorillas and does not definitively rule out the presence of the virus in other members of the troop, according to the press release.

“Aside from some congestion and coughing, the gorillas are doing well,” Lisa Peterson, executive director, San Diego Zoo Safari Park, said, adding that those gorillas remain quarantined together and are eating and drinking.

Officials said it was suspected that the gorillas acquired the infection from an asymptomatic staff member, despite following all recommended precautions including Covid-19 safety protocols from public health authorities as well as wearing personal protective equipment (PPE) when near the gorillas.