LOS ANGELES, Jan. 11 (Xinhua) -- Several gorillas at the San Diego Zoo Safari Park in California, the United States, have tested positive for COVID-19, in the first known instance of natural transmission to great apes, zoo officials said on Monday.

Officials of the San Diego Zoo Global, which operates the San Diego Zoo and the San Diego Zoo Safari Park, said in a press release that they initiated the process of testing fecal samples from the gorillas through the California Animal Health and Food Safety Laboratory System after two of the gorillas began coughing on Jan. 6.

The U.S. 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," said Lisa Peterson, executive director, San Diego Zoo Safari Park, adding that those gorillas remain quarantined together and are eating and drinking.

Officials noted that it's suspected 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.

"Research studies have verified that some non-human primates are susceptible to infection with SARS-CoV-2, but this is the first known instance of natural transmission to great apes and it is unknown if they will have any serious reaction," officials said.

The San Diego Zoo Safari Park, a major tourist attraction in the county, has been closed to the public since Dec. 6, 2020 under regional stay-at-home order in response to the COVID-19 surge.

There have been reports of animals being infected with COVID-19 worldwide. U.S. Centers for Disease Control and Prevention (CDC) noted on its official website that there is risk of people spreading the virus that causes COVID-19 to animals and the first U.S. case of an animal testing positive for COVID-19 was a tiger at a New York zoo.