Officials at Atlanta zoo says 18 of 20 gorillas at the zoo have contracted COVID

Western lowland gorilla at Zoo Atlanta in Atlanta, U.S.

Zoo Atlanta had announced the first positive tests among the western lowland gorillas on Friday after employees noticed the gorillas had been coughing, had runny noses, and showed changes in appetite. A veterinary lab at the University of Georgia returned positive tests for respiratory illness.

Officials at Atlanta's zoo reported Tuesday that at least 18 of the zoo's 20 gorillas have tested positive for COVID-19, an outbreak that began only days before the institution had hoped to get a veterinary vaccine for the primates.

The National Veterinary Services Laboratories in Ames, Iowa, has confirmed that at least four of the gorilla samples have tested positive for the delta version of the virus, according to Zoo Atlanta.

Monoclonal antibodies are being used by the zoo to treat gorillas that are in danger of acquiring viral problems, according to the zoo.

Officials say there's no evidence that the gorillas can pass the virus back to humans and visitors are too far away to be infected by gorillas.

Because the gorillas live close together in four troops, zoo officials say it's impossible to keep infected animals isolated.

Zoo officials say they believe an asymptomatic employee who cares for the gorillas passed on the virus. The employee had been fully vaccinated and was wearing protective equipment such as a mask and gloves.

"According to our guidelines for animal care staff, if they have the slightest symptoms of a cold they are to stay home," said Dr. Sam Rivera, the zoo's senior director of animal health. "It so happens that the animal care team member, the following day she developed signs that she suspected might be consistent with COVID and was tested and was positive."

Zoo Atlanta's Senior Director of Animal Health, Sam Rivera, said the gorillas will be vaccinated with a veterinarian vaccine that the zoo had been waiting for prior to the gorillas' positive testing.

In January, the virus was treated in eight gorillas at the San Diego Zoo Safari Park. All of the San Diego silverbacks recovered after being given an experimental antibody treatment.