JOHANNESBURG - A lioness sprung at the throat of a zoo worker on a rural conservation farm and killed him, apparently because security gates were left open, the Johannesburg Zoo said Tuesday.

Colleagues heard Joe Ramonetha screaming on Monday and rushed to help but it was too late, said zoo spokeswoman Letta Madlala. The 63-year-old was declared dead on arrival at the hospital, she said.

Ramonetha had been feeding the 11-year-old lioness or cleaning out her enclosure on the zoo's conservation farm in Parys, about 75 miles (120 kilometres) from Johannesburg, Madlala said.

Investigators found "It was evident that a terrible accident had occurred - gates were left open - the incident thus occurred due to human error," the zoo's executive manager for education, Louise Gordon, said in a statement later. She said there had not been a fatal animal attack at the zoo in 50 years.

"The staff are trained to close all gates on entering the facility, to let the lions out into the outside camp area, secure the entrances to the night rooms and passage before any work commenced," Gordon said. "Joe was attacked in the staff passage and sustained a fatal bite wound to his throat."

Madlala said Ramonetha had worked for the zoo since 1970 and is survived by his wife and four adult children.

Zoo managers would meet to decide the fate of the lioness called Nyanga, she said.

South Africa has about 2,700 free-ranging lions and 4,000 lions in captivity.