To help manage the expanding zoo, players can employ maintenance workers, zookeepers and tour guides. Bird owners, pet shop employees, zookeepers, and veterinarians are at risk of the infection. As of 2015, the zoo had four full-time zookeepers, one part-time zookeeper, and several volunteers. Dingle's wife Muffye operated the zoo with seven full-time zookeepers. JAZA started conferences for veterinarians and zookeepers in 1953, and for aqarists in 1956. The zoo holds various youth-directed programs, including Junior Zookeepers, birthday parties and holiday events. Wilhelma receives gorilla juveniles rejected by their mother and reared by the zookeepers. Some zookeepers train the animals to make caring for them easier. Animal collections requiring wild animal care takers or zookeepers have existed since about 3,000 B.C. Sometimes, when animals are unhappy in their exhibits, they can attack the zookeepers. He was rescued by zookeepers with relatively minor injuries. Zookeepers were initially stumped over how Ken Allen had managed to escape. Batyr died in 1993 when zookeepers accidentally gave him an overdose of sedatives. Zookeepers lured him down the tree once the park closed that day. Rejected by his mother at birth, he was raised by zookeepers.