(CNN) -- Zoo accidents don't happen often, but when they do, personnel have to be ready.
The National Zoo in Washington has regular unannounced animal escape drills. Zoo curators have emergency teams in case of a wild animal escape, according to wildlife expert Diana Guerrero. And zoos' animal environments sometimes include features such as moats or reinforced enclosures.
"We take into consideration other incidents at other facilities and review them to see if we should review our own safety protocols," said Sarah Taylor, a spokeswoman for the National Zoo.
Nevertheless, the precautions don't always work, such as in the case of the San Francisco Zoo's escaped tiger Tuesday. An escaped Siberian tiger killed a patron at a zoo cafe and injured two others before police shot the animal to death. Watch how a Christmas Day outing turned deadly at the zoo »
Also this year, a 27-year-old zookeeper at Colorado's Denver Zoo, Ashlee Pfaff, was mauled to death by a jaguar. An investigation concluded that Pfaff was at fault; she had left a door to the jaguar's enclosure unlocked and did not follow safety procedures.
In Washington last year, a 30-pound clouded leopard escaped from its brand-new mesh enclosure by digging a hole through the material. The escape was minor -- the cat was found sitting next to its enclosure before the park opened for the day -- but zoo officials responded by reinforcing the mesh.
Still, wildlife expert Jack Hanna said the public should not be alarmed.
"Nearly 145 million people a year visit zoos, and yes, accidents happen," he told CNN's "American Morning" on Wednesday. "But, you know, when you look at the numbers of people that we educate in the zoological world, it's something -- it's a real safe place to go to.
"Safety comes first in any zoological park," he added.
Guerrero, who has worked at the San Diego Zoo's Wild Animal Park and Los Angeles Zoo in California, observes that animals will do what's instinctive.
"When an animal gets out of their territory, they feel very unsafe because that's their home, so they have heightened responses," she said. "They attack first and investigate later." E-mail to a friend