When the boy fell, other visitors immediately told staff members, who responded along with Pittsburgh police. Zookeepers called off some of the dogs, and seven of them immediately went to a back building. Three more eventually were drawn away from the child, but the last dog was aggressive and police had to shoot the animal, officials say.
The zoo was closed, and it was not clear when it would be reopen, authorities said.
Authorities didn't immediately release the name of the boy or his mother, but say she is 34 years old and lives in Pleasant Hills, just outside Pittsburgh. The boy's father arrived on the scene soon after the accident, police said.
The dogs are about as big as medium-sized domestic dogs and weigh 37 to 80 pounds, according to the zoo. African wild dogs are also known as cape hunting dogs, spotted dogs, and painted wolves. They have large, rounded ears and dark brown circles around their eyes and are considered endangered.
The dogs normally live in a 1.5 acre exhibit called the Painted Dog Bush Camp that's part of a larger open area called the African Savanna, where elephants, lions and other animals can be seen. Visitors walk onto a deck that is glassed on the sides, but open in front where the railing is located. Visitors can look out at the dogs below.
Zoo officials said there is also a mesh barrier beyond the railing, but Kraus said it was designed to catch small objects such as cameras, and not humans.
In May, some of the dogs crawled under a fence and escaped into a part of the exhibit that's usually closed. The zoo was on lockdown for about an hour as a precaution.
Police and the Allegheny County medical examiner's office are investigating. Baker said the zoo, which has never had a visitor death, will also investigate. She said no decision has been made yet on the future of the exhibit.
Ten African painted dogs were born at the zoo in 2009, and their mother died of a ruptured uterus shortly after delivering the litter. Five of the pups survived. The mortality rate for painted pups is 50 percent, even when born in the wild to a healthy mother.
It was only the second litter to be hand-raised in captivity, along with one in the United Kingdom, zoo officials said at the time.
Associated Press writer Ron Todt in Philadelphia contributed to this report.