Four children were hospitalized after visiting the Central Florida Fair, which ended in Orlando on March 13. A fifth child was stricken after going to a petting zoo at the Florida Strawberry Festival in Plant City, which also ended March 13.
The potentially dangerous kidney condition - hemolytic uremic syndrome, or HUS - is a rare complication arising from an initial infection most commonly associated with E. coli, a bacterium found in undercooked beef or contaminated food.
The children might have been exposed to the bacteria through the animals' feces, officials said.
Bill Toth, a spokesman for the Orange County Health Department, said not all the children are showing signs of E. coli exposure, and investigators were running additional tests.
Officials said three of the children have tested positive for a different bacterium - Staphylococcus aureus - that can sometimes lead to the kidney problem.
Central Florida Fair manager Charles Price said petting zoo exhibits are inspected by health officials and veterinarians.
"We have hand-washing stations everywhere," he said. "A fair today is not like it was 15 years ago. We are under extreme scrutiny."
An official of the Strawberry Festival wouldn't comment.
Officials were looking into whether a sixth child treated for the same ailment about four weeks ago also came into contact with petting zoo animals.
Last fall, 15 children developed the life-threatening kidney ailment in North Carolina, and a petting zoo exhibit at the state fair in October was determined to be the likely source. In all, 108 people, more than half of them small children, were affected by E. coli traced to the fair, though most had far milder symptoms than the 15.
About 73,000 cases of E. coli infection are reported in the United States each year; an average of 61 prove fatal.