Almost 20 people are confirmed to have been infected with the E coli bacteria at the Brisbane Ekka show, but that number could still double.
So far, 19 people have tested positive to Shiga toxin-producing E coli (STEC) in Brisbane.
Of these, 11 are children under 13 and 12 confirmed they attended the Ekka.
A further 17 cases of gastroenteritis are being tested, with the results likely to be received next week.
Authorities say STEC genes were detected in the bedding of animals in the Ekka's nursery.
It strengthens the argument that animal faeces from the Ekka are the probable source of the outbreak, Biosecurity Queensland chief officer Jim Thompson says.
"Animals typically show no signs of illness and intermittently shed the bacteria in their faeces," he said in a statement.
"This highlights the importance of people practising sound hygiene measures following all contact with animals, their body fluids and excretions."