The farm said it learned Wednesday that people who tested positive had visited earlier this month. It closed the petting zoo after being contacted by the health department. The attraction will remain closed until further notice, while the rest of the farm stays open.

“Testing is still under way, and no source of these illnesses has been confirmed,” the farm said in a statement. It said its “first thoughts are with those who are sick and their families” and wished them a quick recovery.

The farm said health officials inspected its food-service areas and found no problems. It is also arranging animal testing as investigators work to identify all possible sources of exposure, said Sean Conners, the health department’s director of environmental health.

Animals can carry E. coli without appearing sick, Conners said, but animal contact is only one possible route of transmission. The bacteria can also spread through contaminated food, water or surfaces, or from an infected person. The farm said the petting zoo had staff present, handwashing sinks with soap and running water at its entrance and exit, and pens cleaned daily.

STEC symptoms can include severe stomach cramps, diarrhea and vomiting, sometimes with a low fever. They typically begin one to 10 days after exposure, most often within three or four days, according to the health department. Thorough handwashing with soap and water can help reduce the risk of infection.

The farm encouraged anyone who became ill after visiting on or before Sept. 29 to complete the health department’s confidential survey at http://redcap.link/PumpkinFarm.