Middlesex-London Health Unit officials have identified the issue they believe caused dozens of people to become sick.
Almost 60 people were ill after attending a Ducks Unlimited fundraising dinner at the Portuguese Canadian Club in Strathroy.
Analysis of samples from the food and patrons points to the clostridium perfringens bacteria as the culprit.
The bacteria was found in a sample of roast beef from the event.
According to the Public Health Agency of Canada the bacteria appears when food is cooked slowly, cooled too slowly, kept too warm in storage or no re-heated to a high enough temperature.
The health unit is continuing to notify those who attended the dinner on March 30.
