In a report published today, public health investigators say they traced the outbreak to a lunch truck serving food contaminated by the eggs.
The incident, in late 2010 and early 2011, resulted in 91 confirmed Salmonella Enteritidis infections in Alberta, most of which were in Calgary.
The investigators do not list the name of the catering company that sold food through the truck, but they say it bought eggs from an illegal supplier.
The report on the investigation is in this week's issue of Morbidity and Mortality Weekly Report, which is published by the U.S. Centers for Disease Control.
The implicated catering operation's food handling practices were not exactly up to code. Investigators found a bucket used for mixing pooled eggs during the preparation of breakfast sandwiches had not been cleaned for several weeks.
"Numerous sanitation, food-handling and employees hygiene violations were identified," wrote the public health officials, from Alberta Health Services and the Public Health Agency of Canada.
The caterer was prosecuted for breaching Alberta public health legislation and an illegal egg supplier identified through the investigation was eventually fined and jailed for 14 days.
Testing of foods produced by the caterer found Salmonella on pork dumplings and in raw egg mixture taken from the egg bucket. The dumplings were not produced in-house, and the investigators concluded they were contaminated by food handlers at the catering operation.
A note written by the journal editors explained the likely scenario: "Salmonella probably entered the facility via contaminated eggs, with infected food handlers and environmental contamination resulting in transmission to customers."
The journal said the outbreak highlights the potential for lunch trucks to be a source of foodborne illness and the need for health departments to run robust inspection programs both for lunch trucks and their suppliers.
According to the CDC, Salmonella infection can produce fever, abdominal cramps, and diarrhea beginning 12 to 72 hours after consumption of contaminated food or beverages. Symptoms can last four to seven days.
While most people recover without the help of antibiotics, the diarrhea can be severe and some people need to be hospitalized.