Of the 63 people who attended the banquet, 18 developed strep throat less than three days later. When multiple posts appeared on the dance team's Facebook page about ill team members and relatives, a parent got in touch with the state health department.
Health officials interviewed about 100 people by telephone, including those who attended the banquet, people in the homes of attendees and those who did not attend but ate banquet leftovers.
Investigators also conducted DNA analysis of strep bacteria samples taken from the throats of people who became ill. These steps narrowed the possible source of the outbreak to cooked pasta served at the banquet, according to the report published online July 18 in the journal Clinical Infectious Diseases.
"We suspect cooked food was contaminated by respiratory droplets from a person who carried the strep bacteria in the throat when the food was cooling or reheating," lead author Dr. Sarah Kemble, of the Minnesota Department of Health, said in a journal news release. "The food probably was not kept hot or cold enough to stop bacterial growth."
Both the parent who prepared the pasta and a child in the same household reported having strep throat three weeks before the banquet.
"Foodborne illness is not limited to diseases that cause vomiting and diarrhea," Kemble noted.
The rapid communication possible among a large group of people using social media played an important role in the investigation. A more formalized use of social media for disease surveillance and outbreak investigations could benefit public health in some cases, the authors said.
The U.S. National Institute of Allergy and Infectious Diseases has more about strep throat.