Scientists have discovered that a contaminated ingredient used in milkshakes is likely responsible for a Yersinia outbreak in an Australian state.
In January 2023, an outbreak of Yersinia enterocolitica in residential aged care facilities was identified by the Gold Coast Public Health Unit and confirmed using whole genome sequencing.
There were 55 cases, including six staff members. Sick residents ranged in age from 54 to 101, with an average of 87 years old. 41 women and 14 men were affected. The date of onset was known for 46 cases and ranged from January 16 to February 17, 2023, with a peak at the end of January. No cases went to the emergency department. One person, who was ill before the Yersinia infection, died.
There were 11 confirmed, 14 probable and 30 suspected cases of Yersinia enterocolitica. Eleven were confirmed as Biotype 1A, non-typable sequence type (ST) 27. This biotype is generally considered non-pathogenic in humans. The last meeting of the outbreak control team took place in mid-April.
“These outbreaks highlight that Biotype 1A can be pathogenic, especially in vulnerable populations, although symptoms are generally mild,” scientists said.
Probably a contaminated batch
The study, published in Communicable Disease IntelligenceEpidemiological, trace-back and laboratory studies showed that nutritional milkshakes, stored under ideal growing conditions for Yersinia and given to vulnerable residents, were the likely outbreak vehicle.
The source was believed to be a contaminated batch of an ingredient used in the milkshakes at one of the facilities. This ingredient was commonly used in the other facilities and was available to the community.
In January 2023, the Gold Coast Public Health Unit was notified of an outbreak of gastrointestinal illness at a residential aged care facility. Shortly thereafter, one case at another facility and outbreaks at two other locations were reported, all involving confirmed cases of yersiniosis. The reports coincided with an increase in yersiniosis across Queensland.
Cases of Yersinia enterocolitica had been identified in five institutions. Outbreaks had been reported at three of these locations, while the remaining two had a single patient each. All facilities involved in outbreaks were inspected by the Gold Coast Public Health Unit. No employees were working at any affected facilities.
All locations followed safe food handling techniques. There has been no recall of milkshake ingredients because no clear source of contamination was identified.
Milkshake storage
When testing cases at the first facility, Yersinia enterocolitica was identified in stool samples from multiple residents. At one location, Yersinia enterocolitica was found in prepared milkshakes with nutritional value.
Yersinia enterocolitica was isolated from 11 clinical samples and three of 113 food samples, collected from one location. Yersinia was not found in individual milkshake ingredients or in swabs.
Milkshakes were prepared in bulk and stored in jugs at 2 to 8 degrees C (35.6 to 46.4 degrees F) for 24 hours until requested by residents, then discarded when not in use. Fresh milk, powdered milk and vanilla ice cream were often used in nutritional shakes.
“The wave of Yersinia cases in Queensland declined to baseline levels in late February 2023 without specific intervention or food recall, so it was likely a contaminated batch of one ingredient that had been used up,” researchers said.
“This study shows that despite adequate handling and storage of food products, bacteria can persist and cause symptomatic illness in a susceptible population.”
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