Scientists have found a contaminated ingredient used in milkshakes was likely to blame 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. Sick residents ranged in age from 54 to 101 with a median of 87 years old. 41 females and 14 males were affected. Date of onset was known for 46 cases and ranged from Jan. 16 to Feb. 17, 2023, with a peak in late January. No cases went to the emergency department. One person, who was ill prior to Yersinia infection, died.
There were 11 confirmed, 14 probable, and 30 suspected Yersinia enterocolitica cases. Eleven were confirmed as Biotype 1A non-typable sequence type (ST) 27. This biotype is generally considered to be non-pathogenic in humans. The final outbreak control team meeting was in mid-April.
“These outbreaks highlight that Biotype 1A can be pathogenic, particularly in vulnerable populations, although with generally mild symptoms,” said scientists.
Likely a contaminated batch
The study, published in Communicable Diseases Intelligence, revealed that epidemiological, trace-back and laboratory investigations identified nutritional milkshakes, stored at ideal growing conditions for Yersinia and given to vulnerable residents, as the likely outbreak vehicle.
The source was thought to be a contaminated batch of an ingredient used in the milkshakes at one of the facilities. This ingredient was commonly used at the other facilities and available to the community.
In January 2023, the Gold Coast Public Health Unit was informed of a gastrointestinal illness outbreak at a residential aged care facility. Soon after, one case in another facility and outbreaks in two further sites were notified, all with confirmed cases of yersiniosis. The notifications coincided with an increase in yersiniosis across Queensland.
Five facilities had cases with Yersinia enterocolitica. Three of these sites had outbreaks declared, while the remaining two each had a single patient. All facilities implicated in outbreaks were inspected by the Gold Coast Public Health Unit. No employees worked across all affected facilities.
All locations were following safe food handling techniques. A product recall of milkshake ingredients did not occur as no clear source of contamination was identified.
Milkshake storage
Testing of cases at the first facility identified Yersinia enterocolitica in stool samples of multiple residents. Yersinia enterocolitica was detected in prepared nutritional milkshakes at one site.
Yersinia enterocolitica was isolated from 11 clinical samples and three of 113 food samples, from one site. Yersinia was not found in individual milkshake ingredients or from environmental 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 and then discarded if not used. Fresh milk, milk powder, and vanilla ice cream were commonly used in nutritional shakes.
“The wave of Yersinia cases in Queensland reduced to baseline levels by the end of February 2023 with no specific intervention or food recall, so was likely to be a contaminated batch of one ingredient that had been depleted,” said researchers.
“This investigation identifies that, despite adequate handling and storage of food products, bacteria can persist and cause symptomatic illness in a susceptible population.”
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