An outbreak of Cryptosporidium in Sweden in late 2023 was found to be caused by contaminated mixed kale salad from supermarket salad bars.
The outbreak was caused by a previously undetected Cryptosporidium parvum subtype. Infection with this new subtype appeared to result in a higher frequency of fever and vomiting.
In December 2023, reported cases of cryptosporidiosis increased in Sweden, prompting a national investigation into the outbreak and a case-control study to determine its source.
The research team consisted of the Swedish Public Health Agency (Folkhälsomyndigheten), the Swedish Food Agency (Livsmedelsverket) and relevant regional departments for the control and prevention of communicable diseases. The results were recently published in the journal Epidemiology and Infection.
Scientists identified 60 cases between mid-December 2023 and January 2024. The average age was 44 years, ranging from 16 to 81 years, and 73 percent were women. 13 cases have been reported from Sweden’s 21 provinces. Controls were recruited from a national random pool.
In January 2024, an alert was posted on EpiPulse about cases in other European countries. None of the five countries that responded had seen an increase in the number of reports of cryptosporidiosis.
A total of 34 cases and 368 controls were included in the analysis. Compared with the control group, the cases were more likely to have consumed products from salad bars in supermarkets. Patients had also eaten a kale mix salad more often than the control group.
It is not possible to trace back to the grower
Kale has been linked to previous outbreaks of cryptosporidiosis, possibly due to its cultivation above ground level and the challenges of thorough washing.
Of the twenty samples selected for molecular typing, thirteen were of a new subtype of Cryptosporidium parvum. The hospitalization rate of cases during the outbreak was higher than reported in a previous study of cryptosporidiosis cases in Sweden.
Traceability research identified kale producers from Sweden, Belgium and Spain, but no specific grower was identified and no food samples were available for microbiological analysis due to their short shelf life and the long incubation period of cryptosporidiosis.
One food processing company in Sweden supplied all salad products, including kale salad, to salad bars in supermarkets. This company received kale several times a week in quantities of about three to four tons. In November and early December 2023 it was produced almost exclusively by a single grower in Sweden. However, from mid-December there was a shift to deliveries from other growers in Sweden, Spain and Belgium.
Folkhälsomyndigheten posted information about the outbreak in January 2024 and provided updates until it was declared over in mid-February. The company responsible for supplying salad bars to supermarkets was contacted by Livsmedelsverket, which recommended an investigation into routine measures for washing vegetables.
“This study, together with previous studies where kale was the suspected source of contamination, indicates that despite safety procedures in the production chain, Cryptosporidium can still contaminate these products, leading to outbreaks and posing a risk to public health,” researchers said.
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