20b - Outbreak of the novel Cryptosporidium parvum subtype IIγA11 linked to mixed kale salad from salad bars in Sweden, December 2023

Background : In December 2023, notifications of cryptosporidiosis cases increased in Sweden. Molecular typing detected a novel subtype, Cryptosporidium parvum IIγA11. Initial trawling questionnaires identified salad bars in supermarkets and leafy green vegetables as potential sources. In January 2024, we initiated a national outbreak investigation to identify the source and support public health actions.

Methods : We conducted a case-control study including cases defined as domestic laboratory confirmed cryptosporidiosis with symptom onset 15/12/2023-01/01/2024. Controls were recruited from a national random pool and frequency matched by age group, sex and county. We used an online questionnaire with inquiries about salads, vegetables, and salad bar consumption in supermarkets. We calculated adjusted odds ratios (aOR) with 95% confidence intervals (95%CI) using multivariable logistic regressions. 

Results : We identified 60 cases; median age of 44 years (range 16-81), 73% women. The case-control study included 38 cases and 368 controls. Compared to controls (8%), cases (85%) were more likely to have consumed items from salad bars in supermarkets (aOR 58; 95%CI: 22-186). In regards to food items from salad bars, cases (62%) were more likely to have consumed mixed kale salad compared to controls (32%) (aOR 3.6; 95%CI: 1.2-12). Trace-back investigations identified kale producers from Sweden, Belgium and Spain, but no particular grower was identified and no food samples were available. 

Conclusions Mixed kale salad from salad bars in supermarkets was the most likely source of infection. The detection of a novel subtype underscores the importance of adequate typing methods in surveillance and outbreak detection. It is important to understand how the contamination of kale and other leafy vegetables occurs in order to prevent future outbreaks and apply adequate preventive measures.