Swedish pathogen stats show a large Salmonella outbreak

Sweden has revealed human infection data for 2023, including a large Salmonella outbreak that had major economic consequences and caused a shortage of eggs.

The data comes from a report published by the State Veterinary Institute (SVA) with help from the Public Health Agency of Sweden (Folkhälsomyndigheten) and the Swedish Food Agency (Livsmedelsverket).

A total of 1,316 Salmonella infections were reported, compared to 1,137 in 2022. More than half were infected abroad. Thailand was the top country of infection, followed by Turkey, Greece, and Spain. Domestic cases fell slightly.

Incidence was highest in children younger than five years old. For domestic cases, the most common types of Salmonella were Enteritidis, Typhimurium, and monophasic Salmonella Typhimurium. Another 68 different serovars were identified.

Three outbreaks with ten or more cases were found. The most prominent incident sickened 82 people and was traced to eggs produced at CA Cedergren, Sweden’s largest laying hen facility. A closely related strain of Salmonella Enteritidis caused a large outbreak in Belgium in 2022, with hundreds sick, which was also linked to a laying hen site in that country.

In late December 2022 in Sweden, Salmonella Enteritidis was detected in two flocks, and eggs from these birds were recalled. Positive flocks were euthanized, infected sites were cleaned, and biosecurity between remaining flocks was improved. Between February and October, Salmonella was detected in 14 more flocks on the farm. In late 2023, all hens on site were killed and a new round of extensive cleaning and disinfection was undertaken.

Another Salmonella Enteritidis outbreak involved chicken. Austria found the outbreak strain in chicken kebabs produced in Poland. However, these products were not served at restaurants where sick Swedish cases had eaten. Instead, trace back to work on chicken products, which pointed to Thailand as the country of origin.

The outbreak involved 13 Swedish cases and 42 from other European countries. The outbreak strain was similar to a large EU outbreak linked to breaded chicken. Swedish officials said it was likely that this strain is circulating in chicken populations in Europe and Asia.

E. coli and Listeria

939 cases of E. coli were reported, 608 of which were domestically infected. As in previous years, the incidence was highest in children under the age of 5.

A total of 20 cases of STEC-associated hemolytic uremic syndrome (HUS) were reported, of which 14 were infected in Sweden. Nine HUS cases were children under the age of 10. Of 492 typed isolates, 92 different serotypes were identified. The most common were O157:H7, O26:H11, and O103:H2.

Two people who ate cream cheese from unpasteurized milk became ill in July. Isolates from human cases could not be typed, but in a sample of cheese from the same barn, STEC O26 was found. Two international outbreaks were investigated. One was due to STEC O146:H28, with 15 sick in Sweden and cases also in Belgium and Denmark. The second was caused by STEC O157:H7, where Sweden had 11 cases, and infections occurred in the Netherlands and Great Britain. No source could be confirmed in these outbreaks.

Listeriosis was at similar levels as in 2022, with 131 cases in 2023 compared to 125 cases in the year before.

The majority of cases belonged to older age groups. In 2023, the median age was 77, and most sick people were older than 80. Forty-one people died within one month of diagnosis.

An analysis showed that 52 percent of isolates belonged to a cluster. A total of 23 different clusters were found, of which 19 contained identical or closely related isolates identified before 2023.

From May to August, 15 people fell sick in a Listeria outbreak. Patient samples were similar to those taken from four sick people during an outbreak linked to cold smoked salmon in 2022. In August, the outbreak strain was detected in product samples from the same company in Sweden that was part of the 2022 outbreak and in samples from the site and equipment at the facility.

The producer gets salmon raw material from two slaughterhouses in Norway, and one of these sent isolates from their sampling to the Swedish Food Agency. Results showed that the isolates clustered with isolates from human cases, food, and environmental samples from the outbreak, indicating that the outbreak strain entered the production facility in Sweden with raw salmon material contaminated during processing in Norway.

Other highlighted data

A total of 5,676 Campylobacter infections were reported. Travel-related cases increased compared to 2022, while domestic infections remained steady.

For domestic cases, the median age was 50, with a range of a few months to 99 years old. The incidence was highest in the 50- —to 69-year-old age group.

Swedish experts say a correlation between human infections and Campylobacter-positive broiler groups underlines the need for continued preventive measures.

In 2023, 328 cases of yersiniosis occurred, with the majority infected in Sweden. The incidence was highest among children under the age of 5. An outbreak in May included a school in Stockholm. Iceberg lettuce from Spain was the probable source of infection.

A total of 741 cases of cryptosporidiosis were noted. The median age was 35, and 59 percent were women. Due to several outbreaks, most domestically infected cases were reported in September, October, and December.

In September, 126 cases were reported from 12 regions, with almost half being from Stockholm. The investigation showed a link to domestically grown kale distributed to various restaurant kitchens. A small outbreak with four patients was linked to unpasteurized goat cheese.

Ten cases of brucellosis were reported, all of which were infected abroad. For six people, unpasteurized dairy products were the probable source of infection.

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