The number of foodborne infections decreased in Sweden in 2020, according to a report on the surveillance of infectious diseases in animals and humans.

Reports of Campylobacter, cryptosporidium, E. coli, Listeria, Salmonella and Yersinia infections all declined in the past year with at least some of the dro attributed to the direct and indirect consequences COVID-19 has had on surveillance efforts and results.

Disease surveillance is driven primarily by people seeking care and during the pandemic fewer patients have gone to primary care doctors with symptoms consistent with common zoonoses. This is thought to be related to patients choosing to not seek care and a true reduction in disease incidence related to changes in general hygiene such as increased handwashing, physical distancing and reduced travel because of coronavirus related recommendations.

The report was prepared by the National Veterinary Institute (SVA) with help from Folkhälsomyndigheten (the Public Health Agency of Sweden) and Livsmedelsverket (the Swedish Food Agency).

Record Campylobacter low
The pandemic resulted in a record low of campylobacteriosis and a record high proportion of domestic infections. A total of 3,434 cases were reported, which was down from 6,693 in 2019.

A large rise in the number of people infected with Campylobacter was noticed in early August. In parallel to this, several infections were observed in abattoir employees at a large slaughterhouse. There had also been an increase in the proportion of Campylobacter positive batches of chicken from the second half of July, mainly among flocks sent to the slaughterhouse.

One reason cited for the spread among poultry flocks was dirty transport cages that carried the bacteria between chicken farms. A factor that may have made it easier for Campylobacter to gain a foothold at the farms is the practice of thinning, according to the report.

Multi-year Listeria concerns
During 2020, the incidence of listeriosis decreased compared to 2019 but the overall picture shows an increasing trend. In total, 88 confirmed cases were reported compared to 113 in 2019. Overall, 23 people died within one month from diagnosis.

Fourteen different clusters were identified of which 13 contained identical or closely related isolates already seen before 2020. One listeriosis cluster included 19 cases with identical or nearly identical isolates identified since 2014 of which eight cases were in 2020. The majority are from two counties in northern Sweden which indicates the source of transmission is a locally made food product. Additional sampling at several producers was unable to find the outbreak strain.

Another cluster included 26 cases with the same or similar isolates found since 2011 of which four cases were in 2020. The outbreak strain was found in blue cheese and ham sampled from the refrigerators of two patients in 2020, but the source is still unknown.

A rare strain of Listeria monocytogenes in Sweden caused one illness linked to a locally produced cheese. A sample of washed rind cheese was collected from the refrigerator of the person and tested positive for the outbreak strain. The cheese was made from pasteurized milk. Analysis of environmental samples from the dairy showed the site and equipment were contaminated by the outbreak strain.

Salmonella illness from domestic beef
A total of 826 cases of salmonellosis were reported, compared to 1,993 in 2019. Domestic cases decreased by 45 percent from 763 in 2019 to 422 in 2020. The sharp decrease in domestic cases and the record low incidence likely reflects changes in behavior related to the pandemic.

The most common serovars among domestic infections were Typhimurium, Enteritidis and monophasic Typhimurium. Another 55 serovars were identified during 2020.

An increasing number of Salmonella infections linked to domestic beef have been observed. Between 2019 and early 2021, 27 cases belonging to four different outbreaks involving the serovars Agona, Dublin, Düsseldorf, and Reading have been investigated.

Five cases with Salmonella Newport from late autumn were linked to an outbreak investigated by Norwegian authorities. The probable source of infection was iceberg lettuce.

only two outbreaks affected more than 10 people. One was caused by monophasic Salmonella Typhimurium, in which 17 people fell ill between September and November but the source of infection was unknown. The second was caused by the same types of Salmonella Typhimurium as were found among wild birds, cats and dogs and affected 20 people including 12 children.

Long term STEC rise
In 2020, 491 E. coli cases were reported of which 396 were domestically acquired. The long-term trend for STEC infection in Sweden is rising but the figure fell from 756 in 2019.

Shiga toxin producing-E. coli (STEC)-associated hemolytic uremic syndrome (HUS) was reported in 10 cases of which eight were domestically acquired. Six HUS cases were children younger than the age of 10.

For 56 percent of the domestically acquired STEC cases, an isolate could be serotyped. However, for the travel associated cases only 37 percent were typed. In total, 68 different serotypes were identified. The most common were O26:H11, O157:H7 and O103:H2.

One national outbreak investigation was performed during 2020 with seven cases of O103:H11 but no source could be identified.

Other pathogens
A total of 641 cases of cryptosporidiosis were reported with more than 550 infected in Sweden. This is down from more than 1,000 infections in 2019. Many cases in January were a continuation of a national increase from late 2019 wher different vegetables as sources of infections were investigated.

During 2020, 220 Yersinia infections but no outbreaks were reported. This is the lowest incidence since at least 1997 and down from 393 in 2019.

Seven cases of brucellosis were reported, which is less than the average of 13 in the past 10-year period. The low number can partly be explained by a reduction in travel abroad because of the pandemic so less imported infections. For three cases, unpasteurized dairy products were the probable source of infection.