Investigations into several past outbreaks were discussed at a recent event on infectious diseases.

Presentations at the European Scientific Conference on Applied Infectious Disease Epidemiology (ESCAIDE) covered Salmonella, Listeria, Clostridium Perfringens, and botulism outbreaks.

ESCAIDE was held in November in Stockholm, Sweden, and remotely. It was organized by the European Centre for Disease Prevention and Control (ECDC).

In October 2021, the Public Health Agency of Sweden (Folkhälsomyndigheten) detected a nationwide outbreak of people infected with identical monophasic Salmonella Typhimurium strains.

Forty illnesses with onset during October were reported from eight regions with an age range of 5 to 70 years old. More women than men were sick. One region reported a cluster of nine cases linked to a preschool.

In a case-case study, researchers compared exposures of 24 outbreak patients to 47 other salmonellosis patients, reported during mid-September-November 2019 to 2021. Compared to other cases, outbreak patients were more likely to report grocery shopping at one supermarket and consumption of tomatoes and ground beef.

Fresh small tomatoes from one supermarket chain were the most likely source. They were the only item found in all five of the patients’ receipts from this retailer. Almost all outbreak cases reported eating tomatoes. Pre-school shopping receipts included tomatoes but not ground beef.

The outbreak stopped before control measures were taken, supporting the theory that the vehicle of infection had a short shelf life. Traceback by the Swedish Food Agency (Livsmedelsverket) identified suppliers of fresh small tomatoes in Italy and Spain but samples were not available for analysis.

German Listeria outbreak
Another presentation covered a Listeria monocytogenes outbreak caused by shredded vegetables in Hesse, Germany, from 2021 to 2022.

In November 2021, two cases of listeriosis were reported with hospital stays during their suspected exposure periods. Four patients belonged to the cluster from October 2021 to January 2022. The median age was 76 and a half years old.

In February 2022, the outbreak strain was isolated from a retained salad sample in one hospital. Traceback identified a manufacturer who processed vegetables for raw consumption and supplied several hospitals in Hesse. Infections in three patients could be epidemiologically linked to their stay in two hospitals supplied by the company.

The manufacturing site had hygiene deficiencies and the outbreak strain was detected in sliced vegetables such as Chinese cabbage, onions, parsley, cucumber, and leek, and environmental samples like kitchen utensils and from the floor.

Scientists recommended removing industrially pre-cut vegetables for raw consumption from the menu of vulnerable people such as hospitalized patients and more inspections in the supply chain.

Two English incidents
A separate poster detailed an outbreak of Clostridium perfringens associated with roast beef in a restaurant in South West England in January this year.

Contact details were available for 40 of 85 attendees; 31 provided information on food consumption and symptoms, and 15 were defined as cases. The median age was 47 and more men than women were sick.

The incubation period for all cases ranged between 14 and 26 hours. Symptom duration ranged from less than 6 hours to 4 days, with most being sick for 3 days.

Clinical stool and beef samples contained Clostridium perfringens. However, only one stool sample was positive for enterotoxigenic Clostridium perfringens, which is capable of causing gastrointestinal illness. Stool samples were collected between 4 and 9 days after symptom onset which reduces the chances of positive results. One sample had low levels of norovirus. Beef consumption was able to explain three-quarters of cases.

Findings were used to give food safety recommendations to the restaurant, primarily about temperature control when cooking, cooling, and storing beef.

In October 2021, researchers investigated an outbreak of Salmonella Poona in North West England. The outbreak had 13 cases between 2016 and 2021 and was discovered through routine Salmonella surveillance whole genome sequencing (WGS).

Eleven sick people were under the age of 3 and two were adults. The earliest patient in 2016 had attended one nursery while the other 10 child cases from December 2018 to September 2021 had been at another nursery at the time of infection. No other common exposures were identified. Six of 12 children were hospitalized.

Staff sampling at the second nursery identified an asymptomatic staff member who had been employed since 2018 and who tested positive for the outbreak strain. This person had also worked at the first nursery in 2016 when the first case was reported.

“This outbreak highlights the possibility of persistent carriage and shedding of Salmonella Poona and the implications of this wher individuals work with vulnerable groups, necessitating consideration of more enhanced risk management measures such as the exclusion of the individual or changing their duties until the clearance is achieved,” said researchers.

Finally, nine people were hospitalized in a botulism outbreak in Tajikistan in 2020. In 2019, there were 19 cases and three deaths in the country. Botulism is a rare but life-threatening condition caused by toxins produced by Clostridium botulinum bacteria.

The age of patients in the 2020 outbreak ranged from 6 to 44 years old and six were male.

Interviews revealed patients were from the same village in the Dangara region and all reported eating a homemade canned salad at dinner. Investigators retrieved the salad and destroyed it. Consumption of improperly preserved foods was the source of the outbreak, said, investigators.