Researchers have assessed the potential reasons behind a decrease in Salmonella infections in the Netherlands during the COVID-19 pandemic.

The study looked at the impact of coronavirus on salmonellosis from January 2020 to March 2021.

Incidence of Salmonella infections declined significantly after March 2020; in the second, third and fourth quarters of 2020, and in the first quarter of 2021, it decreased by 55 percent, 57 percent, 47 percent and 37 percent, respectively, compared to the same quarters of 2016 to 2019.

The drop was highest among travel-related cases because of restrictions on international trips, according to the study published in Epidemiology and Infection.

An estimated 27,000 Salmonella infections occur annually in the Netherlands with about 70 percent caused by Salmonella types Enteritidis and Typhimurium including the monophasic variant.

Shift in source of infections and Salmonella types
Other changes included an increased proportion of cases among older adults and a larger share of monophasic Salmonella Typhimurium infections compared to Salmonella Enteritidis. This led to decreased contributions of laying hens and more of pigs and cattle as sources of human infections.

In the Netherlands, measures to control the spread of COVID-19 since mid-March 2020 included suggestions to increase hand washing, social distancing, closure of restaurants and educational institutions, restrictions on gatherings and international travel ,and use of masks indoors and on public transport.

From mid-May 2020 to mid-October 2020, some rules were relaxed. This was followed by increasingly stringent measures that put the country into a new lockdown from mid-December 2020 to March 2021.

National surveillance data included 4,788 serotyped Salmonella isolates from 4,772 patients reported during January 2016 to March 2021.

For source attribution analysis, researchers retrieved all serotyped Salmonella isolates from pigs, cattle, broiler chickens, laying hens and reptile pets collected during 2016 to 2020 by the Dutch veterinary services and private clinics as part of routine activities on animals and foods.

In the third quarter of 2020, compared to the third quarter of 2016 to 2019, the proportion of cases among the age groups 15 to 59 and 5 to 14 years old was significantly lower than that in the elderly over 60, while the proportion of male verses female cases was higher in the first quarter of 2021 than in the first quarters of 2016 to 2019.

Contributions of pigs and cattle to human cases in 2020 increased significantly by, on average, 54 percent and 26 percent, respectively, compared to 2016 to 2019. The contribution of laying hens decreased by 17 percent, reflecting the increased occurrence of monophasic Salmonella Typhimurium and the decrease of Enteritidis in 2020.

Infection with Salmonella Enteritidis in the Netherlands is more often associated with foreign travel than infection with Salmonella Typhimurium and its monophasic variant. The proportion of cases caused by the monophasic variant of Salmonella Typhimurium verses Salmonella Enteritidis increased significantly in the second and third quarters of 2020 compared to the same quarters in 2016 to 2019.

Other factors to consider
The decrease among non-travel-related cases was likely because of restrictions on gatherings, including those where food and drinks are served, such as receptions, parties and festivals. Even with takeaway and food delivery options, the shutdown of dine-in services at restaurants, pubs, cafés and bars, including catering services, reduced the exposure to Salmonella via contaminated food, according to the researchers.

Altered healthcare-seeking behavior, testing policies, diagnostic capacity and reporting compliance could have also contributed to the decreased salmonellosis incidence.

In the fourth quarter of 2020, when the second lockdown started, there was a significant rise in the proportion of salmonellosis cases with invasive infection, which are usually more severe. An increase in infections among the elderly was also observed.

Researchers said it was difficult to tell which factors contributed most to the decline in Salmonella incidence.

Changes probably reflect a combination of reduced exposure to Salmonella due to restrictions on international travel and gatherings, closure of dine-in restaurants, catering and hospitality sectors, and changes in healthcare-seeking and diagnostic behaviors, they added.

(To sign up for a free subscription to Food Safety News, click here.)