Scientists have provided information about the types of Salmonella and people infected by the pathogen over a decade in Denmark.

They identified 9,944 Salmonella cases from 2013 to 2022. Results were published in the European Journal of Clinical Microbiology and Infectious Diseases. 

The leading non-typhoidal Salmonella (NTS) serotypes were Salmonella Enteritidis, monophasic Salmonella Typhimurium, and Salmonella Typhimurium. The median age for these cases was 42, with an even sex distribution. A third reported travel prior to the onset of the disease.

The percentage of invasive NTS infection was 8.1 percent. Eleven serotypes were associated with higher invasiveness, with Salmonella Dublin and Salmonella Panama topping the list.

After excluding more than 600 isolates due to no information on serotype, data included 206 typhoidal and 9,122 non-typhoid Salmonella isolates.

Dip during COVID-19
From 2013 to 2019, the annual incidence rate was unaltered, with 18.8 per 100,000 inhabitants, falling to 10.7 and 12.1 per 100,000 inhabitants during 2020 and 2021, respectively. By 2022, the incidence had nearly returned to pre-pandemic levels.

The prevalence of non-typhoid Salmonella cases remained relatively stable until the COVID-19 pandemic in early spring 2020, after which a notable decrease was observed. There were only 570 and 625 reported cases in 2020 and 2021.

During the study period, there was a seasonal peak in incidence during the summer, while the lowest rate was in winter and spring. However, there was a surge in January, likely due to increased travel during the holiday season.

101 cases were included, with 86 patients having two episodes, 10 patients having three episodes, and five patients having four to six episodes, all occurring more than six months apart. Some of these involved identical serotypes, and others were different.

From 2013 to 2022, Salmonella Enteritidis represented over one-fourth of all cases. Overall, 2,404 cases were caused by Salmonella Enteritidis, followed by 1,508 cases of the monophasic variant of Salmonella Typhimurium and 1,230 of Salmonella Typhimurium. Other serotypes, with more than 200 cases included Salmonella Newport, Stanley, Dublin, and Infantis.

Impact of travel-related infections
The highest prevalence of invasive infection was seen in 2013 and lowest in 2020. The most prevalent were Salmonella Enteritidis, with 169 cases; Salmonella Dublin, with 166; Salmonella Typhimurium, with 74; and monophasic Salmonella Typhimurium, with 55. Risk factors associated with invasive infection included increased age and being male.

“The reasons behind this gender difference warrant further investigation, but it may be associated with different behaviors related to food consumption and hygiene practices,” said researchers.

Travel exposure was reported in 3,250 instances, while 3,505 cases were of domestic origin with no travel, and this information was missing for 2,367 cases. 

For serotypes linked with invasive infections, a high proportion of travel-related cases was evident in Salmonella Kentucky, Salmonella Stanley, and Salmonella Java and its monophasic variant.

The travel destinations mentioned by the cases included Southeastern Asia, mostly Thailand; Europe, mainly Spain; Western Asia, such as Turkey; African nations like Egypt; Southern Asia; and Latin America and the Caribbean.

Salmonella Typhimurium and its monophasic variants and Salmonella Napoli and Salmonella Dublin were not frequently related to travel.

“These findings provide critical insights into the epidemiology of Salmonella in Denmark, including incidence trends, prevalent serotypes, and key risk factors associated with invasive illness,” said researchers.