An analysis of major Salmonella types across several countries has revealed they are mainly linked to various livestock sources.
Scientists estimated the contributions of several potential sources of human infection for five frequently reported Salmonella serovars using machine-learning methods based on a collection of genomes from human cases, as well as animal and environmental sources.
This was done across 10 countries: Denmark, England and Wales, France, Ireland, Portugal, Poland, Spain, Sweden, and the Netherlands. The five Salmonella types were Typhimurium, Enteritidis, Infantis, Newport, and Derby.
A total of 3,548 isolates were collected from 2003 to 2021 through routine surveillance in 12 institutes and whole genome sequenced. Non-human isolates were from 17 sources. In total, 1,418 Typhimurium, 984 Enteritidis, 310 Infantis, 374 Newport, and 295 Derby sequences were included. According to the study, published in the Journal of Infection, they were clinical and non-clinical isolates.
Findings by Salmonella type
The majority of human clinical Typhimurium isolates were attributed to pigs. Pigs were predicted as the most frequent source of infection in all countries apart from the Netherlands, where isolates were mainly linked to broiler chickens.
Of 208 clinical Enteritidis isolates, most were connected to poultry layers, followed by broilers. The majority of isolates were attributed to sources in Poland and Spain.
Human Infantis sequences were mainly attributed to poultry broilers. Most clinical isolates were associated with sources in Poland and the Netherlands.
Most human Newport sequences were attributed to reptiles, followed by cattle and layer chickens. More than 90 percent of Portuguese isolates were attributed to cattle.
The majority of human Derby sequences were traced to pigs, followed by turkeys.
Scientists said understanding the proportion of cases exposed to domestic sources of Salmonella as opposed to imported sources is important to inform risk managers, target effective interventions and for control and mitigation strategies.
Human cases are often attributable to countries other than their own, indicating geographical relatedness of sources. Scientists said this shows the value of internationally harmonized Salmonella-control policies in the food production chain.
Poland, the source of a recent Salmonella Enteritidis outbreak in the EU, had a relatively high proportion of sequences attributed to it from other nations. However, the country provided almost 40 percent of non-clinical sequences.
Researchers said national action plans in the poultry sector have reduced the prevalence and human infections. However, implementation of effective Salmonella control efforts in pigs and pork are still missing in most EU countries.