Analysis of the main types of Salmonella in several countries showed that they are mainly associated with various sources of contamination from livestock.
The scientists assessed the contribution of multiple potential human sources of infection for five commonly reported Salmonella serovars, using machine learning methods based on a collection of genomes from humans, as well as animal and environmental sources.
This was done in 10 countries: Denmark, England and Wales, France, Ireland, Portugal, Poland, Spain, Sweden and the Netherlands. The five types of Salmonella were Typhimurium, Enteritidis, Infantis, Newport and Derby.
Between 2003 and 2021, a total of 3548 isolates were collected through routine surveillance at 12 institutes and whole genome sequencing. Non-human isolates were obtained from 17 sources. A total of 1418 Typhimurium, 984 Enteritidis, 310 Infantis, 374 Newport and 295 Derby sequences were included. According to a study published in the journal Infection logthese were clinical and non-clinical isolates.
Results by Salmonella Type
The majority of clinical human Typhimurium isolates have been attributed to pigs. Pigs were predicted to be the most common source of infection in all countries except the Netherlands, where isolates were mainly associated with broiler chickens.
Of the 208 clinical Enteritidis isolates, the majority were associated with laying hens, followed by broilers. Most isolates were attributed to sources in Poland and Spain.
Human Infantis sequences have mainly been assigned to broiler poultry. Most clinical isolates were associated with sources in Poland and the Netherlands.
The majority of Newport's human sequences were assigned to reptiles, followed by cattle and laying hens. More than 90 percent of the Portuguese isolates were attributed to cattle.
The majority of human Derby sequences were found in pigs, followed by turkeys.
Understanding the proportion of domestic Salmonella cases compared to imported sources is important to inform risk managers, identify effective interventions, and control and mitigation strategies, the scientists said.
Human cases can often be traced to countries other than their own, indicating geographic links between sources. Scientists said this shows the value of internationally agreed policies to control salmonella in the food chain.
Poland, the source of the recent Salmonella Enteritidis outbreak in the EU, had a relatively high proportion of sequences attributed to it from other countries. However, the country has provided almost 40 percent of non-clinical sequences.
National action plans for the poultry sector have reduced the prevalence and number of infections in humans, the researchers said. However, most EU countries still lack effective measures to control Salmonella in pigs and pork.
