Study: Local livestock may not be key contributors to Scottish Salmonella outbreaks



A new study may partially exonerate Scottish cattle as the culprits behind outbreaks of resistant Salmonella, as they long have been suspected as reservoirs for drug-resistant strains of the foodborne bacteria.

The findings, which were published in Science, indicate the source of several Salmonella outbreaks in Scotland may be more linked to imported foods and humans rather than the local bovids, which frequently were blamed for the illnesses. During the study, researchers compared genome sequences from 373 samples of illness-causing Salmonella Typhimurium DT104 sourced from humans and animals. They found the strains garnered from humans were mostly different from those taken from area livestock; however, the strains often were very similar to strains isolated in other countries, Nature reports.

The samples had been collected throughout a 22-year period, with most coming from Scotland. Analyses showed that not only are the Salmonella populations in humans and animals distinct from one another, but the number of times the bacteria had moved between the two was low. Humans also exhibited more diversity in Salmonella genes that contributed to antibiotic resistance, according to a statement from the Royal Veterinary College.

“For the first time we’ve determined in detail and on a large scale how Salmonella strains taken from humans and animals in the same setting and over the same time period relate to each other,” said Dr. Alison Mather of the Wellcome Trust Sanger Institute and first author on the study. “Our genomic data reveal how the Salmonella bacteria spread during the course of a long-term epidemic. We found that people have a more diverse source of infection and antibiotic resistance than just the local animals, pointing towards alternative sources.”

Although more research is needed, the team has postulated that imported food and international travel may be “major sources of antibiotic strains of Salmonella”, the statement read.

“Discovering that the animal and human populations of Salmonella were as distinguishable as they were was a great surprise to us,” said Professor Stuart Reid, co-author from The Royal Veterinary College, in a statement. “This finding in no way undermines the importance of prudent antimicrobial use in all species. But our study does demonstrate that greater effort needs to be focused on understanding the natural history of the pathogens and on identifying the major sources of resistance in our global ecosystems.”

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