Researchers sequenced DNA from 373 samples from humans and animals infected with Salmonella Typhimurium DT104 over a 22-year period, mainly from Scotland.
By studying the genetic variation in Salmonella and their drug resistance genes, they found that genetic profiles of the infections in human and animal populations living side by side were more different than expected.
Mather et al reported the Salmonella bacteria largely remained within their original host populations and there were more varied combinations of drug resistance in the human-infecting bacteria, which would have been the opposite if animals had been the source.
"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, first author on the study, from the Wellcome Trust Sanger Institute.
"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."
Travel and imported food source
The team speculated that international travel and imported foods may be major sources of antibiotic resistant strains of Salmonella.
They also found that the estimated number of times that the bacteria had jumped from animals to humans (and vice versa) was remarkably low.
There was also a greater diversity in antibiotic resistance genes in salmonellae isolated from humans.
Professor Nicholas Thomson, senior author from the Wellcome Trust Sanger Institute, said: "Our data provide a very simple message, challenging the established view that local animals are the predominant source of Salmonella infections in Scotland.
“This finding will reinvigorate discussions on the sources of antibiotic resistant Salmonella infections in humans in other environments."
Previously overstated
The findings suggest that the contribution of local animal populations to human infections with S. Typhimurium DT104 may previously have been overstated, said the researchers.
Professor Stuart Reid, co-author from the Royal Veterinary College, said the findings were a surprise.
"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."
Source: Science
Online ahead of print doi: 10.1126/science.1240578
“Distinguishable Epidemics of Multidrug-Resistant Salmonella Typhimurium DT104 in Different Hosts”
Authors: A. E. Mather, S. W. J. Reid, D. J. Maskell, J. Parkhill, M. C. Fookes, S. R. Harris, D. J. Brown, J. E. Coia, M. R. Mulvey, M. W. Gilmour, L. Petrovska, E. de Pinna, M. Kuroda, M. Akiba, H. Izumiya, T. R. Connor, M. A. Suchard, P. Lemey, D. J. Mellor, D. T. Haydon, N. R. Thomson