The study, which was carried out by the University of Glasgow, Scotland, used long-term surveillance data to compare the development of antibiotic resistance to salmonella typhimurium DT104 in people and animals in Scotland. It found that animal and human DT104 populations differed significantly and concluded that, “Local animal populations are unlikely to be the major source of resistance in humans.”
However, American organisation Animal Welfare Approved (AWA) said it asked Richard Young and Cóilín Nunan of the UK’s Alliance to Save Our Antibiotics to examine the research data, and their findings questioned the validity of the study.
Andrew Gunther, programme director for AWA, said: “Having looked at the analysis from Young and Nunan, some of the researchers’ decisions on what data to present is highly perplexing, to say the least.
“We have serious concerns about the way the researchers chose to draw conclusions on the role of antibiotic use in farming, and the emergence of antibiotic-resistant disease in humans. At the very least, this research paper should have been published with far more modest and balanced conclusions.”
One of the key findings of the University of Glasgow study was that of 52 antiobiotic “resistance profiles” found in the samples, only 22 were common to both humans and animals. The authors stated that this is fewer than would be expected if the animal and human microbial communities were well mixed, suggesting that “the risk of resistances passing from animals to humans is lower than previous research has indicated”.
On examining the supplementary data, however, Young and Nunan found that the 22 common profiles accounted for 2,707 of the 2,761 (or 98%) of human isolates and for 2,418 of 2,439 (99%) of animal isolates. “The overwhelming dominance of the common profiles is evidence supporting the opposite view – that the two salmonella populations are well mixed,” Young, a policy advisor at the Soil Association, told GlobalMeatNews.
Additionally, the study stated that of the 22 common resistance profiles, 11 first appeared in humans, six simultaneously in humans and animals and five first in animals. However, no information was provided on which profiles emerged first in which species. This is particularly significant, because one resistance profile alone accounted for 90% of the animal isolates and 70% of human isolates.
“It would have been instructive to know where it first emerged,” said Young. “If it appeared first in humans, then their conclusions are probably right, but it is bad science not to include this data, which they must have written down to make their assessment. If it supported their theory, then I would be surprised that they hadn’t used it.”
Another key argument of the study was that the greater diversity of resistance profiles in humans was evidence that the animal population was probably not the source of resistance in human salmonella. However, Young argued that “resistance diversity in salmonella of animal origin is likely to be increased by the use of antimicrobials in humans, particularly since human-to-human transmission of salmonella is possible. However, resistance diversity in animals is primarily due to farm-animal antibiotic use.” He added that imported food could also explain some of the resistance profiles found in humans.