In a new report, scientists at the University of Glasgow are urging policymakers across the globe to reconsider priorities in efforts to understand and control antimicrobial resistance.

Antimicrobial resistance in humans is often attributed to veterinary use of antimicrobials, but the relative contribution to the problem from animals and humans is poorly understood at the population level, the researchers contend. Despite this, proposals are under consideration by the European Parliament to phase out the precautionary (or prophylactic) use of some antibiotics in animals in the hope that the rate of antimicrobial resistance would be slowed. The plan has been opposed by the British Veterinary Association, which said that the ban would compromise animal health and welfare.

In line with that, the World Organization for Animal Health (OIE), which is the world body responsible for addressing animal diseases, recently warned that a ban on antibiotic use in animal agriculture would leave the world short of protein. (See OIE: Antibiotics for livestock vital to feed world.)

In the United States, the U.S. Food and Drug Administration also came out early this month indicating that "extra-label" use of some cephalosporins in food-animal production will end certain uses of the cephalosporin class of antimicrobial drugs in cattle, swine, chickens and turkeys. The order will go into effect April 5, 2012. The antibiotic will be allowed to treat sick animals and for sick animals with diseases that may not be on the label. (See FDA to end "extra-label" use of some antimicrobials; and Cephalosporin restrictions get mixed reviews.)

At the same time, FDA indicated it would back away from efforts to ban penicillin and tetracycline use in animal agriculture. “FDA is saying ‘we’re going to be more targeted; look at how to protect both animal health and human health and strike a balance,” said Liz Wagstrom, chief veterinarian with the National Pork Producers Council. “I think they are setting a path of tightening the use of these products, but not establish outright bans.”

In the University of Glasgow study, researcher Alison Mather, working with an interdisciplinary team within the college’s medical, veterinary and life sciences departments, exploited long-term surveillance data of Salmonella Typhimurium DT104 from co-located humans and animals in Scotland, demonstrated how animal and human DT104 populations differ significantly in several ways such as prevalence, linkage, time of emergence, and diversity. The findings, published in the Proceedings of the Royal Society B, suggest that the local animal populations are unlikely to be the major source of resistance in humans, and questions policies that restrict the use of antimicrobials in local domestic animals.

Professor Daniel Haydon, Director of the University of Glasgow’s Institute of Biodiversity Animal Health and Comparative Medicine, said: “In our study, there were significantly more human-only types of resistance than we might have 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.

The researchers also found that, “in the majority of resistances which are common to both animals and humans, the resistances appeared first in humans,” Haydon notes. “While it’s inevitable that contact, direct or indirect, between animals and humans will lead to some transmission of disease and resistance in both directions, it appears unlikely that the animal population is the major source of resistance diversity for humans.”

Professor Stuart Reid, the senior author of the work and now Principal at the Royal Veterinary College, London, added: “It remains true that the use of antimicrobials promotes resistance in microorganisms and of course we advocate prudent use in all species but our work does call into question the, at times, singular focus on veterinary usage.”

While the study focused on a single bacterial species, Reid says the findings demonstrate that it’s critical to ensure that “local policies do not impact disproportionately on domestic livestock without considering imported foodstuffs and animals abroad, as well as the medical use of antibiotics. There is still much to be done if we are to understand the problem at the level of the global ecosystem.”

The research was carried out by the University of Glasgow, in partnership with the Scottish Salmonella Shigella and Clostridium difficile Reference Laboratory at Stobhill Hospital; Health Protection Scotland; the Public Health Agency of Canada and the University of Guelph in Canada.

The report, entitled “An ecological approach to assessing the epidemiology of antimicrobial resistance in animal and human populations,” is available here.