Scotland’s cows are no longer the primary suspects in the decades-old epidemic of multi-drug resistant Salmonella. While livestock have been assumed to be a main source of antibiotic-resistant bacteria, a new study in the journal Science suggests that imported foods and foreign travel are likely to blame instead.

Salmonella is one of the most common bacterial pathogens to infect humans and other animals and carries a substantial public health burden. Globally, 90 million people are infected by the bacteria each year. The annual cost of these infections is about $2.7 billion in the United States and €3 billion in the European Union.

Antibiotic resistance only drives up that cost. As microorganisms evolve resistance mechanisms, existing treatments are rendered virtually ineffective, resulting in prolonged illness and increased risk of death. Hospital stays become longer, multiple treatments are often required, and patients with antimicrobial-resistant infections end up paying more for treatments.

An antibiotic resistant strain of Salmonella became a global epidemic in the 1990s. Infection was widespread, and the strain was thought to have made the jump from livestock to humans. To better understand the evolution and transmission of this outbreak, a team of researchers performed whole-genome sequencing of Salmonella samples that were collected over a 22-year period. This detailed level of analysis is able to reveal how closely related the bacteria are and provide some of the details of the epidemic.

Over 250 human and animal samples were collected in Scotland from 1990 to 2011, and an additional 111 samples were collected from other locations to provide global context.

The researchers discovered that Salmonella and its resistance genes were maintained separately in human and animal populations. Transmissions between species were few, and the number of human-to-animal and animal-to-human transmissions were similar, meaning that the majority of human infections in Scotland were unlikely to have come from animals. If local animals were the cause of human infections, the resistance genes from humans would be a subset of those from animals; they aren't.

Sources of infection were more diverse in humans than the local animal population, which helped exonerate the cows and provided the researchers with a new set of suspects: imported foods and international travel. The researchers note that further investigation is necessary before a verdict can be issued. In the meantime, the results highlight an urgent need to improve surveillance data internationally, as well as enhance cooperation between the veterinary and public health sectors. These steps will be needed to understand the sources and routes of transmission for antimicrobial resistant infections.

Science, 2013. DOI: 10.1126/science.1240578 (About DOIs).