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Food-borne illnesses are not always home-grown

Complex international circulation of bacteria may underpin epidemics.

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Scotland's highland cows have been badmouthed as promoters of salmonella outbreaks, but the bacteria may have come from abroad instead.

Scottish cows have a bum rap. For decades, the local cattle have been prime suspects behind the country’s outbreaks of drug-resistant, food-borne illnesses. But research now suggests that humans and imported foods are the real culprits.

A team of researchers compared the genome sequences of nearly 400 samples of diarrhoea-causing Salmonella enterica collected from people and livestock in Scotland. They found that bacterial strains infecting humans were largely distinct from those found in local cattle, but had close ties to strains that had been isolated in other countries.

The results suggest that mass epidemics may spark from a complicated intermingling of bacteria between animals and humans and from exchanges between different countries, the authors say. Their findings are published today in Science1.

“There is a pervading wisdom that local animals are a predominant source of pathogens and resistance,” says study co-author Stuart Reid, a veterinary epidemiologist at the Royal Veterinary College in Hatfield, UK. But as his team's findings show, that may not always be the case. “It’s only if we can treat this as an international issue that we’re going to get to the bottom of it,” he says.

Reid and his colleagues focused on Scottish outbreaks because of the country’s ample collection of bacterial samples obtained from both humans and livestock. The collection was started 23 years ago, when global epidemics of drug-resistant salmonella infections began to arise.

Livestock was assumed to be the source of the epidemics because animals naturally harbour the bacteria. To find out whether this was really the case, the team used whole-genome sequencing to trace the tiny evolutionary steps of the collected bacterial strains. They analysed 142 samples isolated from Scottish patients and 120 from local animals, mostly cows, then compared them with 111 strains collected from people and animals in other countries.

Foreign imports

The team found that strains infecting Scottish patients were different from those in local livestock. And they noted only a few instances in which strains isolated from local livestock had spread to humans. But they also found that strains could spread from humans to animals. “It’s occurring at a low frequency, but in both directions,” explains lead author Alison Mather, an epidemiologist at the Wellcome Trust Sanger Institute near Cambridge, UK.

When they looked at the strains' antimicrobial resistance, the researchers found that bacteria from humans had more diverse collections of resistance genes than those in local livestock. This indicates that local livestock cannot be the sole source of the resistance genes found in the strains found in humans.

The authors therefore suggest that local livestock are not the source of drug-resistant human salmonella outbreaks in Scotland. Rather, they say, foreign strains carried by other humans and in imported food probably entered the country and infected animals and humans separately, then continued to evolve and acquire resistance separately.

A global issue

The authors stress that the study does not imply that antimicrobial resistance developed on farms is less concerning than previously thought, including resistance stemming from the controversial practice of giving antibiotics in feed to promote animal growth.

"We're not saying it's not as bad, we're just saying that there are other sources that need to be considered," says Reid. Though local animals were not a main source of these pathogens, he explains that it does not eliminate the possibility that resistance genes from local farms and foreign farms played a role.

Mark Woolhouse, an epidemiologist at the University of Edinburgh, UK, says that the study clarifies how pathogens and drug-resistance genes spread. “It’s not just multi-bug, multi-drug,” he says, “but multi-country.”

Scotland imports most of its red meat, but the authors say that the country does not have adequate surveillance in place to determine whether imported food is a source of new pathogens. Both Woolhouse and the authors call for Scotland and other countries to boost the monitoring of their food supply.

Lance Price, a genomic epidemiologist at the George Washington University in Washington DC, says that it is not surprising that Scottish cattle are not the source of Scottish outbreaks, because the epidemics were international. He notes, however, that, to eliminate the possibility of a domestically derived outbreak, the authors should have analysed more strains from poultry and pigs, which also carry S. enterica.

“But it underscores that this is a global issue,” he says. “Meat sale and meat trade across borders is making it harder to control antibiotic-resistant pathogens at a local scale.”

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  1. Mather, A. E. et al. Science (2013).

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