Pig farming sent Salmonella around the world
Historical changes in pig farming led to the global spread of Salmonella, resistant to antibiotics. That was the outcome of a new international study led by researchers at the University of Warwick, United Kingdom.
Until now, the impact of intensive farming practices and global trade on Salmonella prevalence around the world and the spread of antibiotic resistance remained unclear. New research provides evidence that intensive farming practices over the last century has enabled the Salmonella bacteria to spread internationally. The overreliance on antibiotics also led to the bacteria evolving to be resistant to antibiotics – posing a huge problem for global health, the researchers say.
Salmonella enterica is a type of bacteria responsible for millions of illnesses, annually, able to contaminate food, water, and food processing facilities. It can enter the food supply chain, with pork being a major source of infection. The pathogen can “jump” from pigs to humans, causing severe, potentially fatal, illnesses. In the study, DNA was analysed from 362,931 strains of bacteria. This led to the detection of nine Salmonella populations that are abundant in pigs.
The scientists linked the expansion of the bacteria to two historical events in the twentieth century. The first is the development of intensive pig farming in the early years of that century. The second is due to the overuse of antibiotics after the 1960s. Europe and the USA contributed the most to international transmissions of the bacteria.
Global pork trade
In a press statement, Prof. Sascha Ott of the Warwick Medical School at the University of Warwick in the United Kingdom, was quoted saying: “Our study has shown how the global trade of pork has played a key role in the evolution of Salmonella, posing direct threats to food safety worldwide.”
Her university colleague, Dr Laura Baxter, added: “Our study showcases the influences of human activities on the evolution and spread of pathogens. Salmonella is not likely to be the only pathogen that has been reshaped by human agricultural practices, so we should also investigate the evolution of other pathogens.”
In the report, the researchers state: “Agricultural production has become increasingly modernised over the past half-century. On the one hand, the pattern of large-scale intensive pig farming has led to the emergence and population expansion of pig-enriched Salmonella; on the other hand, globalised trade exchanges concerning pigs have similarly increased the probability of global transmission of pig-enriched Salmonella enterica serovars or serotypes. In addition, with the use of antibiotics in the process, more and more pig-enriched Salmonella has obtained new antibiotic resistance genes, including many of the previously reported human-specific antibiotic resistance genes. The impact of the development model of modern agriculture on pig-enriched Salmonella is comprehensive and far-reaching.”
Choleraesuis
The scientists initiated the investigation in Salmonella enterica ser. Choleraesuis, a prominent serovar that has been specifically infecting both pigs and humans for over 2,000 years. Compared to previous studies that focus on either its ancient divergence or regional dissemination, they compiled a global data set of “Choleraesuis” strains to give a comprehensive overview of its recent evolution.
The team wrote: “We demonstrated the high occurrences of cross-continental transmissions with a peak in the 1950s-1980s, a period of accelerated globalisation before the use of specific vaccines. The most obvious example is between the Chinese mainland and Chinese Taiwan. Despite their geographic closeness, almost all strains on the Chinese mainland were in clade 1.2 and imported from Europe, whereas strains in Chinese Taiwan were in cluster 2.2.4 and imported from the US. We attributed this to the different trading partners between the two regions. The Chinese mainland mostly imported pork and pig breeds such as Yorkshires and Landraces from Europe during 1950-2000, whilst Chinese Taiwan traded more frequently with the United States.”
Intensive pig farming
The report also noticed the development of intensive pig farming during the beginning of the 1900s, which could increase pig-pig contacts while reducing those between pigs and humans. Such an environment could increase the chance of transmission among the pig populations while reducing spillover between the hosts, facilitating the establishment of host-enriched pathogens.
The team wrote, “Thus, we attributed the increased host jumps and population expansion of Salmonellae in the pig population to the development of intensive pig farming in the twentieth century, which is in need of further research. We showed that the vast majority of the contemporary non-human strains from these nine populations were from pig/pork, and proved that pig is the primary source of their host transfer events.
The chance that strains from these nine pig-enriched populations are transmitted by other non-human hosts is low, although this could still occur. Thus, we hypothesised that pig-related routes, including pork and pigs, the major sources of S. enterica infections were the dominant ways for the transmission of these Salmonellae.”
Major events in evolution
Prof. Ott concluded: “I think the research shows that even across long timescales we can clearly assign the major events in the evolution of Salmonella in pork to human actions. The positive reading of this is that we can be in control and influence future evolution away from highly resistant strains, which are one of the greatest risks to both animal and human health.
The 74% reduction in antibiotic use (and 99% for highest priority critically important antibiotics) in the UK pig industry since 2015, which the Veterinary Medicines Directorate (VMD) reported, is certainly good news in this context, and keeping this positive development going is very important. Going forward, further development of internationally joined-up monitoring systems would be a very positive step.”
Authors of the article in Nature Food are Heng Li, Yilei Wu, Dan Feng, Quangui Jiang, Shengkai Li, Jie Rong, Ling Zhong and Zhemin Zhou, Soochow University, Suzhou, China; Ulrich Methner, Friedrich-Loeffler-Institut, Germany; Laura Baxter, Sascha Ott, University of Warwick, Coventry, UK; Daniel Falush, Chinese Academy of Sciences, Shanghai, China; Zhenpeng Li, Xin Lu and Biao Kan, Chinese Center for Disease Control and Prevention, China; Xiangyu Deng, University of Georgia, Griffin, GA, United States; Yi Ren, Iotabiome, Suzhou, China.
