A recent study has reportedly used whole genome sequencing (WGS) to retrospectively trace the transmission of methicillin-resistant Staphylococcus aureus (MRSA) from animal to human for the first time. Ewan Harrison, et al., “Whole genome sequencing identifies zoonotic transmission of MRSA isolates with the novel mecA homologue mecC,” EMBO Molecular Medicine, April 2013. According to a March 25, 2013, University of Cambridge press release, U.K. and Danish researchers used WGS to examine two separate cases of MRSA infection in Danish farmers and their animals. The results evidently showed that the MRSA strains under investigation carried the novel mecC gene, which allowed researchers to compare the human infections with those found in the livestock and determine that animals were most likely the source of the new strains.
“Having found this new MRSA in both people and animals on the same farm it was likely that it is being transmitted between animals and people. By looking at the single differences in nucleotides, or SNPs, in the DNA sequences of each isolate, it became obvious that in both farms we looked at the human and animal MRSA were almost identical,” said the study’s lead author. “In one case, the results also clearly showed that the most likely direction of transmission was from animal to human.”
According to the authors, their use of WGS not only clarified that MRSA can travel from animal to human—as opposed to the reverse—but raised questions about the use of antibiotics in livestock production. “Our findings demonstrate that the MRSA strains we studied are capable of transmission between animals and humans, which highlights the role of livestock as a potential reservoir of antibiotic resistant bacteria,” said one of the researchers.
Meanwhile, former Food and Drug Administration (FDA) Commissioner David Kessler authored a March 27, 2013, op-ed article urging the agency and Congress to implement a comprehensive system for tracking antibiotic use in livestock production. “In 2011, drugmakers sold nearly 30 million pounds of antibiotics for livestock—the largest amount yet recorded and about 80 percent of all reported antibiotic sales that year,” writes Kessler. “We don’t know much more except that, rather than healing sick animals, these drugs are often fed to animals at low levels to make them grow faster.”
In particular, Kessler supports two recently proposed bills—the Animal Drug User Fee Act and the Delivering Antimicrobial Transparency in Animals Act— that would empower FDA to collect and disclose data about the distribution of animal drugs and how food producers are using those drugs in their daily operations. Opining that “we have more than enough scientific evidence to justify curbing the rampant use of antibiotics for livestock,” Kessler notes that combating resistance will require FDA to track “both the prevalence of antibiotic-resistance bacteria in our food, as well as the use of antibiotics in our livestock.”
“Why are lawmakers so reluctant to find out how 80 percent of our antibiotics are used?,” he asks. “We cannot avoid tough questions because we’re afraid of the answers. Lawmakers must let the public know how the drugs they need to stay well are being used to produce cheaper meat.”