Antibiotics & Agriculture Part 3: The Spread of Resistant Bacteria

The application of antibiotics to livestock has provided a boon to the agricultural industry. Unfortunately an outbreak of Salmonella showed that this application could have some untoward side effects. The farmers and veterinarians not only failed to contain this outbreak of Salmonella, but botched the antibiotic treatment so thoroughly that a multi-drug resistant strain of this pathogen emerged and spread to humans.
Such was the outcry in response to this outbreak that the government set up the Swann report, which attempted to promote more responsible usage of antibiotics. Even though the 1964 outbreak was primarily a result of improper medication for farm animals, the use of antibiotic growth promoters emerged as a specific concern. One of the sole achievements of  this report was to separate the antibiotics used in humans to those used in animals, with specific restrictions on the use of antibiotic growth promoters.

Other countries experienced similar issues. An investigation in the US found that between 1971-1983, the majority of outbreaks of Multi-drug resistant Salmonella stemmed from contact with either farms or animal products. These antibiotic resistant Salmonella proved to be more lethal than their antibiotic sensitive counterparts. In 1977 the FDA decided that it was no longer safe to use certain antibiotics as growth promoters. They tried to stop front-line antibiotics such as penicillin and tetracycline being used as agricultural growth promoters. But for reasons that are unknown, they never followed up on their declarations. It is likely that the FDA simply didn't have the resources or the public support to pass such a law.

In contrast, Northern Europe had begun to implement restrictions on the usage of antibiotics in livestock. Often these restrictions consisted of allowing only one set of antibiotics for the agricultural industry and one for the medical community. But this soon encountered a major setback.
Clinicians began to encounter Vancomycin resistant strains of Enterococci. Vancomycin is often the drug of last resort, and was supposedly tightly regulated so as to prevent resistance developing. These outbreaks often occurred in hospitals, but not always.  When doctors examined patients to find out where this bacterium was coming from, they found something surprising. The source of these Vancomycin resistant Enterococci infections originated from the community. The doctors redoubled their efforts to work out the source of this infection. They checked farm animals, food from shops, sewage outflows, and any other possible place where Enterococci could hide.  What they found surprised them. They found this bacterium in farm animals and food sources and the sewage outflow. They found that not only were these hospital outbreaks traceable to these community sources, but there was a veritable reservoir of vancomycin strains out there that had not yet reached the hospital. But this presented a puzzle.
Vancomycin was only available to hospitals. In accordance with laws, the farms in the area were using different antibiotics. So why were these bacteria in the community, who should never have even seen Vancomycin, suddenly becoming resistant to it ?
The truth is that the bacteria had not specifically developed a resistance to Vancomycin. They had developed a resistance to a drug named Avoparcin. The vancomycin resistance was just a lucky side effect of this. You may not have heard of Avoparcin. This is because it was never meant to be used in humans. It was one of the few antibiotics allowed to be used as a growth promoter. What no-one had foreseen was that it's structure was so similar to vancomycin that it would breed resistance to it. And as a result, one of the key antibiotics to stop hospital outbreaks was rendered useless against the Enterococci.

But why should we worry about these bacteria. Enterococci aren't much of a threat outside of the hospital, and even then they tend not to have multiple drug resistances. Whilst we can worry about Salmonella, we should remember that the best ways of treating Salmonella  don't require antibiotics at all. So why should the spread of these antibiotic resistant bacteria be a worry for us ?

To Be Continued.....

References

Holmberg S., Wells J. & Cohen M. (1984). Animal-to-man transmission of antimicrobial-resistant Salmonella: investigations of U.S. outbreaks, 1971-1983, Science, 225 (4664) 833-835. DOI: 10.1126/science.6382605

Bates J., Jordens J.Z. & Griffiths D.T. (1994). Farm animals as a putative reservoir for vancomycin-resistant enterococcal infection in man, Journal of Antimicrobial Chemotherapy, 34 (4) 507-514. DOI: 10.1093/jac/34.4.507
O'Brien T. (2002). Emergence, Spread, and Environmental Effect of Antimicrobial Resistance: How Use of an Antimicrobial Anywhere Can Increase Resistance to Any Antimicrobial Anywhere Else, Clinical Infectious Diseases, 34 (s3) S78-S84. DOI: 10.1086/340244
http://docs.nrdc.org/health/files/hea_12032301a.pdf