Tuesday, May 4, 2010

Agricultural, Environmental, and Community Sources of Resistance

A new study from the Johns Hopkins Bloomberg School of Public Health gives further evidence that antibiotic resistance is only just the result of antibiotic use and abuse in human medicine, but that a reservoir of resistance, from farm animals and environmental contamination, is a significant contributor as well. A team of scientists led by Henry D. Kalter examined E. coli samples in more than 500 young Peruvian children, then compared resistance levels to a range of factors in the children's households and communities. The results are published in this month's issue of the American Journal of Tropical Medicine and Hygiene.As would be expected, the children's own use of antibiotics, as well as family member use, was a significant risk factor for their carriage of drug-resistant bacteria. Applying an antibiotic to a population of bacteria selects for the resistant ones to survive, and though these bacteria might not immediately cause disease, they may pass these genes to other species of bacteria, for example through conjugation.

But the authors also found evidence for a transfer of resistant bacteria between food animals and humans - specifically, through market chicken raised with antibiotics. Living in a community with more families raising chickens, as opposed to buying their chicken at a market, was a significant protective factor against children's carriage of resistant bacteria. Market chickens were also significanlty more likely to carry antibiotic-resistant bacteria than home-raised chickens, and presumably transferred this resistance to humans in the community though the food chain or direct contact.
Dr. Edward T. Ryan, president of the American Society of Tropical Medicine and Hygiene, explained the study's importance in this respect:
"[The study] improves our understanding of the growing global public health threat of antibiotic resistant organisms, and underscores the critical role that antibiotic use in animals plays in contributing to this threat. The vast majority of the tons and tons of antibiotics ingested each year on this planet are administered to livestock and animals. This study clearly shows that such use comes with a very real cost to human health."
The study also identified a link between environmental contamination with antibiotic-resistant bacteria and increased carriage of resistant E. coli. In fact, as the authors write,
"In these poor communities in a developing country, with inadequate protection of excreta and water, contamination of the environment with antibiotic-resistant bacteria appeared to play at least as great a role in children's carriage of resistant E. coli as did the children's own antibiotic use."
These results corroborate the need for a big-picture approach to addressing antibiotic resistance in both developing and developed nations. The authors cite the intensive use of antibiotics on chickens raised in Peru as a factor contributing to resistance in humans there, but this is hardly an isolated issue - millions of pounds of antibiotics are administered to farm animals every year in the United States, and the presence of an environmental and agricultural reservoir of resistance genes is a threat to the success of antibiotic treatment everywhere.