Researchers Identify Antibiotic-Resistant Bacteria in Pig Manure
A new study has reportedly confirmed the presence of antibiotic-resistant
genes (ARGs) in manure samples harvested from swine farms in China, raising
concerns about the widespread use of therapeutic antimicrobials in livestock
and livestock feed. Yong-Guan Zhu, et al., “Diverse and abundant antibiotic
resistant genes in Chinese swine farms,” PNAS, February 2013. Researchers
apparently used high-capacity quantitative PCR arrays to assess “the type and
concentrations of ARGs at three stages of manure processing to land disposal
at three large-scale (10,000 animals per year) commercial swine farms.”
The results from all the manure samples evidently revealed 149 unique ARGs,
with “the top 63 ARGs being enriched 192-fold (median) up to 28,000-fold
(maximum) compared with their respective antibiotic-free manure or soil
controls.” In particular, the findings suggested that antibiotics and heavy
metals found in the manures had the potential to co-select for resistance traits and further exacerbate “the risks of transfer of ARGs from livestock animals to human-associated bacteria.”
“This study highlights that ARGs in swine farms are not only diverse but are also remarkably abundant, which together offers a higher statistical probability of dispersal, further selection, and/or horizontal transfer in the environment,” wrote the study’s authors, who described their conclusion as alarming. “[U]nmonitored use of antibiotics and metals on swine farms has expanded the diversity and abundance of the antibiotic resistance reservoir in the farm environment… Policies and management tools to facilitate prudent use of antibiotics and heavy metals, including their combined use, in animal industries and animal waste management are needed.”
Meanwhile, the Food and Drug Administration’s Center for Veterinary Medicine (CVM) recently released its third annual report on the sale and distribution of antimicrobials approved for use in food-producing animals. According to CVM, which gathers data from antimicrobial drug sponsors as required by the Animal Drug User Fee Act (ADUFA), the antimicrobials sold for use in domestic animals in 2011 included the following drug classes: aminoglycosides, cephalosporins, ionophores, lincosamides, macrolides, penicillins, sulfas, and tetracyclines. At the same time, however, CVM has warned stakeholders that comparing data on animal and human antibiotics is difficult due to variables such as “the number of humans versus that of food-producing animals during the data period, differences in physical characteristics (such as weight) between humans and the various species of animals, molecular weights and dosages of the different antimicrobials as well as other differences in the conditions of use of the antimicrobials.”
Despite these cautions, the Pew Charitable Trusts has since published an infographic based on the same ADUFA reports behind CVM’s summary, claiming that antibiotic sales for meat and poultry product reached a “record high” in 2011. The infographic specifically alleges that during this period, “29.9 million pounds of antibiotics were sold in the United States for meat and poultry production,” a figure that is 3.9 times greater than the amount of antibiotics sold to treat human illnesses. “These practices are contributing to the emergence of drug resistant superbugs that make infections more difficult and costly to treat. In 2011, more antibiotics were sold for use in meat and poultry production than ever before,” opines the Pew’s report, which urges Congress and FDA “to rein in the overuse of antibiotics in food animal production.”
