A number of articles caught my attention recently regarding the use of antimicrobials in livestock and the potential environmental impact or contribution to antimicrobial resistance.
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CANADA: Environmental effects kept in check on farms
20.jul.07
SPARKplug Submission
Katharine FoundEnvironmental activists have long criticized pharmaceutical use by hog farmers and veterinarians in treating swine disease, saying pharmaceuticals are being overused and errantly contaminating the environment. But new research from the University of Guelph has shown that environmental contamination from antibiotics does not pose appreciable risks to soil and aquatic organisms. Prof. Paul Sibley of the Department of Environmental Biology and Prof. Keith Solomon of the Centre for Toxicology have wrapped up six years of research examining the use of pharmaceuticals in the Canadian hog and cattle industry. They’ve determined that the pharmaceuticals represent negligible environmental risk if used as instructed.
"It’s good news for producers, veterinarians and pharmaceutical companies," says Sibley. "We’ve found evidence that suggests there’s little risk to soil and aquatic biota from using pharmaceutical products, so there’s little need to be concerned." Pharmaceuticals first raised concerns when they were detected in the environment more than a decade ago. It was thought they could cause contamination through simple routine practices such as manure spreading. Animals administered antibiotics excreted them through feces or urine, which was then applied to land and could cause damage to soil systems or migrate into nearby waterways. But was this true? To find out, the research team simulated real-life scenarios in the laboratory and field to study pharmaceutical toxicity. They applied pharmaceuticals directly to soil and water to simulate field exposure. This direct exposure would be similar to an actual worst-case scenario, making it a good test of potential risks, says Sibley. In toxicity, safety (or risk) is often measured as the difference between what is found in the environment and what the pharmaceutical’s toxicity is known to be, he says.
In most experiments, he found that the toxicity effects of pharmaceuticals were in the milligram- to gram-per-litre range. That was significantly higher than the nanogram- to microgram-per-litre range typically detected in soil and water for pharmaceuticals.
Sibley says the long duration of some of the studies helped to accurately assess changes in contamination levels and toxicity over time, ultimately leading to a stronger conclusion that supports environmental safety. "It’s comforting to know that levels of antibiotics in the environment don’t seem to be posing a problem. We’ve tested aquatic, vertebrate, fish and soil communities, and the evidence clearly indicates little cause for concern."
He says there’s been a strong push by consumers and activists over the last decade to reduce pharmaceutical use on pig farms. Programs such as the Canadian Quality Assurance are helping to inform farmers about proper protocols and how to manage the antibiotics given on the farm. The program also helps with traceability should contamination occur.
This study has found that farmers can be reassured that their practices are helping them be safe stewards of the land, says Sibley. "I hope this research still encourages pharmaceutical companies, veterinarians and producers to be more efficient with their antibiotic treatments but, at the same time, eases any thoughts of potential negative environmental effects." This research was funded by the Canadian Pork Council through the Livestock Environmental Initiative, the Canadian Cattlemen’s Association and the Canadian Network of Toxicology Centres
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Swine Manure Unlikely as Major Contributor to Antibiotic Resistance
By: Bruce Cochrane Farm.com 7/26/2007 —
Research conducted by the University of Manitoba indicates swine manure is not to blame for the development of antimicrobial resistance in the environment.
As part of a multidisiplinary research project near La Broquerie, in which hog manure is being evaluated as a fertilizer on pasture and hayland, scientists are monitoring microbial contamination of groundwater.
Dr. Denis Krause, a professor with the Departments of Animal Science and Medical Microbiology says researchers have tracked E. coli, as well as microbial communities rich in particular kinds of antibiotics from groundwater that potentially could be contributed by antibiotics used in the hog industry.
Clip-Dr. Denis Krause-University of Manitoba In terms of the E. coli we find that E. coli from hog manure do not survive on pastures for very long periods of time at all.
We also find that e. coli that are spread from hog manure that have been spread on pasture typically do not find their way into cattle that are grazing the pasture.
We also find that there isn’t a great deal of background antibiotic resistance in microorganisms in the groundwater.
The do not come from any digestive tract source.
That means that there’s a huge amount of background antibiotic resistance in the environment and that’s something that we’ve seen in the literature from other research groups that is in fact quite common.
I think it’s very unfair of people to point a finger solely at the livestock industry to say that they are the major contributors.
There’s a lot of other issues going on and there’s a lot of other sources of microbial contamination in potable water sources other than just the livestock industry.
Dr. Krause suggests it would be extremely difficult to point the finger solely at the hog industry as a major contributor to antibiotic resistance, either in the environment or in human medicine, because there’s already so much background antibiotic resistance, the result of antibiotic producing organisms that we typically find in soil.
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Evaluating the effects of chlortetracycline on the proliferation of antibiotic resistant bacteria in a simulated river water ecosystem.
Appl Environ Microbiol. 2007 Jul 6;
[Epub ahead of print]
Muñoz-Aguayo J, Lang KS, Lapara TM, González G, Singer RS.
Department of Veterinary and Biomedical Sciences, College of Veterinary Medicine, University of Minnesota, St. Paul, MN 55108; Departamento de Microbiología, Facultad de Ciencias Biológicas, Universidad de Concepción, Chile; Department of Civil Engineering, University of Minnesota, Minneapolis, MN 55455; Instituto de Medicina Preventiva Veterinaria, Facultad de Ciencias Veterinarias, Universidad Austral de Chile, Valdivia, Chile.Antibiotics and antibiotic metabolites have been found in the environment, but the biological activity of these compounds is uncertain, especially given the low levels that are typically detected in the environment. The objective of this study was to estimate the selection potential of chlortetraclyine (CTC) on antibiotic resistance of aerobic bacterial populations in a simulated river water ecosystem. Six replicates of a 10-day experiment using river water in continuous-flow chemostat systems were conducted. Each replicate used three chemostats, one serving as a control to which no antibiotic was added and the other two receiving low and high doses of CTC (8 ug/L and 800 ug/L, respectively). The addition of CTC to the chemostats did not impact the overall level of cultivable aerobic bacteria (P=0.51). The high-CTC chemostat had significantly higher tetracycline-resistant bacterial colony counts than both the low-CTC and control chemostats (P<0.035). The differences in resistance between the low-CTC and control chemostats were highly non-significant (P=0.779). In general a greater diversity of tet resistance genes were detected in the high-CTC chemostat and with a greater frequency than in the low-CTC and control chemostats. Low levels of CTC in this in vitro experiment did not select for increased levels of tetracycline resistance among cultivable aerobic bacteria. This finding should not be equated with the absence of environmental risk, however. Low concentrations of antibiotics in the environment may select for resistant bacterial populations once they are concentrated in sediments or other locations.
PMID: 17616621 [PubMed – as supplied by publisher] -
Team Tracks Antibiotic Resistance from Swine Farms to Groundwater
To reach Tony Yannarell, call 217-333-8809; e-mail: acyann@uiuc.edu. To reach R.I. Mackie, email: r-mackie@uiuc.edu.
The routine use of antibiotics in swine production can have unintended consequences, with antibiotic resistance genes sometimes leaking from waste lagoons into groundwater.
In a new study, researchers at the University of Illinois report that some genes found in hog waste lagoons are transferred – "like batons" – from one bacterial species to another. The researchers found that this migration across species and into new environments sometimes dilutes – and sometimes amplifies – genes conferring antibiotic resistance.
The new report, in the August issue of Applied and Environmental Microbiology, tracks the passage of tetracycline resistance genes from hog waste lagoons into groundwater wells at two Illinois swine facilities.
This is the first study to take a broad sample of tetracycline resistance genes in a landscape dominated by hog farming, said principal investigator R.I. Mackie. And it is one of the first to survey the genes directly rather than focusing on the organisms that host them. Mackie is a professor in the department of animal sciences and an affiliate of the Institute for Genomic Biology.
"At this stage, we’re not really concerned about who’s got these genes," Mackie said. "If the genes are there, potentially they can get into the right organism at the right time and confer resistance to an antibiotic that’s being used to treat disease."
Tetracycline is widely used in swine production. It is injected into the animals to treat or prevent disease, and is often used as an additive in hog feed to boost the animals’ growth. Its near-continuous use in some hog farms promotes the evolution of tetracycline-resistant strains in the animals’ digestive tracts and manure.
The migration of antibiotic resistance from animal feeding operations into groundwater has broad implications for human and ecological health. There are roughly 238,000 animal feeding operations in the U.S., which collectively generate about 500 million tons of manure per year. Groundwater comprises about 40 percent of the public water supply, and more than 97 percent of the drinking water used in rural areas.
Tony Yannarell, postdoctoral research associate in the Institute for Genomic Biology, right, with undergraduate research assistant Shazan Ahmed, junior in molecular and cellular biology, tracked the passage of tetracycline resistance genes from hog waste lagoons into groundwater wells at two Illinois swine facilities.
Federal law mandates that animal facilities develop nutrient management plans to protect surface water and groundwater from fecal contamination. Most swine facilities hold the effluent in large, water-filled lagoons until it can be injected into the ground as fertilizer. Thanks to a change in the law in the late 1990s, new lagoons must be built with liners to prevent seepage. Swine facilities in operation prior to the new regulations are allowed to continue using unlined lagoons, however.
Some of these lagoons leak.
The researchers extracted bacterial DNA from lagoons and groundwater wells at two study sites over a period of three years. They screened these samples for seven different tetracycline resistance genes.
They found fluctuating levels of every one of the seven genes for which they screened in the lagoons. They also found that these genes were migrating from the lagoons to some of the groundwater wells.
It should be noted that many genes that confer antibiotic resistance occur naturally in the environment. Tetracycline is itself a bacterial product, employed by Streptomyces bacteria long before humans discovered its usefulness.
In order to determine the origin of the tetracycline resistance genes found in the groundwater, the researchers conducted a genetic analysis of one gene family, tet(W), in samples from the lagoons and from groundwater wells below (downgradient of) and above (upgradient to) the lagoons. They found that the variants of tet(W) genes in the upgradient, environmental control wells were distinct from those of the lagoons, while the wells downgradient of the lagoons contained genes consistent with both the background levels and those in the lagoons.
"There’s a human impact on these sites that is superimposed on a natural signal," said postdoctoral research assistant Anthony Yannarell, an author on the study.
One of the two hog farms, "Site A," was more impacted by resistance genes from the lagoon, due to its hydrogeology. The site included two layers of sand – at about two meters and eight meters below the surface – through which groundwater flowed.
"Every time we looked in the lagoon, we saw all of the genes we were looking for," Yannarell said. "At Site A, all the wells that were closest to the lagoon almost always had every gene. As you got further from the lagoon you started to see genes dropping out."
The resistance genes were present at much higher levels – "an order of magnitude higher," said the authors – in the lagoon than in the contaminated wells. Most were diluted as they moved away from the lagoons in the groundwater.
There was one notable exception. A gene known as tet(C) was found at higher levels in some of the groundwater wells at Site A than in the lagoon. Its heightened presence was not consistent with background levels, indicating that something in the environment was amplifying this one gene, which had originated in the lagoon.
Perhaps the gene had migrated to a new organism, Yannarell said, to find a host that was more suited to conditions in the groundwater.
"What we are seeing is that the genes can travel a lot further than the bacteria," Mackie said. "It’s a matter of getting the DNA into the right organism. It’s a relay race."
Other authors on the study are postdoctoral research assistant S. Koike; Illinois State Geological Survey geochemist I.G. Krapac; research assistant H.D. Oliver; USDA Agricultural Research Service scientist and professor of crop sciences J.C. Chee-Sanford; and visiting professor of animal sciences R.I. Aminov.