Feeding antibiotics to livestock creates an ever-increasing number of resistant bacteria, including many that can harm humans
A recent study from the Maryland School of Public Health has found a simple way to help overcome the health problems caused by antibiotic resistance: stop adding antibiotics to animal feed. The study found that when poultry and beef are produced without these antibiotics, bacterial resistance quickly declines.
Feeding antibiotics to livestock creates an ever-increasing number of antibiotic-resistant bacteria, including many that cause disease in humans. Yet up to now, no one has been able to say how quickly the damage can be undone by ending this practice or if it can be undone at all.
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The study, by a team that included researchers from Pennsylvania State University and Johns Hopkins Bloomberg School of Public Health as well as the Maryland School of Public Health, found an immediate and substantial decrease in antibiotic-resistant bacteria on poultry farms that had just switched from conventional to organic practices. (1) And this happened in the very first flock raised organically.
"While we know that the dynamics of antibiotic resistance differ by bacterium and antibiotic, these findings show that, at least in the case of enterococci, we begin to reverse resistance on farms even among the first group of animals that are grown without antibiotics," said lead researcher Amy R. Sapkota, an assistant professor with the Maryland Institute for Applied Environmental Health in a university press release.
But this study's findings won't become reality on their own. Meat and poultry farmers must embrace it. For that to happen, consumers will have to persuade them through the power of the pocketbook.
Over Half a Century of Antibiotics in Animal Feed
Including antibiotics in animal feed became routine soon after the rise of feedlots. Meat animals are fed antibiotics because doing so increases their weight and makes meat production cheaper. As far back as 1946, it was known that healthy animals fed low doses of antibiotics -- doses strong enough to kill some of the animals' internal bacteria but not all of them -- grew to market weight on less feed than animals that weren't fed antibiotics.
"You start giving [cattle] antibiotics, because as soon as you give them corn, you've disturbed their digestion, and they're apt to get sick, so you then have to give them drugs, says Michael Pollan, author of In Defense of Food: An Eater's Manifesto. "That's how you get in this whole cycle of drugs and meat.... Once they start eating the [corn], they're more vulnerable. They're stressed, so they're more vulnerable to all the different diseases cows get." (2)
Because of this, meat animals are also fed antibiotics protectively, to prevent the spread of disease before disease even occurs. Meat animals are raised under highly crowded conditions, which means that on the typical meat or poultry farm, one sick animal can easily infect all the others. This is similar to what occurs with people living in overcrowded conditions, where disease rates traditionally soar.
All these antibiotics add up. The Union of Concerned scientists estimated that 70 percent of all antibiotics in the U.S. (24.6 million pounds annually) are fed to healthy livestock, not taken by people. (3)
When antibiotics are fed to farm animals, even at the low levels used to aid growth, the remaining bacteria within the animals tend to acquire resistance to them. This was seen as early as 1951 in streptomycin-fed turkeys. (4) But it didn't set off many alarm bells. The main impact of antibiotic resistance is seen in infections in humans, but back in the 1950s an infection that couldn't be cured with streptomycin could still be cured by using penicillin or some other antibiotic. But that was about to change.
The Spread of Antibiotic Resistance
Bacteria spread antibiotic resistance to other bacteria, even bacteria of a different species, by genetic transfer. This means that a harmless intestinal bacterium could make a disease-causing bacterium antibiotic resistant. Sewers are excellent breeding grounds for such transfer. This began to set off the alarm bells. But it wasn't until the advent of methicillin-resistant Staphylococcus aureus -- MRSA -- that the alarm bells really began ringing.
(1) Sapotka AR, Hulet RM, Zhang G, et al. Lower Prevalence of Antibiotic-resistant Enterococci On U.S. Conventional Poultry Farms That Transitioned to Organic Practices. Environmental Health Perspectives, Aug 10, 2011. Published online ahead of print. http://dx.doi.org/10.1289/ehp.1003350 Last accessed September 10, 2011.
(2) PBS interview with Michael Pollan, part of the Frontline show, "Modern Meat" http://www.pbs.org/wgbh/pages/frontline/shows/meat/interviews/pollan.html Last accessed Sept 10, 2011.
(3) Mellon M, Benbrook C, and Benbrook KL. Hogging it: Estimates of Antimicrobial Abuse in Livestock. January 2001. Union of Concerned Scientists website http://www.ucsusa.org/assets/documents/food_and_agriculture/hog_front.pdf Last accessed September 10, 2011.
(4) Starr MP and Reynolds DM. Streptomycin resistance of coliform bacteria from turkeys fed streptomycin. Proceedings of the 51st General Meeting, Society of American Bacteriology, Chicago, IL. 1951:15-34.
The Case of MRSA
Methicillin is an antibiotic that was developed to treat infections by the bacterium Staphylococcus aureus, which had begun to develop resistance to many other antibiotics. At a Boston hospital in 1968, there was an outbreak of Staphylococcus infections that couldn't be cured by methicillin or several other antibiotics the first outbreak of MRSA in the U.S. MRSA has since proven to be an extremely troubling source of infections, first in hospital patients and later in communities.
There are only a limited number of antibiotics that can be given to humans to treat infections. And with disease organisms becoming resistant to multiple antibiotics, this number was rapidly dwindling. Antibiotic resistance was no longer a theoretical topic for scientists to discuss. It was now an extremely serious health problem.
A 2007 study published by the CDC found that 20 percent of all human MRSA in the Netherlands is of animal origin, from pigs and likely from cows. And the researchers strongly doubt that this problem is confined to the Netherlands. In other words, antibiotic-resistant bacteria from farm animals have been linked to antibiotic-resistant infections in humans. (5)
The issue of whether antibiotic-resistant bacteria from farm animals have already caused infections in humans is really moot. The ease with which resistance spreads from one species of bacteria to another means that the pool of drug-resistant bacteria created on animal farms is bound to spread to humans sooner or later. Unless of course, these resistant bacteria can be reduced or eliminated.
This was the idea behind the Maryland School of Public Health study. Could drug-resistant bacteria be discouraged by a reduced reliance on antibiotics?
Short-Circuiting Antibiotic Resistance
Bacteria don't choose to become antibiotic resistant. When they are growing in the presence of an antibiotic that would normally kill them, those that are able to resist the antibiotic survive and breed, while those that don't die. It's a survival trait, pure and simple.
But what happens when that antibiotic disappears? Antibiotic resistance, whether caused by a protein that inactivates the antibiotic or a pump that transports it out of the cell, comes at an energy cost to the bacteria.
When antibiotics are removed from feed and no antibiotic is present, the energy spent on antibiotic resistance is no longer a survival trait, it's a liability. Non-resistant bacteria now are more energy efficient and should be at an evolutionary advantage and grow better, eventually replacing their resistant cousins. At least that's the theory. The Maryland researchers decided to put this theory to the test.
The Maryland Study Design
The researchers compared bacterial antibiotic resistance in five large conventional Mid-Atlantic poultry farms to that of five large Mid-Atlantic farms that had recently converted to organic growth practices and been certified as organic. Among the many requirements for a farm to be certified as organic, poultry cannot be fed any antibiotics or growth hormones.
They looked at the resistance of two species of Enterococcus, Streptococcus-like bacteria found in the intestine and colon of both poultry and humans. These bacteria normally cause no harm but can cause infection when they spread to other parts of the body. Because Enterococcus species are a major source of hospital-acquired infections in humans and because they can spread resistance to other bacterial species through genetic transfer, they're medically important bacteria.
The researchers collected bacterial samples from poultry litter, feed, and water and tested their resistance to 14 antibiotics. They looked at the two most common species of Enterococcus, E. faecalis and E. faecium. These bacteria spread from farm animals, chiefly through animal waste, into air, soil, and water.
A Large and Immediate Decrease in Resistance
Perhaps the most important study finding was the marked drop in bacteria that were multi-antibiotic resistant on the newly organic farms, since multi-drug resistant bacteria create the greatest problems in human infections. While 84 percent of the E. faecium on conventional farms were multi-drug resistant (resistant to three or more antibiotics), only 17 percent were on the organic farms. Multi-drug resistance in E. faecalis showed a similar pattern, 42 percent on conventional farms and only 10 percent on the organic farms.
Resistance to the vast majority of antibiotics tested was lower on the newly organic farms. For example, 81 percent of the E.faecium on conventional farms were resistant to tetracycline, compared to only 12 percent on the newly organic farms. And while all the declines seen weren't this spectacular, the researchers emphasize that all this occurred in the very first flock grown organically. The decreases are only expected to grow larger in the next year or two.
The results of the study are summarized in the table below.
There are likely still sources of residual antibiotic-resistant bacteria remaining on the newly organic farms, even after the extensive cleaning required to be certified as organic. These should diminish over time and lead to even lower rates of resistance. And even organically-grown poultry can still have major exposure to antibiotics. They can't be fed antibiotics, but this prohibition only applies from the first day of life onward. Breeder facilities and hatcheries are allowed to inject antibiotics into eggs and the chicks that hatch are still certified organic.
Banned in Europe
Other countries have already acted to curb antibiotic feeding to livestock. Most notably, the European Union banned feeding of all medically important antibiotics to livestock in 1998 and followed that with a total ban on all antibiotics in 2006.
There haven't been many studies done on the effect of banning antibiotics in livestock in Europe and Asia. Two that have been done firmly support the Maryland study's results.
Taiwan banned the use of Avoparcin as a feed additive in 2000. Avoparcin is an analogue of the antibiotic vancomycin; bacteria that are resistant to one of these antibiotics will almost always be resistant to the other. Testing the same two bacterial species that were tested in the Maryland study, a 2007 study found that vancomycin resistant enterococci on chicken farms decreased nearly fourfold from 2000 to 2003, dropping from 13.7 percent to 3.7 percent and 3.4 percent in the two Enterococcus species. (6)
A 2001 study from Denmark tested the effect of the 1998 ban on medically important antibiotics. It compared the antibiotic resistance of E. faecium from broilers in 2000 to the pre-ban level in 1997. It found that resistance to four medically important antibiotics -- avilamycin, avoparcin, erythromycin and virginiamycin had each dropped greatly, to between one half to one-fourteenth of their 1997 level by the year 2000. (7)
Both these studies offer the same lesson as the Maryland one: Stop feeding antibiotics to livestock and antibiotic-resistance in bacteria will quickly decline.
What Does the Future Hold?
People have been given a vision of a future where antibiotic resistance begins to fade, instead of becoming a more serious medical problem with every passing year. But expecting industry to take the lead ignores the lessons of the past. Historically, businesses have been loath to change lucrative practices for the sake of the social good without some type of pressure that forces them to do so. That leaves it up to consumers to make this vision real.
Americans have said no to the food industry before: they rejected New Coke simply because they didn't like the taste, and the Coca-Cola company ultimately had no choice but to bow to their wishes.
There's no question that if enough people reject antibiotic-fed meat and poultry, the industry will have to change their practices. But organic meat and poultry do cost more. And some people simply can't afford to pay much more. Buying at farmers markets or directly from the farmer can help, since prices will be lower. And as the organic market share grows, prices should also begin to drop.
For those who can afford the price, the question becomes simpler: what kind of world do you want to live in? If you'd like one where antibiotics can still cure infections, it might be time to start buying organic meat and poultry. And there could be a bonus: many people say that organic meat tastes much better.
Image: REUTERS/Jessica Renaldi.
(5) van Loo I, Huijsdens X, Tiemersma E, et al. Emergence of Methicillin-Resistant Staphylococcus aureus of Animal Origin in Humans. Emerging Infectious Diseases Dec. 2007; 13:1834-1839. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2876750/. Last accessed September 10, 2011.
(6) Lauderdale T, Shiau Y, Huang H, et al. Effect of banning vancomycin analogue avoparcin on vancomycin-resistant enterococci in chicken farms in Taiwan. Environmental Microbiology March 2007; 9:819-823.
(7) Aarestrup FM, Seyfarth AM, Emborg H, et al. Effect of Abolishment of the Use of Antimicrobial Agents for Growth Promotion on Occurrence of Antimicrobial Resistance in Fecal Enterococci from Food Animals in Denmark. Antimicrobial Agents and Chemotherapy July 2001; 45:2054-2059 http://aac.asm.org/cgi/content/full/45/7/2054 Last accessed September 10, 2011.
This article originally appeared on TheDoctorWillSeeYouNow.com.
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