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roundtable: the evolving enemy Watch Show 4:
"The Evolutionary Arms Race"
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How concerned should we be about becoming infected with resistant bacteria from foods like chicken, beef, and even aqua-cultured fish that may have been grown on diets treated with antibiotics? Will eating less of these foods, perhaps by switching to "organic" chicken, reduce our risk, and do vegetarians have a reduced risk for this reason?
If the use of antibiotics in a widespread fashion at low doses in agriculture, particularly in the raising of animals, is so common and yet so potentially dangerous from the standpoint of evolution of resistance, why is it still the practice? What are the costs and benefits of this practice?
Several people have submitted questions not about antibiotics, but about all of the liquid soaps that are now for sale in supermarkets that are labeled "antibacterial." Do these work like antibiotics in causing further resistance? If so, how, and if not, how is their action on bacteria different than just plain soap?
Americans now seem to be getting fanatic about sterility, and people are trying to raise their children in an almost "germ-free" environment. We have a question from a woman who says that she has several friends, young mothers, generally, who seem to overuse antibacterial products. She asks: Is it not true that the presence of some "germs" in moderation is actually good for the development of our immune system?
In one of the programs in the Evolution broadcast series, there was the story of the leafcutter ants that culture Streptomyces bacteria on their bodies and seem to use those bacteria to apply antibiotics to keep their fungal farms parasite free. This has been inferred to mean that somehow the Streptomyces and the antibiotics they produce have been evolving with the enemy, and this is a kind of use that appears to have been going on for 15 million years without permanent resistance being developed. Is there anything we can do to put ourselves in that situation versus the more static situation we're in now?
Could you give an explicit description of how evolutionary theory informs integrated pest management? I think the cross-fertilization here between bacteria and antibiotic resistance and agricultural pests and insecticide resistance is very interesting, and I don't think many people make that connection.
Some physicians, who are quite impassioned, say they would like to prescribe fewer antibiotics, but their patients demand them. Since in effect they cannot be absolutely positive there isn't a bacterium involved, when, say, a parent brings in a young child who is ill, and they can't be absolutely certain that there won't be bacterial complications as a result of a viral illness, they would like to know how the public will be educated and who is going to take on the job, so that patients will basically get off the doctors' backs and let their colds run their course.
Another part of that question is that doctors are afraid of being sued if they don't prescribe an antibiotic and a bacterial infection does develop. So from the medical perspective of overuse of antibiotics, how might we address this basic problem in the interface between healthcare providers and the consuming public?
   

 

Q: If the use of antibiotics in a widespread fashion at low doses in agriculture, particularly in the raising of animals, is so common and yet so potentially dangerous from the standpoint of evolution of resistance, why is it still the practice? What are the costs and benefits of this practice?
Panelist Responses: < back to intro page
George Beran
This is certainly a major issue. We do use antibiotics for basically three purposes in livestock production. One is for the treatment of disease, one is for the prevention of disease -- and these are both very important -- and the other is for purposes of growth promotion and feed efficiency. The three are somewhat interlocked, and you can't really separate them fully, but we are moving as rapidly as possible to limit the use of antibiotics in sub-therapeutic feeding, which is the term that we use.

Well over half of all of the antibiotics that are used now for growth promotion and feed-efficiency purposes are antibiotics which have a different mechanism of action than those that are used in human medicine and in treatment of disease. Now, having said that, that still leaves a tremendous amount of overlap between these drugs and drugs that are used in human medicine. And with the use of about 20 million pounds of antibiotics in animal production each year, we recognize that the group that overlaps is certainly very large.
Tamar Barlam
Over the last several decades, the trend on farms has been to larger farms, more intensive farming operations. The use of antibiotics for disease prevention, as well as for growth promotion, is because of these operations, for a variety of reasons. There is more disease on these large, intensive farms, and they've gone to antibiotics to try to keep control of that and to try to basically make sure that the system can keep going economically. So it's primarily economic forces that are moving agriculture towards these large farms and towards this massive amount of antibiotic use.

I can certainly agree that a lot of the antibiotics currently used in agriculture are not related to any human medicines, and as those ones should be all the way on the back burner in terms of antibiotics that we phase out. But there are lots of antibiotics, such as the penicillins, tetracyclines, erythromycin, that are exactly the medicine that people take, or other drugs that are very closely related, and I think we need to focus on those.

In Europe, where they don't use antibiotics in this way, some of the poultry houses are as large as in the States. And they have had to make some hard choices; they have had to really focus on bio-security and excellent hygiene and such. So far there hasn't really been the will to do so here in the U.S.

One of the factors that has held back the public health advocates who would like to see this agricultural use of antibiotics decline is the difficulty in obtaining accurate data on the amount of antibiotic use. There's no on-farm monitoring, no sentinel sites to see what the bacterial flora is -- in other words, [to see] how many resistant bacteria are there.

So we've been limited in our access to some of the information that would make the connection very powerfully between antibiotic use on farms and a risk to public health. But despite being hampered in this way, every month, practically, more information from the scientific journals, from good research, comes out -- ever since the information that was documented by Doctor Levy back in the '70s -- that this is a problem. So I think it's economic forces that encourage people to try to slow down the phasing out of antibiotics. There are information gaps that we have to close, but I think that these problems will be addressed over the next years, because I think there's more and more concern about this use of antibiotics on farms.
Stephen Palumbi
Well, I think that one of the issues that has to be brought up here is that the economics of the situation seems to have focused on just what's going on in the farm. And as Dr. Levy has said, the problem is that the antibiotics leak out of those farms into the general environment. Many, many studies have shown that those antibiotics generate the evolutionary resistance in bacteria in the soil and in the environment, and those bacteria can get into people.

So the economics of the situation has to go beyond an individual farm income and growth rate. It also has to include the impact on the environment and on the people living in the environment of exposure to resistant bacteria. If those economics were folded into the economics of using antibiotics for growth enhancement in livestock, I don't know what the result would be. I would guess in many cases it would not be economical to treat animals with antibiotics, if in fact you had to figure in the costs to the environment and to the people around those farms.
Stuart Levy
I think this is a very important point. I must say, since the subject on the table is evolution, that antibiotics are a critically important human influence on evolution. I mean, many of them are natural products, but in fact it is we who have harvested these and are using them, and are therefore pushing evolution towards resistant forms. Now this is important, because that impacts how we can treat people and animals that have disease, and how well we can control disease.

While we are addressing misuse of antibiotics in people, there is a very current issue here, and that is those of you who are out there stockpiling them even as we speak. This is not a good idea. Hopefully people are not using them, but certainly they are keeping them and might be tempted to use them at the first sign of any flu-like symptom. And that is going to create in your environment the same kind of resistance problem that is created by inappropriate use of antibiotics in farm animals. This is a serious issue, because these antibiotics are very valuable drugs of last resort for treating conditions like pneumonia, urinary tract infections, and even things like ruptured appendices. And if we stockpile these drugs and increase the level of resistance to them by inappropriate use, they may not be available and may not be effective when we really need them.

These antibiotics are powerful. They will destroy the susceptible bacteria and allow the resistant ones to propagate. That now makes the rare resistant ones, which might have occurred due to one unusual event, very successful in that environment. Quite clearly, any growth inhibitory agent, any chemical or natural product that is going to kill off certain bacteria and allow others to survive, is going to change the environment.

And while some of the antibiotics that are used for growth promotion and feed efficiency in animals are not those that we use in human medicine, we have learned all too clearly in the past decade that a couple of the important antibiotics for human use are related to those that are being used for growth promotion in animals. In fact, that has allowed for the propagation of resistance to the new drugs that we want to introduce to people. Therefore, really, we should look into a new way of raising animals without using antibiotics to promote growth, with more in the way of probiotics, exclusion, and new, hygienic ways of raising the animals. That way we can preserve these antibiotics for the treatment of the sick animal, for the prevention of disease.
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