Antibiotics, Food and Humans

Introduction

Antibiotic resistance has become an increasingly serious health problem in the U.S. and throughout the world, one that has existed almost since the introduction of penicillin.(1) Bacterial resistance to antibiotics can occur in one of two ways: bacteria can have an intrinsic resistance to specific antibiotics, or resistance can develop from either a mutation or a transfer of genes between bacteria.(2) The current increase in antibiotic resistance has occurred for a variety of reasons: overcrowding that facilitates interpersonal transfer of bacteria, increased travel that disseminates resistant strains globally, and the overuse of antibiotics in humans and animals.(3, 4) The overuse of antibiotics in agriculture, which can negatively affect human health, is the focus of this paper. 40% of antibiotics used worldwide are given to animals as feed additives,(5-7) while in the U.S. estimates average around 50% (6, 8, 9) with 90% of agricultural antibiotics being used as growth promoters.(10) About 80% of food animals have received medication for at least part, if not most, of their lives.(5)

Agricultural antibiotics used not to fight infection, but to promote growth and health, are called Antimicrobial Growth Promoters (AGP). In the European Union is it illegal for antibiotics used therapeutically in humans to be used as growth promoters, but there is no similar law in the U.S.(6) Although many of the drugs that are legal for use as growth promoters in animals are not used in humans, many belong to the same classes as drugs that are used for humans. (6) When we look for the primary causes of antibiotic resistance, we have drugs that are improperly used by humans, combined with overuse in agriculture, where the drugs are used to promote growth and not necessarily therapeutically.(11) This is a concern, because overuse and abuse of antibiotics is associated with serious human illness.(4, 8, 10, 12-14) There are also concerns about the health effects of antibiotic residues, which could cause allergic reactions, the reduction of healthy gut bacteria, the emergence of resistant bacteria in animals and the transfer of the resulting resistance to humans.(5)

The paradox of antibiotic use is that it is indiscriminate and kills “good” bacteria with the “bad”, which can make people (or animals) more susceptible to infection by dangerous microbes.(7, 15, 16) Also humans can become seriously ill if they develop food poisoning from eating meat contaminated with bacteria resistant to multiple antibiotics.(4, 17, 18)

Methods

The information gathered for this paper started with what I already had at home: some books, several articles from Discover, Scientific American and New Scientist magazines, and a selection of articles copied from various journals regarding antibiotic resistance. From there I did a search in the PubMed database via EndNote for articles on antibiotic resistance and agriculture. I then went to the Health Sciences Library (several times) and searched the available journals, looking to see if the articles I found in PubMed were pertinent to my subject, copied the articles that I felt would be helpful, and finally added them to the pile of articles I already had.

Results

Many food animals are given antibiotics as Antimicrobial Growth Promoters (AGP) to make the animals grow bigger, faster, and to reduce incidence of disease.(6) Such effects are in the interest of ranchers and farmers, because it allows them to get the animals to market sooner which is important, because for the last 20 years the average return on a steer was $3 a head, a very slim margin of profit.(19)

In plant agriculture there is comparatively minimal use of antibiotics, and that is primarily to manage bacterial diseases in fruit. Despite this low level of application, plant pathogens have still developed resistance to the antibiotics that are used.(16, 20)

It is slightly different with cattle. Part of the reason cattle are fed antibiotics is because their diet makes them ill. Once cattle have reached the age of about six months, instead of continuing to graze, they are sent to feedlots and fed corn (19) supplemented with rendered remains of pigs, horses, and poultry as well as cattle blood. In the U.S. cattle remains were banned from cattle feed in 1997 but this did not limit the use of other animal remains or cattle blood as a protein additive.(21) Cows are ruminants, which means they have the ability to digest grass, but the modern cattle industry does not allow cattle to graze, instead feeding them a supplemented corn diet, which is a much richer food than a ruminant's stomach can handle. This diet leads to a variety of illnesses, such as feedlot polio, liver abscesses, and rumenitis—diseases that are rarely found in grazing cattle. Because these diseases are ubiquitous in cattle kept in feedlots, the cattle are treated with antibiotics in their food.(19)

Several studies have looked at the prevalence of E. coli O157:H7, a bacteria that can cause serious illness in humans, in cattle feedlots although the results are greatly varied. A 2002 study by Smith et al collected fecal samples directly from the rectums of cattle from five different feedlots, and found E. coli isolates in 23% of the animals tested, with a range of 0.7 to 79.8%. They also found bacteria present in all of the pens observed, although not all animals in a pen carried the bacteria. Bacteria were also recovered from some of the available water and food. They found that muddy pen conditions were associated with a higher number of fecal organisms, possibly because such conditions are more hospitable to the bacteria.(22)

A 2001 study of four Kansas feedlots by Gallard et al found a much lower prevalence of E. coli. This study collected fecal samples from the ground in the pens, and samples from the water and feed, over a period of eleven months. They found only 0.19% prevalence of E. coli in the fecal samples, and only one contaminated water sample. These samples were tested for antibiotic resistance, and it was found that a third of the samples containing bacteria were resistant to one or more antibiotics, including antibiotics that are not approved for growth promotion.(23)

These two studies found widely divergent rates of bacterial contamination of the feedlots, but there are some possible reasons for this. First is the manner in which the samples were collected. In the study by Smith et al, the samples were collected directly from the animals, while the Gallard study collected samples from only five fecal pats in each pen (each pen contained up to 250 animals). Collecting the samples from the ground instead of the animals may have made a difference in the number of bacteria collected. Another difference could be related to a factor mentioned by Smith et al, which is that bacterial contamination was related to pen conditions.(22, 23) Gallard et al did not mention pen conditions, but considering much of the Midwest was under drought conditions in 2000 (24) when the samples were most likely to have been collected, it is possible that this could have been a factor in the lower rate of E. coli present in the samples.

A United States Department of Agriculture (USDA) study published in 2000 found that the presence of E. coli in the guts of cattle varied from 1% in the winter to 50% during the summer, and that 83% of the cattle in the study had been exposed to E. coli,(21, 25) while different research at packing plants found E. coli present on 3.56% of the hides and 0.44% of the carcasses sampled.(21, 25, 26) In one slaughterhouse in Nebraska, it was estimated that stomach contents are splattered on the carcass for up to 20% of the animals slaughtered.(21) These rates of E. coli contamination are important because many of these carcasses are then ground, which creates a high likelihood of contamination of the end product--hamburger. A study looking at the incidence of E. coli in frozen beef patties found 7 to 10% of the patties to be contaminated with the bacteria.(26) E. coli can cause illness with only a few bacteria,(26) and because there are multiple ways that bacteria can enter the food product, the presence of even low levels of E. coli in a cattle population can be potentially dangerous.

Two different studies headed by van den Bogaard have looked at rates of resistant bacteria in poultry, and the transmission of resistant bacteria to farmers and slaughterers. Both studies used the same methodology: Fecal samples were collected from poultry farmers—chicken farmers in one study, and a mixture of chicken and turkey farmers in the other—as well as fecal samples from the poultry. Fecal samples were also obtained from people working in slaughterhouses. These samples were then tested for antibiotic resistance.(27, 28)

The 2001 study found that poultry farmers who treat their animals with antibiotics and those who work in slaughterhouses are more likely to have endemic antibiotic resistant bacteria, when compared with poultry farmers whose animals are not being treated with antibiotics. These bacteria may also be spreading to those who consume such poultry.(27)

The 2002 study, which looked at 752 different E. coli isolates, found that half were resistant to one or more antibiotics. They also found two instances where humans carried bacteria resistant to an antibiotic that is used in animals, showing a transfer from food animals to humans.(28)

As both of these studies simply looked at the prevalence of antibiotic resistance in the E. coli populations of the samples taken, it is difficult to find fault with the methods of collection, since problems would have affected both resistant and non-resistant bacteria alike, or with analysis, since the results were simply prevalence data, from the raw data displayed in the paper.(27, 28)

Manure may also contain antibiotics and resistant bacteria. In 1997, livestock in the U.S. produced 1.4 billion tons of manure.(29) This means that even a low level of bacterial presence is significant. Manure containing resistant bacteria can then enter the environment through run-off or leaching. A 2001 study found that bacteria living in the soil and groundwater beneath pig farms have acquired tetracycline resistance.(30) An outbreak of E. coli O157 at a county fair in Ohio, may have been due to water contamination by animal wastes,(31) and contaminated manure can taint vegetables grown using such as fertilizer, causing foodborne illness.(32, 33)

A group has developed a mathematical model for studying the impact that antibiotic use in animals has on human antibacterial resistance concluding that agricultural antibiotic use has “a large impact on prevalence in humans over time” because human resistance to the antibiotics develops much sooner. They call this “years of wasted medical potential”.(9)

Discussion

Antibiotics are used in food production for a variety of reasons. They are used as growth promoters, to increase health and reduce illness.(6) It is cost effective for farmers to use growth promoters, because they are readily available, inexpensive, and allow the animals to put on weight quickly, which creates increased profits. Because profits in the cattle industry have been notoriously low for the past several decades,(19) this is probably the driving force behind the continued use of pharmaceuticals and feedlots.

Antibiotic use in agriculture occurs in cattle ranching, poultry and pig farming, and even commercial fruit production, but the uses in each may differ. In poultry and pig farming, the use is primarily to increase growth and decrease illness. Use in plants is typically to manage bacterial diseases in fruit production, and is only a small proportion of antibiotic use in agriculture.(20) Use in cattle, however, is quite different. Initial use is similar to the growth production use in poultry and pigs, but once the cattle have been sent to feedlots, the use becomes therapeutic as well as growth promoting. The corn diet supplemented with animal protein that is given to cattle actually causes ill health, because cattle are not designed to eat such a rich diet.(19) In this situation antibiotics are used to treat illness, but it is an illness brought about by the diet fed to the animals, and exacerbated by their living conditions.

Antibiotic resistant bacteria can contaminate food primarily in one of two ways. The first is through the meat from the animals. Research has shown that it is common for bacteria, commonly E. coli or salmonella, to contaminate the meat of the slaughtered animals. The presence of these bacteria by themselves can cause illness in humans who the eat the resulting meat, but if these bacteria are resistant to antibiotics, which could be occurring in up to a third of contaminated meat, then there is the distinct possibility of serious illness.(23)

Bacterial presence in manure is the second way antibiotic resistant bacteria can enter the food chain. Research has found resistant bacteria in manure from animals treated with antibiotics.(23, 25, 27, 28, 34, 35) If this manure is then used on fields as fertilizer, the resistant bacteria can contaminate crops grown on that field.(12, 32, 33) Contamination of water supplies, including wells, lakes, and rivers can also occur, and this could sicken hundreds of people, and possibly even kill immune suppressed individuals.(30)

The research that I reviewed has shown that bacteria resistant to antibiotics are present in poultry fed antibiotics as well as those who farm or slaughter those animals.(14, 27, 28) Other research has found that E. coli O157:H7 is present in cattle and that up to one third of those bacteria may be resistant to antibiotics.(23, 34, 35) This resistance could then be spread throughout the community.(8) Bacteria present in food, be it from contamination by water, individuals or from processing, have caused numerous cases of food borne illnesses in the U.S.(8, 12, 14)

There are alternatives to antibiotic use. Some involve the way the animals are treated and some involve the way the resulting food product is treated.

Irradiation of food has become mainstream in recent years, but there have been other discoveries, some practical and some less so, about how to treat food to reduce bacterial contamination. One discovery was that “blasting” meat with water at explosive pressures kills bacteria and has the added benefit of tenderizing the meat.(36) A more practical solution that can be implemented by individuals is through the use of certain herbs and spices. Traditionally garlic has been seen to have antimicrobial properties, and recent research has borne this out.(37-39) A study has also discovered a number of spices that can reduce E. coli in ground meat. The most effective, in order of potency, are: clove, cinnamon, garlic, oregano, and sage.(40) These spices can then be used by families in conjunction with other practices to reduce bacterial contamination.

Of course the more rational solution would be to keep harmful bacteria from contaminating food in the first place. Changes in the agriculture industry to reduce the use of antibiotics would be easier to implement in some areas than in others. One solution is to change the conditions under which food animals are raised. Reducing overcrowding and increasing access to fresh air are two steps that could substantially help to increase animal health. Reduction of antibiotic use in poultry farming has already been attempted, by spraying chicks with the “good” bacteria that they would encounter living under normal conditions. These bacteria then out-compete the harmful bacteria. This was found to reduce illness, as well as antibiotic use.(7)

It is possible that a similar system could be established in other areas of agriculture where antibiotics are used solely as growth promoters. Stopping the spraying of fruit trees with antibiotics should also be possible to implement without dire consequences.

The situation with cattle is different, however, because it would require a change in the feedlot system, possibly one of allowing the cattle to graze or be fed hay until it is time for slaughter instead of sending them to feedlots for fattening after six months of age. Ecologically, this would make more sense, for cattle are designed to graze on grasses, and live on marginal land that may not be suitable for other types of farming. This would also help to reduce the amount of waste requiring disposal, another serious problem with feedlots. It seems wasteful to feed corn that requires intensive farming and fertilization to a creature designed to eat grass. This change would, however, reduce the speed with which cattle grow and also cause a shift in the agricultural production and use of corn, and neither of these problems will be easily addressed.

We have only a limited number of antibiotics, and those come in only a few classes, so bacteria that become resistant to one antibiotic may develop resistance to all the drugs in that class.(6, 41) The fact that bacterial resistance that develops in animals subsequently develops in humans,(14) leads one to conclude that the use of antibiotics in animals is associated with the development of antibiotic resistance in humans.

Regarding regulation, policymakers must view the benefits of animal health and the economic benefits to farmers and pharmaceutical companies (8) against the somewhat less commercial benefits of reduced antibiotic resistance. Unfortunately there are powerful Political Action Committees (PACs) and lobbyists that have fought regulation of agribusiness on all counts. A recent example of this is the legislation to allow poultry not fed organic feed to be labeled as organic. This means that feed containing antibiotics and pesticides can be fed to poultry and the meat can still receive the organic label.(42) The battle is also being lost in the courts. In December of 2001 a U.S. Court of Appeals ruled that the USDA could not shut down a processing plant whose product contained high levels of salmonella. The court said “the presence of salmonella in meat products does not render them 'injurious to health'”.(35) And regulations for livestock waste, which were issued in December 2002, although requiring feeding operations to obtain permits, absolve corporations of liability for illegal spills by subcontracted growers and allow those producers to write their own pollution management plans.(43)

In 2002 the World Health Organization (WHO) made recommendations for the use of antibiotics in animals including: ending the use of antimicrobial growth promoters that are used in human medicine, evaluation and authorization of antibiotics before they are used in food animals, phasing out antibiotics from agriculture whenever possible, a database of both imports and exports of potential antimicrobials, and a database of residues found in foods.(44) Despite these recommendations and the recommendations of other doctors and organizations, the Food and Drug Administration (FDA) has only proposed guidelines for the industry, not rules, and those guidelines do nothing to regulate drugs that are currently being used, instead focusing upon future drugs that may or may not ever be developed.(45)

With the current situation, and despite the many calls for the restriction of antibiotic use in agriculture, it is unlike this change will occur in the U.S. in the near future unless there is a commercial push for change, because it has become apparent in recent years that changes in the food industry are more likely if they are pushed by McDonald's, because the FDA and the USDA are typically ignored by many growers and processors.

A federal investigation of American feed mills and rendering plants found that many companies had not been taking the threat of mad cow—or the FDA's new feed regulations—very seriously…The federal government's apparent inability to keep prohibited feed away from cattle prompted McDonald's Corporation to take action…On March 13, the McDonald's corporation announced that its ground beef suppliers would be required to supply documentation showing FDA feed rules were being strictly followed… IBP, Excel, and ConAgra immediately agreed to follow McDonald's direction.(21)
If you're looking for any kind of change in how we run our animal agriculture, you're more likely to get it by pressuring McDonald's to do something that you are by pressuring the USDA…if there were an outcry today about hormones in meat or antibiotics in agriculture and McDonald's took note and felt pressure…it would be gone from all of the cattle industry overnight.(19)

Current patterns of antibiotic use in food animals is a problem that leads to antibiotic resistance in animals, which can lead to antibiotic resistant illness in humans through association with treated animals, eating meat from treated animals, or contact with food and water contaminated by manure from treated animals. Unfortunately there looks to be little hope for resolution of this problem in the near future.

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