Prescription for Trouble: Using Antibiotics to Fatten Livestock

Overuse of antibiotics in animal agriculture has led to antibiotic-resistant strains of bacteria that make treating illness more difficult

If you get food poisoning, will the antibiotic prescribed by your doctor be able to fight the infection? We regularly hear news stories heralding promising new drugs or drug therapy. Ironically, concealed in the din of information about new drugs looms a health crisis growing out of the loss of old drugs.

Once, a storehouse full of medicines such as penicillin and streptomycin could handily fight off most infections from bacteria and other microorganisms. Now, once-vulnerable bacteria have evolved resistance, and many of these drugs are losing their effectiveness. Health experts agree that there is serious danger of losing some of the most precious drugs—antibiotics, a subgroup of a larger group of threatened agents known as antimicrobials. Some strains of tuberculosis, for example, are now resistant to all available antimicrobial drugs. Unfortunately, tuberculosis is not the only resistant microorganism on the public health horizon.

Why are these drugs losing their power? Because they're being overused. Bacteria become resistant to antibiotics through overexposure to them. Hardy strains of the bacteria survive the exposure and pass on that resistance trait to successive generations. And they also pass the trait across to other bacteria that are unrelated, including some that cause human disease. Eventually the antibiotic wipes out all the vulnerable bacteria, and only resistant bacteria remain. Then the drug is no longer effective.

Preserving the effectiveness of antibiotics and other antimicrobials will require changes in all major areas of use: human medicine, veterinary medicine, and agriculture. But agricultural uses deserve special attention, since they account for 70 percent of the antibiotics and related drugs used in the United States, and since they provide resistant bacteria with a direct route into people's kitchens.

From Feedlot to Kitchen

Resistant bacteria that develop in CAFOs (confined animal feeding operations) can be transferred to the general human population via food. The government, public health officials, and physicians are increasingly concerned about foodborne diseases caused by Campylobacter and Salmonella bacteria. According to the Centers for Disease Control and Prevention (CDC), nationwide there are 76 million cases of foodborne illness a year and 5000 deaths from viral and bacterial pathogens.

As resistant strains of bacteria emerge, they have easy passage to humans—right through the grocery store. Campylobacter, for example, is carried into kitchens on poultry and can cause illness when people eat raw or undercooked poultry meat. While this does not always cause severe illness, the CDC estimates that there are two to four million Campylobacter infections per year, resulting in as many as 250 deaths each year in the United States. Furthermore, about one in a thousand Campylobacter infections leads to Guillain-Barré Syndrome, a disease that can cause paralysis. Thus, the emergence of drug-resistant Campylobacter is a serious public health concern.

In fact, the use in poultry of fluoroquinolones, a precious class of antibiotics, led to the development of resistant Campylobacter strains. Before fluoroquinolones were approved for use in agriculture in the United States, no fluoroquinolone resistance was reported in people unless they had previously taken the drugs for illness or traveled to a country that permitted their use in agriculture. But after the antibiotics were approved for agricultural use, resistant strains began emerging in samples taken from both humans and poultry. The correlation of the emergence of resistance with the use in animal systems was important evidence that agricultural use was the culprit. The FDA recognized the seriousness of the threat and banned fluoroquinolones from veterinary use in September 2005.

Antimicrobial use in agriculture can also compromise human therapies when bacteria develop cross-resistance—when their resistance to one drug also makes them resistant to other, related drugs. This has happened in Europe with vancomycin, one of the drugs of last resort for treating certain life-threatening infections. Data suggest that rising levels of vancomycin-resistant bacteria in hospitals may have resulted from use in agriculture of avoparcin, a drug chemically related to vancomycin. Because avoparcin and vancomycin are similar in structure, bacteria resistant to avoparcin are resistant to vancomycin as well.

Similar phenomena are apparently occurring as a result of the use of antimicrobial drugs in the United States. The effectiveness of synercid, a drug of last resort for the treatment of vancomycin-resistant infections, is threatened because of the use of virginiamycin as a growth promoter in chickens and pigs in the United States. Virginiamycin is chemically related to synercid, and bacteria resistant to the one drug also appear to be resistant to the other.

While the links between animal agriculture and human disease are complicated and in need of additional study, evidence is strong enough for scientists and public health organizations to call for reduced use of antibiotics in agriculture. The CDC has concluded that, in the United States, antimicrobial use in food animals is the dominant source of antibiotic resistance among foodborne pathogens.

Keeping Drugs Useful, Keeping Meat Cheap

What can be done so that these drugs remain useful? Aren't antibiotics necessary to preserve the health of the livestock? While some uses of antibiotics in livestock operations are a matter of animal health, other uses have an economic motive. Especially troubling is their use not to cure sick animals but to promote "feed efficiency," that is, to increase the animal's weight gain per unit of feed. These drugs are also regularly added to the feed and water of animals that are not sick in order to prevent diseases caused by overcrowded and unsanitary CAFO conditions. These nontherapeutic uses translate into relatively cheap meat prices at the grocery store.

But is this economic motive an essential use of these drugs? First, the economic advantage appears to be minimal. The National Research Council estimated that a ban on nontherapeutic use (that is, any use in livestock that are not sick) would increase per capita costs by about $5-10 per year. That is a price most people would willingly pay to preserve a robust arsenal of medicines against infectious disease.

Second, using antimicrobial drugs is not the only way to lower meat costs. The same report suggests that adopting other methods of maintaining animal health, comfort, and well-being could reduce drug use and cut costs. Such methods might include reducing overcrowding, controlling heat stress, providing vaccination to prevent disease, and using beneficial microbial cultures.

The Challenges of Reducing Antibiotic Use

Although reducing or eliminating nontherapeutic uses of antibiotics is a straightforward solution to the problem of resistance, it will be difficult to implement. Eliminating this use of antibiotics challenges the standard operating procedures of a large and powerful industry.
The nontherapeutic use of antibiotics is ingrained in livestock and poultry operations because producers believe that chickens, cows, and pigs—particularly those that are not healthy to begin with—gain weight faster when these drugs are added to their feed.

In addition, livestock producers have bought into the myth that bacteria that cause illness in humans develop resistance only in medical settings. While no one denies that unwise use of antibiotics in human medicine is a source of serious resistance problems, this view has prevented recognition of one of the best opportunities to cut back on these drugs—in nontherapeutic agricultural applications.

Agricultural use for growth promotion and prevention of diseases due to overcrowded CAFO conditions accounts for the vast majority of the antibiotics and related drugs used in the United States. This enormous amount of drugs is delivered to animals under conditions conducive to the development of resistance. Large numbers of similar animals are raised in CAFOs that characterize contemporary agriculture. Chicken houses, for example, can contain 50,000 birds. And the Environmental Protection Agency estimates there are about 11,000 operations with over 1,000 beef cattle, 700 dairy cattle, 2,500 hogs, or 30,000–125,000 chickens.

In such large operations, antibiotics are often delivered to animals in food and water over extended periods. Bacteria are constantly being exposed to the drugs and eliminated from the populations. It is hard to imagine how resistance would not develop under these circumstances. Indeed, industrial livestock systems are hog heaven for resistant bacteria.

What's Next

The battle against emergence of antimicrobial resistance will take place on many fronts: in hospitals, in doctors' and veterinarians' offices, and on farms. The most sensible approach is to identify and reduce nonessential uses of antibiotics and reserve as many of these drugs as possible for wise use in human and veterinary medicine. Obvious nonessential uses, such as nontherapeutic use in livestock operations, should be the first target in the effort to save antibiotics. The CDC and the World Health Organization have called for an end to the nontherapeutic use in animals of drugs that are used to treat human disease or that are related to such medicines.

Last revised date: August 11, 2009

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