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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 -- drugs that are most familiar as 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 provide resistant bacteria with a direct route into people's kitchens.

From Feedlot to Kitchen

Bacteria that become resistant in agricultural, particularly livestock, operations can be transferred to the general human population via food. According to a 1998 National Research Council study, The Use of Drugs in Food Animals: Benefits and Risks, the reported incidence of bacteria-related food-borne illness is increasing. The government is increasingly concerned about food-borne diseases caused by Campylobacter and Salmonella. As resistant strains of bacteria emerge, they have easy passage to humans -- right though 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 Centers for Disease Control and Prevention (CDC) estimate 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 Guillan-Barre syndrome, a disease that can cause paralysis. Thus, the emergence of drug-resistant Campylobacter would be a serious public health concern.

In fact, Campylobacter is becoming resistant to the fluoroquinolones, a precious class of antibiotics, as a result of agricultural use. Only recently were fluoroquinolones approved for use in poultry in the United States. Before this use, 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 now, resistant strains are emerging in samples taken from both humans and poultry. The correlation of the emergence of resistance with the use in animal systems is important evidence that agricultural use is the culprit.

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, so that 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 antimicrobial growth promoters in agriculture. The CDC has concluded that, in the United States, antimicrobial use in food animals is the dominant source of antibiotic resistance among food-borne pathogens.

Beefing Up Food

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. This so-called subtherapeutic use translates 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 study estimated that a ban on such subtherapeutic use in livestock would increase per capita costs between $5 and $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.

Reduction Problems

Although reducing or eliminating the use of antibiotics to promote growth is a straightforward solution to the problem of resistance, this will be difficult to achieve. Eliminating this use of antibiotics challenges the standard operating procedures of a large and powerful industry.

The subtherapeutic use of antibiotics is ingrained in livestock operations because it works. Chickens, cows, and pigs -- particularly those that are not healthy to begin with -- do gain weight faster when these drugs are added to their feed, and those gains translate into higher profits. 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 most attractive opportunities to cut back on these drugs -- in subtherapeutic agricultural applications.

Agricultural use, much of it for growth promotion, accounts for 40 percent of the antibiotics sold in the United States. This enormous amount of drugs is delivered to animals under conditions congenial to the development of resistance. Large numbers of similar animals are raised in the concentrated facilities that characterize contemporary agriculture. Chicken houses, for example, can contain 20,000 birds. And the Environmental Protection Agency has identified 6,600 operations with at least 1,000 beef cattle or 700 dairy cattle or 2,500 hogs or 100,000 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 their subtherapeutic use in livestock operations, should be the first target in the effort to save antibiotics. Indeed, the CDC and the World Health Organization have called for an end to the use for growth promotion in animals of those drugs that are used to treat human disease or that are related to such medicines. UCS is taking a careful look at the health risks of industrial agriculture and will be working to reduce the subtherapeutic use of antibiotics in livestock.