Introduction | Antibiotics in Human Medicine | The Antibiotic Arsenal | Antimicrobial Use in Food Animals | Solutions to the Resistance Problem
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Antibiotics are routinely added to feed and water to prevent disease and to promote growth in food animals. Photos courtesy of the Agricultural Research Service of the USDA.
No one knows for sure how much of the antibiotics used in this country are consumed by animals being raised for food, such as cattle, dairy cows, pigs, and poultry. One estimate places the number at 20 million pounds of antimicrobials each year.
What is known is that the use of large quantities of antibiotics in producing the nation's food supply contributes to antibiotic resistance.
Bacteria can move between ecosystems, animals, and humans. Antibiotic resistant bacteria from the intestines of animals enter the food supply and can then be introduced into the human intestine when food is consumed.
There is some evidence that resistance genes carried by the bacteria in animals can be transferred to bacteria that are normally human-specific. For example, the use of the antibiotic avoparcin as a growth promoter in food animals has been linked to the subsequent appearance of vancomycin-resistant enterococci in human intestines.
Some antibiotic resistant Salmonella cases have been traced back to meat from animals fed antibiotics.
Epidemiological information indicates that food of animal origin is the source of the majority of foodborne bacterial infections caused by non-typhoid Salmonella, Campylobacter, Yersinia, and E. coli O157:H7.
Food animals are often given the same drugs used in humans. Antibacterials are also used in veterinary medicine, fish farming, and agriculture, and have been found in food, soil, and water. Many are available at feed and pet stores without a prescription.
Most of the antimicrobials given to food-producing animals each year are not used to treat sick animals.
Instead, antibiotics are routinely added to feed and water to prevent disease and to promote growth. This long-term, low-dose exposure to antibiotics is more likely to result in resistant bacteria than short-term antibiotic use to treat sick animals.
The practice of giving subtherapeutic doses of antibiotics to prevent disease and promote growth dates back to the 1950s, but even now the mechanisms are not well understood. The controversy over the practice dates back almost as far.
In the 1960s, scientists began raising concerns about the emergence of multiple drug-resistant strains of bacteria and the possibility of cross-resistance with therapeutic antibiotics used in humans.
Food and Drug Administration (FDA) officials proposed restricting the use of penicillin and tetracycline (antibiotics commonly used in humans) in 1977, but were overruled by Congress, which requested that the National Academy of Sciences (NAS) conduct a study.
The NAS concluded that no restrictive actions should be taken on the sub-therapeutic use of antibiotics in animal feed. A 1999 report once again concluded that there was no immediate public health risk from antimicrobial use in food animals, but did acknowledge that “there is a link between the use of antibiotics in food animals, the development of bacterial resistance to these drugs, and human disease, although the incidence is very low.”
In the meantime, Danish food animal producers voluntarily gave up use of antibiotics as growth promoters altogether in 1998. (Denmark had banned such use of specific antibiotics in the 1970s and 1990s.) Sweden banned the use of all antibiotics as growth promoters in 1986. In 1999, the European Union banned the use of four antibiotics as growth promoters.
Here in the U.S., the FDA proposed a ban on the use of fluorquinolones in poultry in late 2000. Fluorquinolones are important in the treatment of human infections because of their broad spectrum of activity against a range of infectious bacteria and their safety and ease of administration.
Many scientists are concerned about potential problems from the apparent relationship between the use of the agents in food-producing animals and the emergence of Salmonella serotypes with reduced susceptibility to fluorquinolones in humans. Since their introduction for use in poultry, there has also been a significant rise in fluoroquinolone-resistant Campylobacter jejuni isolated in live poultry, poultry meat, and humans in the U.S., U.K. and Netherlands.
FDA has proposed a regulatory framework for antimicrobial drugs used in food-animal production, and is developing a guideline document for industry. The framework seeks to rank drugs by their importance to human medicine and to provide a risk-based framework for their use in animals. FDA's Task Force on Antimicrobial Resistance expressed “particular concern that certain drugs, of classes currently viewed as critical for human medicine, are already being used in food animals.”
The Task Force is part of a recent government-wide effort to combat antimicrobial resistance.
Legislation introduced into Congress in June 2002 would prohibit the nontherapeutic use in feed animals of eight specific antimicrobial drugs that could select for resistance to drugs used in human medicine.
The American Veterinary Medical Association recommends using narrow spectrum drugs when possible, limiting treatment to only sick or at-risk animals rather than dosing whole herds or flocks, and using drugs important to human health only after careful consideration. The association calls for further research to determine the risks of sub-therapeutic levels in animal feed to promote growth before prohibitions are imposed.
In the U.S., the Department of Agriculture collects information about resistant bacteria in animals as well as antibacterial drug residues in food. While the levels of antibacterials in food that might promote resistance in humans are not known, their use can contribute to the pool of resistant pathogens.
The National Antimicrobial Resistance Monitoring System (NARMS) monitors the occurrence of drug resistant microbes in both humans and animals. This coordinated network of public health laboratories and federal agencies collects samples of specific microbes found in people and animals and sends them for testing to determine if they are resistant to antibiotics. The results of these tests are compared with data from previous years to look for changes in resistance patterns. NARMS reports are published annually.
Surveillance for antibiotic resistance in agricultural settings is being expanded to all 50 states and a study is being launched of resistant pathogens found on retail foods. The Environmental Protection Agency is reviewing current scientific data on the effect of antimicrobial pesticide products.
Five years after Denmark banned the use of antibiotics as growth promoters in food animals, resistance rates of bacteria to these antibiotics fell significantly.
Following Denmark's ban of the antibiotic avoparcin in food animals in 1995, the resistance rate for this and related antibiotics fell from 72.7% in 1995 to just over 5% in 2000.
One of the concerns raised by calls to reduce or prohibit antimicrobial use in food animals is that production costs will rise.
A University of Iowa study of the effect of Sweden's ban on antimicrobials for growth promoters in the hog industry found an estimated net increase of consumer costs of about $0.12 +/- 0.06/kg retail meat, half of which was due to the antibiotics ban and half to animal welfare legislation.
Because of differences between farming and food production practices in Scandinavia and the U.S., it can't be assumed that the exact same outcomes would be seen following a ban of antimicrobial use on American farms and feedlots.
The impact of such a ban needs to be studied to examine issues such as the incidence of disease in flocks without preventive antibiotics, the effect on food production and the general food supply, and how to address possible economic losses for farmers.
This examination of possible outcomes and ways to address them should include participation by farmers, food producers, scientists, the pharmaceutical industry, and public health officials. For example, if the proposed legislation becomes law, steps could be taken to help compensate farmers and food producers for losses associated with the transition away from antimicrobial use.
The University of Iowa report on potential effects on the hog industry estimated that a ban in the U.S. would cause production costs to initially rise by $6.04 per hog, tapering to an increase of $5.24 after 10 years. The decrease in net profit would be $0.79 per hog after 10 years because of higher prices charged to consumers. Retail pork prices would increase by $0.05 per pound.
Introduction | Antibiotics in Human Medicine | The Antibiotic Arsenal | Antimicrobial Use in Food Animals | Solutions to the Resistance Problem
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