Scientists: Reduce, don't ban, antibiotics

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Is the high-pitched political wrangling over the use of antibiotics in livestock obscuring a legitimate public health concern?

Bold proclamations of doom by natural-food advocates notwithstanding, no one knows for sure the extent to which agriculture contributes to diseases' resistance to drugs meant to treat illness in humans and animals.

Even scientists who would abolish the non-therapeutic use of antibiotics in farm animals concede that people are mainly to blame for an overuse of the drugs that researchers say caused 65,000 more deaths in the U.S. in 2008.

Exposure sources

Food is one of many potential sources of exposure to antibiotics for humans, who can also be exposed to the agents in such innocent things as Lysol and hand soaps, asserts Chad Mueller, an Oregon State University beef cattle systems researcher.

Still, many scientists -- including those in ag circles -- suspect that use of the drugs in livestock contributes to the resistance problem, and suggest that better management of herds can be part of a solution.

"Our goal is ... to reduce the need to ever use the drug while still protecting the safety of food to the consumer," said Doug Call, associate professor at Washington State University's College of Veterinary Medicine.

"One (solution) is to ... do everything we can to minimize the probability of having illness by cleaning up operations that are poorly run," he said. "Then you have less need for therapeutic use. We also need to battle a culture of using subtherapeutics, to educate producers that this doesn't really help you in the long run."

An antibiotic is a natural or synthetic medicine that attacks microbes such as bacteria and fungi. The U.S. Food and Drug Administration allows antibiotics to be used in animals to treat and prevent illness or to allow the animal to more efficiently digest feed. Subtherapeutics are drugs administered below the dosage level used to treat disease, such as in animal feed.

It's that second use -- to promote the growth of animals -- that draws the most ire from advocacy groups such as the Union of Concerned Scientists, which claims that about 25 million pounds a year are used for such nontherapeutic purposes alone.

Groups such as the Alliance for the Prudent Use of Antibiotics argue these treatments are a big reason for increases in drug-resistant infections in people, and the groups have allies in Congress and in the Obama administration. Legislation to eliminate nontherapeutic use of antibiotics is expected to move forward in 2010.

Data unclear

It's unknown how much antibiotics are used in agriculture because the government does not track it. The Animal Health Institute, a pharmaceutical industry group, estimates that 20.2 million pounds were produced for farm and companion animals in 2003.

The U.S. Centers for Disease Control and Prevention has estimated that about 150 million prescriptions for antibiotics are given to people each year, and as many as 50 million of those are unnecessary.

"I don't think anybody has a good handle on ... the relative contribution of antimicrobial use in a different setting to the antimicrobial resistance we see in humans," said Jean Patel, deputy director of the CDC's office of antimicrobial resistance.

What is well known, Patel said, is humans' exposure to resistant germs in health-care settings such as hospitals and through over-prescription of the drugs by doctors. The CDC has undertaken public education campaigns to address both of those issues, as well as one aimed at farmers.

The exposure doesn't stop there. Federal regulators are beginning to take notice of pharmaceutical residues in the nation's drinking water. And sewage treatment plants are "loaded with everything from cosmetics to anti-cancer drugs to antibiotics," providing an ample surrounding for germs to develop resistance, said John Maas, a beef extension veterinarian at the University of California-Davis.

Even resistance itself is a moving target, noted Ellen Jo Baron, associate director of clinical microbiology at Stanford University's School of Medicine. A consensus group of microbiologists, physicians and pharmaceutical industry representatives meets each year to determine resistance criteria, she said.

"The definition of resistance is not clear cut," Baron said, explaining that the scientists must take into account such variables as dosing.

Use varies

In agriculture, the use of antibiotics varies from animal to animal and farm to farm. OSU's Mueller estimates that 7 to 10 percent of producers use feed-grade antibiotics to promote growth, and more of it is used for swine than for ruminants.

With cattle on grazing land, antibiotics are typically only given when an animal is ill, said Larry Forero, a University of California researcher in Redding, Calif.

In feedlots, Maas said, three compounds are typically used as feed additives -- monensin, which increases feed efficiency, and tylosin and chlortetracycline for preventing disease. None have much relevance to human medicine, he said.

Poundage estimates for antibiotics given to livestock are often misleading, Maas argues, because they represent entire bags of feed and the active ingredient is typically only a tiny percentage of the mixture.

The Union of Concerned Scientists' 25 million-pound estimate refers to active ingredients, counters Margaret Mellon, the group's director of food and environmental programs. The estimate was made after considering a variety of factors, including which drugs are approved for which species, what dosages are used and the lifespans of animals given the drugs, she said.

"This is an enormously important public debate about the use of the drugs and the overuse in animals and humans," said Mellon, who coauthored the 2000 book, "Hogging It: Estimates of Antimicrobial Abuse in Livestock."

Forero compares the drugs' use in feedlots to immunizations of schoolchildren who can pass germs in close quarters.

"It's a tool to keep those animals healthy," he said.

Ag has role

Dr. Stuart Levy, an author and nationally recognized expert on antibiotic resistance at Tufts University in Boston, concedes that "finger-pointing to animal usage as the cause for human problems of resistance is a gross error."

But agriculture certainly contributes, Levy said -- particularly with strains of drug-resistant salmonella, E. coli and campylobacter, which have been proven to be transmitted from animals to people.

Such things as runoff from farms into streams and the use of manure on vegetables contribute to the broader problem, said Levy, president of the Alliance for the Prudent Use of Antibiotics.

Antibiotics' growth-promoting qualities were discovered "by accident" in the 1950s, largely because the drugs were preventing illness, he said.

"The practice has continued, but the question is do you still need it? Europe has proven that you don't need it," Levy said.

"When I entered the field 30 years ago, the argument from the industry was that the farm was on a different planet than people," he said. "If you want to look at what is the environmental impact of this, it's enormous, so if you don't need it, get rid of it. There are those that don't accept that. They're asking for more data and more data, but there are those of us who feel the data is there."

Stanford's Baron agrees.

"I think there's so much pressure on these enormous factory farms ... to have fast-growing, healthy animals and they're using antibiotics," she said. "When you cram all those animals together in an unnatural environment, it's much more likely disease would spread quickly.

"I think there's economic reasons why antibiotics are used in our livestock, but I think it's a bad thing," she said.

At least some concerns about agriculture-related resistance problems have merit, acknowledges WSU's Call. A "fairly strong smoking gun" exists when it comes to a class of synthetic drugs called fluoroquinolones, which interfere with the ability of bacteria to replicate DNA.

"The challenge with this class of drugs is that bacteria can become resistant by some very simple mutations in the chromosome," Call said. "You might imagine that if you put a large number of bacteria under pressure from the drug, some of them will come up with a mutation and become resistant. In a couple of generations or one generation ... that resistant population expands in the niche."

One fluoroquinolone, Baytril, was recently banned for use in poultry because of evidence that it spawns resistance in campylobacter, which typically doesn't affect chickens or turkeys but is harmful to humans.

A majority of antibiotics used in livestock belong to classes of drugs that are also used to treat human illness, asserts the National Center for Zoonotic, Vector-Borne and Enteric Diseases. Those include tetracyclines, sulfonamides, penicillins, streptogramins and other classes, explains the agency, which is part of the CDC.

That doesn't mean putting heavy restrictions on the use of antibiotics in agriculture would necessarily be effective, Call said. In some tests when antibiotics are taken away or when none were ever given, scientists still notice an increase in resistance, he said.

"The question becomes what has happened with these strains?" he said. "Is the horse already out of the barn?"

Adding to the resistance problem is the fact that in many developing countries, antibiotics can be bought right off the shelf, he said.

That said, scientists need to help agriculture reduce its impact on antibiotic resistance, perhaps by genetically making it more expensive to the bacteria or by enhancing delivery of natural antibody-laden colostrum to calves, he said.

"We have to have a cheap and easy way to help producers make those judgment calls," Call said. "There's a variety of ways that can be addressed, but it needs to be more aggressively pursued. ... On the other hand, there have got to be better ways to solve the problem than to call for blanket restrictions that don't do any good."

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