New study adds to concerns about animal-to-human resistance to antibiotics


Yet another study has found stuff you don’t want to eat in stuff that you eat.

On April 15, scientists reported that the meat bought at supermarkets is often contaminated with Staphylococcus aureus bacteria that are resistant to antibiotics used to fight human disease.

The study, published in the journal Clinical Infectious Diseases, found staph on 47% of 136 samples of beef, chicken, pork and turkey from 26 grocery stores in five U.S. cities. Of those bacteria, 96% were resistant to at least one type of antibiotic and more than half were resistant to at least three.

The advice for consumers remains unchanged: Cook meat thoroughly — heat kills bacteria — and wash items such as cutting boards and knives that come into contact with meat.

The larger concern is what all this means for public health.

Lance Price at the Translational Genomics Research Institute in Flagstaff, Ariz., and coauthors concluded that the resistant staph on meat was probably coming from the animals — and not, say, a worker’s unclean hands. This seems to point the finger at antibiotic use in agriculture.


“That’s the most logical explanation,” says Dr. Gail Hansen, a trained veterinarian and senior officer in human health and industrial farming at the Pew Charitable Trusts, based in Washington, D.C., which funded the study. (The Pew trust opposes routine use of antibiotics in agriculture.)

Antibiotic overuse is a problem because the drugs trigger an arms race: Sensitive bacteria won’t grow in their presence but drug-resistant forms will — and multiply. The quantity of antibiotics used in raising animals for food dwarfs that used by humans by a factor of four. And, for the most part, the drugs aren’t used to treat sick animals but are administered routinely.

Here’s a closer look at the issue.

What’s the deal with antibiotic resistance?

This is a growing problem in the U.S. and around the world. Infections caused by resistant bacteria are harder to treat, leading to longer — potentially more severe — illnesses. When infections are no longer susceptible to first-line medicines, pricier therapies must be used.

Estimates vary, but extra costs resulting from antibiotic resistance in the U.S. run well into the billions. “We have a crisis in clinical medicine,” says Ellen Silbergeld, an expert in the health and environmental impacts of industrial food animal production at Johns Hopkins Bloomberg School of Public Health in Baltimore.

One notable antibiotic-resistant organism — methicillin-resistant Staphylococcus aureus, or MRSA — is causing plenty of grief in hospitals and communities right now. “MRSA doesn’t just spring out of the ground,” Silbergeld says, but scientists don’t know much about where, outside of hospitals, this dangerous bacterium comes from. Now that antibiotic-resistant staph has been found on meat, it should be investigated as a potential source of MRSA infections in the community, she says.


Why do meat producers use antibiotics in healthy animals?

Antibiotics have been shown to promote growth. Nobody really knows the reason: It could be through protection against disease, or how food is digested.

And giving antibiotics keeps infections from spreading in the crowded conditions of large food animal operations, should one animal get sick.

As long ago as 1969, a report commissioned by the British government recommended a ban on the use of antibiotics as growth promoters for animals. Since then calls for restrictions, particularly on antibiotics used to combat human infectious disease, have come from a slew of organizations, including the World Health Organization, the Centers for Disease Control and Prevention, the American Medical Assn and the Union of Concerned Scientists. Yet the practice remains.

Denmark, the world’s largest pork exporter, has made changes with good results. From 1992 to 2008, antibiotic use in pig-rearing was cut by more than half (as measured per kilogram of pig). During that same period, overall production increased, animal growth rates improved and death rates didn’t change, according to a 2010 report in the American Journal of Veterinary Research.

But Dr. Scott Hurd, a trained veterinarian and researcher at Iowa State University in Ames, says industry numbers show that only 13% of antibiotics used in food animal farming are for growth promotion and adds that preventive use is necessary in certain situations. “It’s always based on previous experience, when you know the animals are going to get sick,” he says. He notes that in Denmark, the amount of antibiotics used for treatment has doubled, indicating that a lot more animals are getting sick.

Hurd adds that changing practice in the name of public health would be counterproductive. “We lose a lot of our tools to keep animals healthy,” he says. “Even marginally ill animals are more likely to be contaminated with Salmonella and Campylobacter, so you have the potential to decrease public health.”

Can antibiotic-resistant bacteria jump from animals to humans and cause disease?


Yes. The first documented instance of antibiotic-resistant staph transmission from animals to humans was in the Netherlands in 2003: A child had a MRSA infection and the only risk factor was that she lived on a pig farm. The bacterial strain she contracted matched that of the pigs, and her family also were found to be carriers, although they weren’t sick.

“Now this strain makes up 30% of community-acquired disease in the Netherlands,” Price says. “It’s really emerging quickly.”

In the U.S., a 2009 study published in the journal PLoS ONE found MRSA in 49% of 299 pigs tested at two different pig farms, as well as in nine of 20 tested farmworkers there. “We think pigs are reservoirs and transmit to humans,” says study lead author Tara Smith, an infectious disease researcher at the University of Iowa in Iowa City. “But we don’t know that for sure.”

What’s being done to tackle the issue of antibiotic resistance?

The biggest effort in the U.S. is the National Antimicrobial Resistance Monitoring System, which involves the CDC, Food and Drug Administration and the U.S. Department of Agriculture. Currently, four bugs are tracked — Salmonella, Campylobacter, E. coli and Enterococcus — but not staph.

The FDA has drafted guidelines (not yet finalized) for judicious antibiotic use in food animal farming that include measures to limit use of drugs used as human medicines and add veterinary oversight. But this is only advice and not enforceable.

Rep. Louise Slaughter (D-N.Y.) has authored the Preservation of Antibiotics for Medical Treatment Act, which calls for the phasing out of non-therapeutic use of medically important antibiotics in livestock.


A variety of campaigns are in place to reduce antibiotic use in medicine. For example, the CDC’s Get Smart campaign urges healthcare providers to adhere to prescription guidelines and works to educate patients about judicious use of the drugs.