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The same commercial animal farms that provided the breeding grounds for the novel H1N1 A virus that caused the current “Swine Flu” pandemic are home to another dangerous threat to human health: antibiotic resistance.
In the United States, feed animals – poultry, swine, cattle and sheep – are routinely fed low doses of antibiotics through their water or food troughs to promote growth and expedite weight gain. A series of large scale studies conducted by the Department of Agriculture in 1999, 2001 and 2006 revealed that over 80 percent of swine farms, cattle feedlots and sheep farms administer antibiotics for non-therapeutic purposes (i.e., not used to treat diseases)i. Many of the antibiotics used on animals are identical or closely related to those used to prevent infections among humans, including tetracyclines, macrolides, bacitracin, penicillins, and sulfonamides. Bacteria in animals (as in humans) are able to develop antibiotic resistance when exposed to low doses of drugs over a long period of time, contributing to the rise of pathogens that are able to defeat our shared antibiotic arsenal.
For over a decade, multiple scientific studies have confirmed that the use of antibiotics in agricultural animals contributes to the development of resistant bacterial infections in humans. A March 2003 National Academy of Sciences report stated that “a decrease in antimicrobial use of human medicine alone will have little effect on the current situation of growing antibiotic resistance. Substantial efforts must be made to decrease inappropriate overuse in animals and agriculture as well.”ii Similarly, in 2000, a WHO report on infectious disease stated “Since the discovery of the growth-promoting and disease-fighting capabilities of antibiotics, farmers, fish-farmers, and livestock producers have used antibiotics in everything from apples to aquaculture. This ongoing and often low-level dosing for growth and prophylaxis inevitably results in the development of resistance in bacteria in or near livestock, and also heightens fear of new resistant strains “jumping” between species...”iii
Despite laudable efforts by the medical community to curtail human overuse of antibiotics, the U.S. is among the last developed countries to implement the same control polices on their use in animals. An estimated 70 percent of all antibiotics sold in the United States are used on healthy livestock, according to a study by the Union of Concerned Scientists.iv Globally, it is estimated that half of the antibiotics produced in the world are not used to improve human health, but for usage on the farm.v
Animal to Human Antibiotic Resistance: Routes of Transmission
Resistant bacteria in animals can be transmitted to humans via three pathways: through consumption of meat, from close contact with animals, or through the environment (in streams, ground soil etc).
Via food: Recent studies have found that meat in U.S. grocery stores is widely infected with antibiotic resistant bacteria. Five out of 90 samples of retail pork in the state of Louisiana tested positive for methicillin-resistance Staphylococcus aureus or MRSA – an antibiotic resistant staph infection – according to a 2008 study in Applied and Environmental Microbiology.vi Another recent study found MRSA in one out of 320 samples of retail meat in the Washington D.C. area. In 2007, the USDA issued a fact sheet on the recently identified link between antimicrobial drug use in animals and MRSA infections in humans and disturbing new studies reveal that hog farms are a source of a deadly new strain of MRSA (ST398).
Via working with animals: People working in the livestock industry may be exposed to the resistant bacteria by handling animals, feed or manure. Workers can then pass the resistant bacteria to family and community members.
Via the environment: Nearly two million pounds of manure produced by livestock each year in the U.S. contaminate groundwater and soil. The manure from antibiotic-fed livestock contains resistant bacteria, creating a huge pool of resistant pathogens available for mutation to bacteria forms that cause human diseases. The U.S. Geological Survey reported in 2002 that antibiotics were present in 48 percent of streams tested nationwide – and almost half of the streams tested were downstream from agricultural operations.vii
However, drug resistance is not limited to bacteria. A 2005 Washington Post exposé revealed that for years Chinese poultry farmers had been lacing their chicken’s water supply with an antiretroviral drug, amantadine, to prevent economic losses caused by birth flu infection. The overuse of amantadine by Chinese farmers has been blamed for the emergence of widespread viral resistance to a life-saving drug in a pandemic – sacrificing a whole class of antiretroviral drugs.viii
The strongest links between animal antibiotics and resistance in humans may be data from the European experience. In 1998 the European Union (EU) banned feeding of antibiotics to animals that are valuable for human health and, in 2006, the EU banned all antibiotics and related drugs to livestock for growth promotion purposes. Following introduction of these measures, there was a decrease in the levels of antibiotic resistance found in farm animals, in meat, and within the general human population.ix
Policy Actions to Prevent Future Spread
Although there has been heightened public concern over the use of antibiotics in feed animals in recent years, the debate is not new among concerned scientists and U.S Federal Agencies. The FDA came close to banning the practice over 30 years ago, but powerful farm and pharmaceutical lobbies crushed the effort. A recent attempt from the FDA to withdraw the fluoroquinoline antibiotic enrofloxacin from use in poultry due to concerns about its link to human antibiotic resistance was challenged by pharmaceutical manufacturers. The court battle dragged on for more than five years and cost taxpayers millions of dollars in legal fees.x All the while, the resistance-causing antibiotics continued to be used. In contrast, in countries like Norway, Sweden and Australia, where quinolones are reserved exclusively for human use, quinolone-resistant bacteria are practically unknown.xi
Many of the current drugs used for non-therapeutic purposes in animals were approved before the FDA began its current processes of in-depth research and consultations of the potential health risks to humans. The FDA currently has no established schedule for reviewing existing drug approvals that were passed before the current review regime was in place. A 2004 GAO report revealed that research on the human health risk is crippled by a lack of data on the types and amounts of antibiotics used in animals and lack of coordination between agencies charged with its monitoring and regulation.xii
Major reductions in antibiotic use can be achieved by terminating existing approvals of medically important human antibiotics for non-therapeutic purposes in animals. But cancelling the approvals of existing drugs used in animal agricultural has proved to be immensely difficult– as it requires sufficient data to prove their harm on human health, coupled with political will and coordination among the agencies that regulate such drug use – the USDA, CDC, HHS and FDA. Past ineptness in inter-agency coordination and action has left legislating a ban through Congressional action as the only seemingly viable option.
Current legislation to prevent the use of antibiotics important to human health from being used non-therapeutically in animals was introduced in March 2009 by the only microbiologist in the House of Representatives, Congresswoman Louise Slaughter of New York, and in the Senate by Edward Kennedy. The bill is currently awaiting committee action.
"We're up against a pretty strong lobby. It will come down to whether members of Congress want to protect their constituents or agribusiness," said Slaughter in a March 2009 interview with Reuters. "We are losing the ability to treat humans. We have misused one of the best scientific products we have."
Among the newly appointed heads of key U.S. agencies involved in antibiotic regulation, the general public, congressional leaders, and the medical community, there is a growing understanding of the potential dangers of inaction both in terms of human health and the economic toll that unabated non-therapeutic use of antibiotics in animals will have in the future. The U.S. position on the use of antibiotics in livestock differs from many of its major trading partners. Export bans on U.S. produced meat may occur in the future. The dangerous rise of bacterial resistance among animals and humans will continue unabated unless key agencies and congressional leaders enact and implement legislation to ban this dangerous practice.
iAnimal and Plant Health Inspection Service. Feedlot ’99. Part 3: health management and biosecurity in US feedlots, 1999. Washington, DC: US Department of Agriculture, December 2000.
iiSmolinski MS, Hamburg MA, Lederberg J, eds. 2003. Microbial Threats to Health: Emergence, Detection, and Response. (Washington DC, National Academies Press).
iiiWorld Health Organization, 2000. Report on Infectious Diseases.
ivMellon MG, Benbrook C, and Benbrook KL. 2001. Hogging It! Estimates of Antimicrobial Abuse in Livestock (Cambrirdge, MA: Union of Concerned Scientists).
viPu S, Han F, Ge B, 2008. Isolation and Charaterization of Methicillin-resistant Staphyloccus Aureus strains from Louisiana Retail Meats. Journal of Applied Environmental Microbiology, 2008 Oct 31; 75(1): 265-7.