Experts wrangle over effects of antibiotics use in livestock
by Michael Schroeder and Jenni GlennThe Journal Gazette
Dr. Tim Barman, director of veterinary services for Cooper Farms in Ohio, prescribes antibiotics to keep disease outbreaks among 12,000 baby turkeys from getting “bigger and bigger.”
Farms often use antibiotics to promote growth in livestock, which health groups say contributes to drug-resistant bacteria in people. Baby turkeys are sectioned off in pens at Cooper Farms. A benefit of using antibiotics to treat illness rather than promote growth, Barman said, is that healthy animals grow faster.
The bacteria: Methicillin-resistant Staphylococcus aureus, or MRSA, is a type of bacteria that’s resistant to certain antibiotics such as methicillin, oxacillin, penicillin and amoxicillin. Common in nature, MRSA can exist on the bodies of humans and animals – on the skin, in the nose – without causing damage.
Deaths: In cases where MRSA enters the body, such as through a wound, it can lead to serious infections. MRSA is responsible for an estimated 94,000 serious MRSA infections and 19,000 deaths in the U.S. annually.
Signs and symptoms: Most staph infections, including those caused by MRSA, appear as a bump or infected area on the skin that may be red, swollen, painful, warm to the touch, perhaps draining. The symptoms may also include a fever.
MRSA strains: MRSA infections usually fall into one of two categories: hospital-acquired or community-acquired. When MRSA was discovered, people typically contracted it at hospitals and health clinics. In the past two decades, however, the number of MRSA infections in people who have had no connection to any health care setting has increased. But hospital-acquired MRSA still makes up the bulk of known MRSA infections.
The strain of MRSA that is regularly isolated from livestock, often referred to as ST398, is different from the strains responsible for hospital-acquired and community-acquired MRSA.
Livestock and MRSA: A Denmark study several years ago identified MRSA in both swine and people who worked with swine. Those who worked with the infected pigs were more likely to be infected than those who didn’t. MRSA has also been isolated in cattle and poultry. But the exact relationship between resistance to certain antibiotics used in livestock, such as pencillin G, and resistant bacteria infecting people is unclear and the subject of considerable debate among health and farm officials.
Legislation update
The Preservation of Antibiotics for Medical Treatment Act of 2009 was introduced and referred to the House Committee on Energy and Commerce and Rules Committee on March 17. It hasn’t moved since that day.
The debate on the use of antibiotics in farm animals is decades old. Similar legislation has failed to gain traction. Fort Wayne – In the wake of an international flu outbreak, pigs have been unfairly vilified.
The so-called swine flu, which now appears more benign than first thought, is typically passed from person to person, not pig to person.
But the new flu strain, a mix of human, pig and bird flu strains, is a reminder that people and animals share much, including disease and infection.
More than 350,000 people call Allen County home, and even the healthiest are susceptible to the flu and other ailments. And humans are not the only potential germ carriers here.
In addition to pets such as cats and dogs, about 13,400 cattle, 36,700 hogs and 2,400 egg-laying chickens are kept here, according to U.S. Department of Agriculture figures.
The implications of that aren’t lost on health organizations such as the American Medical Association, which supports federal legislation to restrict the use of antibiotics in livestock primarily for treating disease.
Proponents of the legislation say rampant antibiotic use contributes to the spread of antibiotic-resistant bacteria in people. Such bacteria include MRSA, or methicillin-resistant Staphylococcus aureus.
MRSA is responsible for an estimated 94,000 serious infections and 19,000 deaths in the U.S. annually, according to a 2007 report by the Centers for Disease Control and Prevention.
But industry officials and other opponents say banning certain antibiotics used in livestock in the U.S. – something already done in Europe – could have unintended consequences, such as driving up food costs and endangering public health.
Why it matters
Experts on both sides of the debate say people’s health – not just animals’ – could be affected by antibiotic use in livestock. The overuse of antibiotics, sometimes to help animals gain weight, could cause problems. But so could underuse.
Frequent use of antibiotics makes it easier for bacteria to build resistance to drugs. Some resistant bacteria can be passed between animals and people, but the extent to which people become infected because of resistance built up in livestock is unclear.
People-to-people transmission is blamed for most MRSA infections. Still, Dr. Deborah McMahan, Allen County health commissioner, says it’s important to use antibiotics responsibly in people and animals. Doctors are getting better about not using antibiotics unnecessarily in people, McMahan said, but there’s room for improvement.
MRSA is a complex problem, she said; it demands an approach that considers all potential contributors – major and minor. But without more data, McMahan isn’t sure how antibiotic use in a rural Allen County farm affects someone in downtown Fort Wayne.
Marianne Ash, a trained veterinarian, is director of biosecurity and preparedness planner for the Indiana State Board of Animal Health. Ash said "very, very few" human MRSA infections are of the animal strain. The issue, if it could be called that, isn’t even on the board’s radar, officials there said.
Based on current science, Ash doesn’t believe a ban on using antibiotics for growth promotion is warranted.
Veterinarians are more likely to be MRSA carriers than the general population. The same is true of doctors who treat people. Only when the drug-resistant staph infects a person, perhaps entering through an open wound, does it cause problems.
But Ash said pets are more likely to spread MRSA to people than farm animals.
"The real issue is with companion animals rather than livestock," she said.
Antibiotics’ benefit
Antibiotics keep livestock healthy and free of pathogens that could wind up in the food supply and cause food poisoning, said Ron Phillips of the Animal Health Institute, a trade association representing pet and livestock pharmaceutical manufacturers. Without these tools, farmers would have limited ways to treat illnesses in their herds and flocks.
Without antibiotics, Adams County farmer Ben Rediger estimates 30 percent of the 900 young dairy cows he raises would die. He uses the antibiotics to treat cows for respiratory illnesses, diarrhea or infections.
"They get sick just like people," he said.
Rediger said he uses the expensive medications sparingly. He watches for signs that a heifer isn’t feeling well and then administers antibiotics.
Those favoring limits on antibiotic use wish all farmers would be so particular.
But others advocate more widespread use of antibiotics. They say low doses of antibiotics keep animals healthy and reduce the risk of food poisoning for people.
Risks of antibiotics
Antibiotic resistance increases almost every time a drug is prescribed, regardless of whether the patient is a person or an animal, said Paul Ebner, an assistant professor of animal sciences at Purdue University. But Ebner said it is difficult to trace the origins of human antibiotic-resistant illnesses, partly because people overuse many medications.
Studies are examining whether antibiotic use on farms affects human health. But the American Medical Association isn’t waiting to push for limits.
The association opposes farmers using low-dose antibiotics as pesticides or growth promoters. Proposed legislation would require those wanting to use new animal drugs for purposes such as growth promotion to first demonstrate that the antibiotics won’t harm people.
The effectiveness of antibiotics "is being compromised by bacterial resistance, arising in part from the excessive use of antibiotics in animal agriculture," the association said in a letter supporting proposed legislation.
A strain of MRSA recently found in pigs and pig farmers in Illinois and Iowa is the same strain found in one-fifth of all documented human cases in the Netherlands.
The strain, sequence type 398, or ST398, hasn’t proved prevalent in the U.S. But its discovery here raises questions about the potential role of livestock in the spread of MRSA among people.
The study published in January in the online Public Library of Science journal was the first to document MRSA in swine and swine workers in the U.S. As far as the researchers knew, it was also the first to report the presence of the ST398 strain in the U.S.
The findings suggested that "agricultural animals could become an important reservoir for this bacterium," according to lead study author Tara Smith, an assistant professor of epidemiology at the University of Iowa, and other researchers.
But because similar antibiotic protocols were in place at all farms studied, the study’s authors said they couldn’t speculate on the relationship between antibiotic use in the pigs and MRSA.
How much is used
Various organizations – some defending antibiotic use and some concerned about it – estimate that between 30 percent and 70 percent of all antibiotics are used on farms, said Bob Martin, former executive director of the Pew Commission on Industrial Farm Animal Production.
The Animal Health Institute, a trade industry for animal medication manufacturers, uses the 30 percent figure. The Union of Concerned Scientists, which supports restrictions on nonessential livestock antibiotics, uses the 70 percent figure.
Phillips, of the Animal Health Institute, said pet and livestock owners spend about $5 billion a year on antibiotics, vaccines and other medicines.
In 2005, 24.4 million pounds of antibiotics were sold for use in livestock and pets, according to the Animal Health Institute. Most livestock antibiotics are used to treat diseases, but in 2007 about 13 percent were used to help animals grow faster. That use has drawn criticism from health organizations.
USDA figures show antibiotics might be used more widely for growth promotion than farmers care to admit. About 55 percent of hog farms feed antimicrobials – drugs to fight bacteria or viruses – to hogs that have been weaned from their mother’s milk, according to a 2006 agency survey of hog farmers.
Nearly 18 percent of dairy farms feed weaned dairy cows antimicrobials for disease prevention or growth promotion, the USDA found in a 2002 survey.
The agency did not have figures for beef cattle or poultry operations.
Mixed signals
Denmark banned growth-promoting antibiotics a decade ago, and opponents of antibiotic limits point to an increase in sick animals that needed antibiotic treatments. They say current farm practices would be more effective than a ban.
A ban in the Netherlands had similar results.
A 2002 World Health Organization report found the Danish ban was associated with slower growth and an increase in death and diarrhea in weaned pigs, but the changes weren’t seen in the older pigs.
The organization also said the ban could have indirectly increased salmonella resistance to the drug tetracycline. After the ban, farmers used the drug more often to treat sick animals. An increased resistance could lead to more human salmonella infections.
But the World Health Organization said the Danish ban produced "no serious negative effects." It found no reason why, under similar conditions, other countries couldn’t discontinue the use of antibiotics solely for growth promotion. In 2006, Europe banned the use of growth-promoting antibiotics.
The Danish ban could provide valuable future data on whether there is a relationship between human antibiotics resistance and doses given to animals, said Ebner, the Purdue assistant professor.
But there isn’t a simple cause-and-effect relationship between livestock antibiotic use and drug-resistant illnesses, he said. The relationship appears to be more complex than that.
