Antibiotic-Resistant MRSA in Livestock May Spread to Humans

Feb. 21, 2012— -- Livestock in the United States may be building resistance to deadly bacterial infections, and those superbugs may be easily transferrable to humans, according to a new study published in the journal, mBio.

Methicillin-resistant Staphylococcus aureus, or MRSA, is a strain of staph bacteria that does not respond to antibiotics used to treat staph infections. About two out of every 100 people carry this strain of staph, according to the National Institutes of Health, and infections can be minor to severe. The more severe infections occur most frequently in health care settings, according to the CDC, and they can quickly become life-threatening.

In 2003, scientists discovered a strain of the bacteria called ST398, and today, the strain can be found in pigs, turkeys, cattle and other livestock. The strain, which causes skin and respiratory infections, regularly infects people who handle the livestock.

Now the new genome analysis found that the MRSA strain found in livestock in 2003 likely came from an antibiotic sensitive strain of MRSA in humans.

"Most of the ancestral human strains were sensitive to antibiotics, whereas the livestock strains had acquired resistance on several independent occasions," Ross Fitzgerald in Center for Infectious Diseases at the University of Edinburgh, who reviewed the research, said in a press release.

Once the strain infected livestock, the strain likely changed into several different types, some of which are resistant to various antibiotics, said Fitzgerald, and it is now a two-way street.

"The overuse of antibiotics in food animals for growth promotion allows for various strains of antibiotic-resistant bacteria," said Dr. Marcus Zervos, chief of infectious diseases at Henry Ford Health Systems in Detroit. "If we continue to use antibiotics in food animals, especially for unneeded reasons, the infections will become antibiotic resistant and make their way into people."

Zervos said about 20,000 Americans die each year due to MRSA complications, meaning there are more deaths related to the staph infection than AIDS each year. About 16,000 people in the United States die of AIDS each year, according to the CDC.

The use of antibiotic growth medication has expanded as the meat and livestock industry moved to more mass production. The drugs are added to animal feeds to help the animals grow larger for slaughter, lower the percentage of fat in the livestock and boost protein content. They are also meant to prevent bacteria, including E.coli, Salmonella and enterococci from developing in the animal, but the controversial practice promotes antibiotic resistant bacteria strains to form in the animals' internal systems.

Many experts agree that the United States should follow Europe's lead by banning the feeding of all antibiotics and other drugs in livestock to promote growth. On Jan. 1, 2006, the European Union prohibited such products to prevent antibiotic resistance in humans and in animals.

"We haven't endorsed that approach in the U.S., but I think Europe is further ahead than we are," Zervos said. "It's not just the tonnage of antibiotics we use here, but we also use every major class of antibiotics that is used in food animals is used in people, and it doesn't look like there's any trend in reducing that unneeded use."

Nevertheless, Jennifer Koeman, a veterinarian and director public health at the National Pork Board, defended the regular use of antibiotics in animals.

"Antibiotics are administered to livestock to protect their health and welfare, which helps ensure food safety and human health," Koeman said. "Farmers work closely with veterinarians to develop a comprehensive herd health program, which may include antibiotics."

While bacteria like MRSA in meat is killed once it is cooked at high heat, experts said farm workers and other handlers of the livestock are most at risk of contracting the infection.

"It's pretty unlikely that someone would get MRSA after cooking the meat, but if you don't wash your hands thoroughly after handling the raw meat, there's potential to acquire MRSA," said Dr. William Schaffner, chief of preventive medicine at Vanderbilt University School of Medicine.

Schaffner said he believes there is little likelihood of contracting MRSA from eating meat, but the concern of antibiotic resistance in humans is of great concern, as nationwide sales of antibiotics for humans and animals continues to grow.

Experts said the excessive use of antibiotics in the United States to treat a wide variety of illnesses, including viruses (which do not respond to antibiotics) and the overuse of antibiotics in food products may cause continuing resistance to antibiotics.

"These findings are a result of inaction to do something to control antibiotic use in the food animals," Zervos said.