MRSA strain in food animals concerns experts

By Dr. Ananya Mandal, MDFeb 23 2012

Researchers have announced this Tuesday that a bacteria strain that causes a hard-to-treat staph infection probably developed its antibiotic resistance in food animals.

The strain of staph, known as methicillin-resistant Staphylococcus aureus, or MRSA CC398, most often infects farm workers who come in contact with infected pigs, turkeys, or cows. Now the strain of bacteria has been found in about half of meat samples taken throughout the country. The researchers say that the indiscriminate use of antibiotics in livestock used for food is to blame for the infection's drug resistance.

“We are watching this emerge in real time, and it's emerging really quickly,” says Lance Price, lead author of the report that will appear in mBio published by the American Society for Microbiology. The strain is also showing the potential to pass from human to human, increasing the chance of an outbreak. Price says it's unlikely this is the only strain that has developed an antibiotic resistance in animals. “I imagine this has happened multiple times in the past and it'll happen multiple times in the future,” he says.

The CC398 strain of MRSA first appeared in 2003, and is found in pigs, cattle and poultry in the United States, researchers said. It’s in nearly half of all meat in the U.S. food supply, according to the American Society for Microbiology. Most of the time, it can be killed by cooking food thoroughly.

Doctors are beginning to prescribe fewer antibiotics for fear of creating superbugs, but their use in food animals isn't any less important to drug-resistance development, Price says. “We have tons of messages out there to tell physicians to stop over prescribing, to tell parents not to ask for antibiotics every time their child has a stuffy nose,” Price says. “Meanwhile, we're using 29 tons of antibiotics for food production. Those examples couldn't be more polar opposites.”

Livestock are commonly fed a cocktail of pro-growth hormones, antibiotics and other pharmaceuticals to help them grow faster and prevent infection in the crowded spaces where they spend their lives. Among several concerns, opponents of this practice say profligate antibiotic use can force microbes to mutate and become more dangerous. This is apparently what happened with CC398.

“The most powerful force in evolution is selection. And in this case, humans have supplied a strong force through the excessive use of antibiotic drugs in farm animal production,” said Paul Keim, a co-author on the study and director of Northern Arizona University’s Center for Microbial Genetics and Genomics. “It is that inappropriate use of antibiotics that is now coming back to haunt us.”

In Congress, Rep. Louise Slaughter has been pushing legislation that would regulate antibiotic use in animals to be used for food. “I cannot stress the urgency of this problem enough,” she said in a statement last year. “When we go to the grocery store to pick up dinner, we should be able to buy our food without worrying that eating it will expose our family to potentially deadly bacteria that will no longer respond to our medical treatments.” Last week she asked more than 60 fast-food companies to voluntarily disclose whether they raise their animals with antibiotics or not. “Very simply, consumers have a right to know what's in their food,” she said.

“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.

“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.”