Deadly Livestock Superbug Jumped To Humans
February 22, 2012

Deadly Livestock Superbug Jumped To Humans

A strain of the potentially deadly antibiotic-resistant bacterium known as methicillin-resistant Staphylococcus aureus (MRSA) has jumped from food animals to humans, researchers reported Tuesday in the online journal mBio.

The study, led by the Translational Genomics Research Institute (TGen), suggests that MRSA CC398 likely started as a non-resistant (antibiotic-susceptible) strain in humans, but developed resistance to antibiotics after spreading to farm animals.

MRSA is a well-known cause of a variety of invasive infections that can rapidly turn life threatening.  Roughly two percent of people carry this strain of staph, with infections ranging from minor to severe, according to the National Institutes of Health (NIH).  The more severe infections typically occur in health care settings, and can quickly become lethal, according to the U.S. Centers for Disease Control and Prevention.

In 2003, a novel form of MRSA known as ST398 emerged in livestock, but now regularly infects farm workers and others who come into contact with infected livestock.  This can result in acute skin and soft tissue infections, respiratory infections, and bacteremia (also called sepsis).  The ST398 strain can now be found in pigs, turkeys, cattle, and other livestock, and has been detected in 47% of meat samples in the United States.

In the current study, researchers used whole genome sequencing, a cutting-edge testing method in which billions of molecules of DNA are spelled out in exacting detail, to trace the likely history of MRSA CC398.  They found that the jump of Staph from humans to food animals was followed by the bacterium becoming resistant -- first to tetracycline, and then methicillin, two important antibiotics for treating Staph infections.

MRSA CC398 is often referred to as “pig-MRSA” or “livestock-associated MRSA” because it frequently infects people with direct exposure to swine and other food animals.

“Our results strongly suggest that food animals-associated MRSA CC398 originated in humans as MSSA (methicillin-susceptible S. aureus),” the researchers concluded in their report.

Once in animals, the microbe became resistant to tetracycline and methicillin, likely as a result of the routine antibiotic use that characterizes modern food-animal production, the researchers said.

The findings illustrate a tight link between antibiotic use on the farm and potentially lethal human infections.

“Our findings underscore the potential public health risks of widespread antibiotic use in food animal production. Staph thrives in crowded and unsanitary conditions. Add antibiotics to that environment and you´re going to create a public health problem,” said Dr. Lance Price, the study´s lead author and Director of the TGen´s Center for Food Microbiology and Environmental Health.

“Retracing the evolutionary history of MRSA CC398 is like watching the birth of a superbug – it´s simultaneously fascinating and disconcerting.”

“MRSA CC398 was discovered less than a decade ago and it appears to be spreading very quickly.”

Dr. Paul Keim, one of the study´s authors and Director of TGen´s Pathogen Genomics Division, said the study describes evolution in action.

"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.”

“We can't blame nature or the germs. It is our inappropriate use of antibiotics that is now coming back to haunt us. The good news is that this study clearly shows the way to change evolution and remove this and other similar threats to our health," said Dr. Keim.

Dr. Frank Aarestrup, Head of the Microbial Genomics and Antimicrobial Resistance unit at the Technical University of Denmark, said the study provided valuable information about the origins and future risks of this MRSA strain.

“Given its rapid emergence and trajectory of increasing importance in humans, the evolutionary history of MRSA CC398 has relevance for the epidemiology of MRSA and global health,” he said.

The researchers note that additional study is needed to further assess the risks.

“Further analyses are required to estimate the number of independent genetic events leading to the methicillin-resistant sub lineages, but the diversity of the (MRSA) subtypes is suggestive of strong and diverse antimicrobial selection associated with food-animal production,” said Dr. Robert Skov, the study´s Senior Microbiologist and a member of the Department of Microbiological Surveillance and Research at the Statens Serum Institut in Copenhagen, Denmark.

The findings were published February 21 in the online journal mBio.


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