Why MRSA evolved to spread from farms to humans

Kathleen Blanchard's picture
Modern day farming practices linked to MRSACC398 evolution.
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A genome wide study finds a potentially deadly strain of MRSA - methicillin resistant Staphylococcus aureus - is spreading from farm animals to humans.The bacterium, specifically known as MRSA CC398, is resistant to both methicillin and tetracycline, which are two common drugs used to treat staph infection that can become deadly if it enters the bloodstream. It is also referred to as "pig-MRSA" or "livestock-associate MRSA" because it usually infects farm workers in direct contact with animals.

The study, conducted by the Translational Genomic Research Institute (TGen), published in the online journal mBio, links the overuse of antibiotics in livestock as the cause of the superbug.

Staphylococcus aureus, or 'staph', which at one time was treatable in humans, has jumped from to animals and then back to humans, developing antibiotic resistance in the process.

It happens when bacteria share their DNA or when their DNA becomes encoded to become drug resistant. Sometimes DNA hops around to make a bacteria stable and indestructible.

The researchers analyzed 89 genomes in 19 countries.

Paul Keim, director of TGen’s Pathogenic Genomics Division said in a press release, “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. It is that inappropriate use of antibiotics that is now coming back to haunt us.”

MRSA is destroyed by cooking, but the concern is that bacteria gets on hands and can be spread after handling raw meat. The bacteria can linger on surfaces and utensils, potentially causing sources of infection.

Lance Price, NAU faculty member and director of the Center for Food Microbiology and Environmental Health at the Translational Genomics Research Institute, described the study as seeing “evolution in action”.

Genomes of MRSA were taken from pigs, turkeys and chickens from 19 countries on four continents.

The finding is of particular significance because of the high toll MRSA infections take on health and associated mortality rates.

The CDC reports that in 2005 there were 278,000 hospitalizations related to MRSA in the U.S. alone, which included infections acquired outside of the hospital.

In the same year, 94,000 people were infected with the bacteria for the first time. Approximately 19,000 people who developed their first invasive infection from methicillin resistance staphylococcus aureus died.

The researchers say it’s likely that MRSA CC398 was treatable with antibiotics before it jumped to animals where it developed resistance from anti-microbials given to livestock.

The use of antibiotics in livestock is banned in Europe, and there has been much concern of their use in the U.S.

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Even for healthy the people, MRSA and other emerging ‘super bugs’, could claim lives because antibiotic development hasn’t been a focus of pharmaceutical companies.

Edward Schwarz, PhD and colleagues from the University of Rochester Medical Center have developed a vaccine that can prevent bacterial infection drug resistant staph infection that can also find its way into implants after joint replacement surgery that is still being investigated.

Price said the study was “like watching the birth of a superbug—it is simultaneously fascinating and disconcerting,” because the bug seems to be spreading rapidly since it was discovered ten years ago.

In January, 2012, University of Iowa Healthcare scientists highlighted the prevalence of MRSA in pork products in the U.S. that they say was higher than previously known, but the finding didn’t get much media attention.

Why are antibiotics allowed for use in livestock?

Especially if it’s a major public health issue.

Well, New York Times columnist Nicholas D. Kristof said this, in a March 2009 opinion article:

“The answer is simple: politics. Legislation to ban the nontherapeutic use of antibiotics in agriculture has always been blocked by agribusiness interests.”

The current study investigators say antibiotics in an environment of filth and crowded conditions – where, unfortunately most livestock are raised - is “going to create a public health problem.”

The reason why MRSA CC398 is spreading from farm animals raised for food to humans is from overuse of antibiotics that are allowed to be given to animals that aren’t even sick – but they’re at high risk for infection because our farming practices are filthy and the animals could become diseased quickly.

Humans gave farm animals a treatable Staphylococcus strain that evolved into MRSA CC398 from modern farming practices and overuse of antibiotics.

Citation:
mBio
"Staphylococcus aureus CC398: Host Adaptation and Emergence of Methicillin Resistance in Livestock"
Lance B Price, et al.
February 21, 2012

Resources:
CDC
MRSA Surveillance

University of Rochester Medical Center
“Bone Infection and MRSA Vaccine”
February, 2012

pLoS One
“MRSA in Conventional and Alternative Retail Pork Products”
Ashley M. O'Brien et al.
January 19, 2012

The New York Times
Pathogens in our Pork
March, 2009

Image credit: Wikimedia commons

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