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Methicillin Resistant Staphylococci

Posted May 10, 2010

It seemed like a miracle! When the first human infection was cured by penicillin, doctors and veterinarians thought they would never again lose a patient to a bacterial disease. But this optimism was short-lived as various organisms found ways to thrive despite antibiotic therapy and became resistant to these wonder drugs. In 1961, a Staphylocccus aureus strain from a patient in England was found to be resistant to methicillin; medical professionals were alarmed as this drug had always overcome this species in the past. Within twenty years, similar resistant strains appeared in the United States and the battle against the methicillin resistant staph (MRS) had begun.

"During the year 2005 in the United States, 19,000 people died of infection with methicillin resistant Staph aureus, while 16,000 died of AIDS" noted Dr. Audrey Cook, clinical associate professor at the Texas A&M College of Veterinary Medicine & Biomedical Sciences, "and over 30 percent of Staph aureus isolates cultured from infected human patients were methicillin resistant."

"An infection with MRS usually occurs when there is a breach in the body's natural defenses, such as a surgical wound, skin break or IV catheter" said Cook. "The consequences of MRS depend on which part of the body is infected. If the organism gets into the surgical site on a knee, we'll see a bone infection. If it gets into the blood stream, we see an infection of the heart valves. If it gets into the lung, we'll see pneumonia."

So how do these infections occur? In order to answer this question, we need to understand a little bit about the biology of Staphyloccocci. These are very common bacteria, and numerous species and strains have been identified. Staph aureus is commonly found on healthy people and animals, while other staph species, such as Staph pseudintermedius, are more often found on our pets. Both species of bacteria can be either sensitive or resistant to methicillin. The methicillin resistant types are only a problem if they get into the tissues or organs; they are harmless when sitting on intact skin.

"MRS are carried by a small percentage of healthy people and animals" said Cook. "These individuals are 'colonized' which means that the bacteria don't just sit on the skin, but actually make colonies and reproduce. The most common place for colonization to occur is the front part of the nose, just inside the nostrils. As the bacteria can multiply rapidly, very large numbers of organisms can be present. From the nose, the organism can move around and contaminate the environment. Infections occur if the organism gets into a vulnerable individual and causes disease. So, the bacteria can move from people to people, people to animals, and animals to animals. It is important to understand that a healthy person who is exposed to MRS has essentially no risk of infection, but may be contaminated and spread the bacteria or become transiently or permanently colonized."

MRS can survive for long periods in the environment and on clothes and equipment. Fortunately, simple steps can limit contamination and minimize the risk of actual infection. Frequent hand-washing is one of the best defenses, along with maintaining a clean, dust-free environment.

"We don't know which people or animals may be colonized or contaminated with MRS" said Cook. "But if we pay attention to our own hygiene and keep our environments clean, we can minimize the risks."

"At present, we still have a few antibiotics which can overcome MRS" said Cook "but we can't take these for granted. It's important for those of us who treat patients to be vigilant and aware of how the bacteria are spread. We need to make sure that pet owners understand that these infections can be serious and to follow through with treatments and rechecks as instructed. The bottom line is, be aware! MRS are bacteria that move freely from species to species and we place ourselves, each other and our patients at risk if we are casual about these organisms."


Pet Talk is a service of the College of Veterinary Medicine & Biomedical Sciences, Texas A&M University. Stories can be viewed on the Web at Suggestions for future topics may be directed to

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