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MRSA Strain That Eludes Traditional Tests Found in Cow’s Milk, Study Finds (Bloomberg, Jun 3, 2011):

British scientists have discovered a new strain of the drug-resistant germ known as MRSA in cow’s milk and some evidence that the animals could be a source of the infection in humans.

University of Cambridge researchers were led to the discovery while studying an infection of the animals’ udders, according to an article published today in the U.K journal The Lancet Infectious Diseases. Tests showed that people in Scotland, England and Denmark carried the new variant of MRSA, or methicillin-resistant Staphylococcus aureus, and that the bacteria can elude the usual means of detection.

Tests that search for the mecA gene, which enables the bacteria to resist treatment with some antibiotics, can miss the new, genetically different strain, researchers said. Relying solely on gene testing can lead doctors to prescribe medicines that are powerless against MRSA, the researchers said.

“It’s important that any of the MRSA testing that is based on detection of the mecA gene be upgraded to ensure that the tests detect the new gene found in the new MRSA,” said Mark Holmes, the lead researcher, in a statement.

While the new strain can cause disease in people, the risk of becoming infected is low, the scientists said. Further study is planned to determine how prevalent the new strain is and where it’s coming from, the researchers said.

Molecular Testing

Standard molecular screening for MRSA searches for the mecA gene. The new strain has a mecA gene that’s only 60 percent similar to the original version, leading to a false negative result when tested, the study found.

Testing should be updated to identify the genetic change, researchers said. The majority of testing in British hospitals is performed by seeing if the bacteria will grow in the presence of antibiotics, Holmes said.

“Even though this new strain isn’t picked up by the current molecular tests, they do still remain effective for the detection of over 99 percent of MRSAs,” said Angela Kearns, head of the U.K. Health Protection Agency’s Staphylococcus Reference Laboratory, in a statement.

The researchers also found evidence that cattle could be a key reservoir of the new strain of MRSA. The samples found in humans were either of a strain thought to be unique to animals or other types detected in cattle, the study said. In England, the researchers found a geographical association between human and animal isolates of MRSA. Pasteurization of milk will prevent infection, they said.

Staphylococcus aureas bacteria live on the skin and in nasal passages. Of the people with the bacteria present, about 1 percent have MRSA, according to the U.S. Centers for Disease Control and Prevention. MRSA causes infection when germs enters the body through a cut, a sore, a catheter or a breathing tube, according to the U.S. National Institutes of Health.

MRSA Infections

MRSA infections can occur in hospitals or in otherwise healthy people who come into contact with the bacteria in settings such as locker rooms or daycare centers, the NIH said. Infections cause a red, painful, swollen area on the skin, and symptoms of severe cases include chest pain, chills, fatigue, fever and shortness of breath.

Serious MRSA infections acquired while in the hospital may be treated with medicines including Cubist Pharmaceuticals Inc. (CBST)’s Cubicin, Pfizer Inc.’s Zyvox and generic drugs such as doxycycline and tetracycline, according to NIH.

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