The annual meeting of the Infectious Diseases Society of America is taking place this week in Vancouver. This afternoon, one of the researchers presenting there released a startling bit of news: the spread in Ohio (and likely elsewhere in the United States) of a MRSA strain that is common in the rest of the world but so unusual in the US that it was last seen here in the 1990s and has never caused infections here.
The strain is ST239, sometimes called the Brazilian clone, and it’s serious stuff: It causes major epidemics and is responsible for something like 90 percent of all the MRSA infections in hospitals in Asia. Its hallmark is that it very multi-drug resistant — including to just about all the oral antibiotics that are used against MRSA.
Brief recap — disease geeks, go on ahead and we’ll catch up: MRSA stands for methicillin-resistant Staphylococcus aureus, the most common drug-resistant infection in the world (and, shameless self-promotion alert, the subject of my recent book). All MRSA strains, at a minimum, are indifferent to the beta-lactam antibiotics, a group of drugs that share a four-cornered arrangement of atoms at the center of their base molecule. That structure allows the beta-lactam drugs to disrupt staph’s cell membrane, killing the bacterium. When staph evolved a work-around for that attack, it became resistant not just to methicillin, which hasn’t been sold for years, but to all the other beta-lactams as well — of which there are dozens; they are the fundamental, basic antibiotics used in medicine every day. Over the years, MRSA has also gained the ability to defuse the attack of additional drugs and drug families, by exchanging with other bacteria bits of DNA containing additional resistance genes. At the same time, the pace of new-drug approvals has slowed dramatically.
OK, back to today’s news.
At IDSA this afternoon, Dr. Shu-Hua Wang, an assistant professor of medicine at Ohio State University, revealed the results of an analysis of MRSA strains that showed up in a surveillance network that links Ohio State and seven rural hospitals nearby. Between January 2007 and January 2010, 7 percent of the 1126 MRSA strains they found in sick patients turned out to be ST239 — the first ST239 infections ever recorded in the US.
The infections were serious: half of the patients had bloodstream infections and one-fourth had pneumonia. And they were unusually difficult to treat: The bacterial samples were indifferent not only to the beta-lactams, but to a wide array of additional drugs: clindamycin, tetracycline, trimethoprim/sulfamethoxazole (TMP/SMX, usually called Bactrim or Septra), moxifloxacin, and gentamicin. They were susceptible only to vancomycin, the MRSA drug of last resort for decades, and one newer drug, linezolid (Zyvox).
That is a dismaying list. Here’s why: In the search for drugs that still work against MRSA, medicine has increasingly turned to older drugs — ones that until recently were never been used against the bug, because there were newer, better drugs available. Tetracycline and TMP/SMX are two of the great remaining hopes for treating MRSA infections without rolling out the really big-gun drugs that should be reserved for life-threatening emergencies. If the resistance factors carried by ST239 knock out the older generics, MRSA infections will become yet harder to treat. As appears to have happened in Ohio:
“Twenty-two percent of the patients experienced relapse and failure of their drug regimens,” Wang said in a briefing for the media. “Another 22 percent died within 30 days.”
The 77 patients infected with ST239 tended to be male and older, and they were apparently already ill: 74 percent had been hospitalized before, 44 percent had had surgery, 29 percent had been in a nursing home, 17 percent had been on dialysis. Where the ST239 first came from and how it spread isn’t clear, Wang said, except that the surveillance network results show the earliest cases to have been treated at Ohio State. It may have spread to the rural hospitals from there.
The most worrisome implication of today’s news may not be the appearance of ST239 itself — it is so common in the rest of the world that its return to the US may just have been a matter of time — but rather the possibility that public health may not be able to keep track of it. In everyday medicine, isolates are tested enough to identify the organism and to determine its susceptibility to antibiotics; those results deliver enough information to determine a patient’s best treatment. Multi-locus sequence typing, the test that identified ST239 in the Ohio patients, is expensive and complex and delivers more information than a clinician really needs; it’s most used in academic research labs. So it is entirely possible that patients could become infected with this strain without the strain being detected, unless the physician treating them happened to put together the puzzle pieces of the unusual resistance pattern.
This possibility seems to have been on Wang’s mind in Vancouver. To track the spread of ST239, “Increased molecular surveillance is needed,” she said. But she offered some workarounds — less expensive, more commonly used assays — whose results could be used to cast a net of detection for the spread of this new strain.
MRSA image from the Public Health Image Library, CDC; poster previously sold by Threadless, original design by Olly Moss
