Maryn McKenna

Journalist and Author

Community MRSA rates rising, and epidemics converging

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A study published Tuesday in Emerging Infectious Diseases makes me happy, despite its grim import, because it confirms something that I will say in SUPERBUG: Community MRSA strains are moving into hospitals, blurring the lines between the two epidemics.

The study is by researchers at the excellent Extending the Cure project of Resources for the Future, a group that focuses on applying rational economic analysis (think Freakonomics) to the problem of reducing inappropriate antibiotic use. (Here’s a post from last year about their work.)

Briefly, the researchers used a nationally representative, commercial (that is, not federal) database of isolates submitted to clinical microbiology labs, separated out MRSA isolates, divided them into whether they originated from hospitals or outpatient settings (doctors’ offices, ambulatory surgery centers, ERs), and analysed them by resistance profile, which has been a good (thogh not perfect) indicator of whether strains are hospital or community types (HA-MRSA or CA-MRSA). They cut the data several different ways and found:

  • Between 1999 and 2006, the percentage of staph isolates from outpatient settings that were MRSA almost doubled, increasing 10% every year and ending up at 52.9%. Among inpatients, the increase was 25%, from 46.7% to 58.5%.
  • Among outpatients, the proportion of MRSA isolates that were CA-MRSA increased 7-fold, going from 3.6% of all MRSA to 28.2%. Among inpatients, CA-MRSA also increased 7-fold, going from 3.3% of MRSA isolates to 19.8%.
  • Over those 7 years, HA-MRSA did not significantly decrease, indicating that CA-MRSA infections are not replacing HA-MRSA, but adding to the overall epidemic.

So what does this mean? There are a number of significant aspects — let’s say, bad news, good news, bad news.

Bad: CA-MRSA strains are entering hospitals in an undetected manner. That could simply be because patients entering the hospital are colonized by the bug and carry it with them. But it could also be because healthcare staff who move back and forth between outpatient and in-patient settings — say, an ambulatory surgical center and a med-surg ward — could be carrying the bug with them as well.

Good: If they are detected (analyzed genotypically or for drug sensitivity), CA-MRSA strains are less expensive to treat because they are resistant to fewer drugs, and some of the drugs to which they are susceptible are older generics, meaning that they are cheaper.

Very Bad: The entrance of CA-MRSA strains into hospitals risks the trading of resistance factors and genetic determinants of transmissibility and colonization aptitude in a setting where bacteria are under great selective pressure. Several research teams have already seen this: In several parts of the country, CA-MRSA strains have become resistant to multiple drug families.

Is there a response? The work of Extending the Cure focuses on developing incentives that will drive changes in behavior around antibiotic use. These results, lead author Eili Klein told me, call for developing incentives for creating rapid diagnostic tests that will identify not just that a bug is MRSA, but what strain it is, so that it can be treated appropriately and not overtreated.

The results also underline the need for something that is particularly important to me: enhanced, appropriately funded surveillance that will define the true size of the MRSA epidemic and delineate the behavior of the various strains within it. Right now, surveillance is patchy and incomplete, done partially by various CDC initiatives and partially by the major MRSA research teams at academic medical centers. As we’ve discussed, there is no national requirement for surveillance of patients, and very few state requirements; there is no incentive for insurance companies to pay for surveillance, since it benefits public health, not the patient whose treatment the insurance is paying for; and there is a strong disincentive for hospitals to disclose surveillance results, because they will be tarred as dirty or problematic. Yet to know what to do about the MRSA epidemic, we first have to know the size and character of what we are dealing with, and we do not now.

The cite is: Klein E, Smith DL, Laxminarayan R. Community-associated methicillin-resistant Staphylococcus aureus in outpatients, United States, 1999–2006. Emerg Infect Dis. DOI: 10.3201/eid1512.081341

MRSA in meat in Louisiana: pig meat, human strain

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On Nov. 3, I posted on an enterprising group of TV stations in the Pacific Northwest who had retail meat in four states tested for MRSA. I said at the time that it was the first finding of MRSA in meat in the US that I knew of.

Turns out that I was wrong by three days. On Oct. 31, the journal Applied and Environmental Microbiology published an electronic version of a study that they will be printing in the paper journal on some future date. Journals do this when a finding is so important or timely that it should see the light immediately, rather than wait through the additional weeks or months of print production.

And this finding is certainly timely. Shuaihua Pu, Feifei Han, and Beilei Ge of the Louisiana State University Agricultural Center have made what appears to be the first scientifically valid identification of MRSA in retail meat in the United States. But — and this is an important point — it is not the swine strain, ST 398, that has been found in meat in Canada and Europe, and in hospital patients in Scotland and the Netherlands, and in pigs in Iowa; and in humans in New York, though that strain was drug-sensitive.

Instead, what the researchers found (in 5 pork and 1 beef samples, out of 120 bought in 30 grocery stores in Baton Rouge, La. over 6 weeks in February-March 2008) was USA300, the dominant community MRSA strain, and USA100, the main hospital-infection strain. In other words, they found meat that had been contaminated during production by an infected or colonized human, not by a pig. As they say:

…the presence of MRSA in meats may pose a potential threat of infection to individuals who handle the food. … (G)reat attention needs to be taken to prevent the introduction of MRSA from human carriers onto the meats they handle and thereby spreading the pathogen.

As we’ve discussed before, the primary danger from MRSA in meat is not that people will take the bug in by mouth (though that is a danger, since S. aureus because of its toxin production can cause severe foodborne illness — and these researchers found, overall, an S. aureus contamination rate of 46% of their pork samples and 20% of their beef samples). Rather, the danger is that people handling the raw meat will be careless in preparing it, and will colonize themselves by touching the meat and then touching their own noses or mucous membranes, leading to a possible future infection. As reader Rhoda pointed out in a comment last week, people could also infect themselves directly, by getting MRSA-laden juice or blood into an abrasion or cut.

So: Be careful in the kitchen, keep meat separate from other foods, wash cutting boards and knives, and (say it with me, now) wash your hands, wash your hands, wash your hands.

The cite for the new paper: Pu, S. et al. Isolation and Characterization of Methicillin-Resistant Staphylococcus aureus from Louisiana Retail Meats. Appl. Environ. Microbiol. doi:10.1128/AEM.01110-08. Epub ahead of print 31 Oct 08.

Housekeeping note: This is the 16th post I’ve written on MRSA in food animals and/or meat. Providing all the links to the previous posts is starting to obstruct the new news. So if you are looking for all those past posts, go to the labels at the end of this post, below the time-stamp, and click on “food.” You should get something that looks like this.