25 November 2009

Community MRSA rates rising, and epidemics converging

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

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