Maryn McKenna

Journalist and Author

MRSA colonization – the long-term risk

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One of the ongoing puzzles of MRSA’s behavior is the significance of colonization, that situation of MRSA living on the skin — or in the nostrils or other locations close to the body’s external surface — without causing illness. It’s not known how frequently MRSA colonization occurs, for one thing: The long-standing estimate of 1% of the population has been challenged by a number of recent studies.

Another persistent question has been whether the risk of illness and death changes as colonization continues. It has been established that up to one-third of newly colonized carriers will become seriously ill within a year of their acquiring the bug (Huang, SS. et al., Society for Healthcare Epidemiology of America Annual Meeting 2006, abstract 157 – not online that I can find)— but what happens beyond that? Does the risk of illness persist or decrease?

In Clinical Infectious Diseases, the same team that defined the risks of recent colonization report that there are significant risks to long-term carriage as well: 27% of invasive illness in the second year and 16% thereafter, based on a review of 281 patients who were followed for at least one and up to four years at Brigham & Women’s Hospital, a Harvard Medical School teaching hospital. These patients become very ill, and in addition use a significant amount of health-care resources:

At our hospital, there are 2–3 times as many hospital admissions involving patients previously known to harbor MRSA than there are hospital admissions of individuals who are newly detected as MRSA carriers each year.

What is the precipitating event that tips MRSA carriage over into MRSA illness? It may be health care. In other words, the long-term carriers do not become ill with MRSA disease and then come to the hospital. Instead, they come to the hospital for some other reason, and the surgery, IV placement, dialysis etc. they receive allows their MRSA strain to slip past the protective barrier of their skin and begin an invasive infection.

We submit that these high risks of MRSA infection among culture-positive prevalent carriers are not only preferentially detected because of hospitalization but may, in fact, be incurred because of the device-related, wound-related, and immunologic declines associated with a current illness.

This raises the question of whether any admitted patient found to be colonized should undergo the routine known as decolonization before any other procedures are performed — and whether institutions and insurance companies will be open to the additional hospital days and drug costs that will represent.

The cite is: Datta, R. and Huang, SS. Risk of Infection and Death due to Methicillin-Resistant Staphylococcus aureus in Long-Term Carriers. Clinical Infectious Diseases. 2008 47:176-81.

Great post elsewhere on antibiotic use in animals

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There will be a bit of a blog break, as I’m traveling for a week. But here as a walk-off is an excellent post from the marvelous public-health blog Effect Measure about the complexities (to be kind) of food companies declaring antibiotic use in food animals.

Very short version of the story: Massive chicken producer Tyson advertised its chickens as being “raised without antibiotics”; the chicken eggs were actually being treated with gentamicin before hatch (therefore technically not being “raised”; the US Department of Agriculture objected, then backed down, then objected again after Tyson’s competitors acted on their own.

Tyson announced it is “voluntarily” withdrawing the label. Which is more than the USDA did, apparently, its hand having been forced by Tyson’s competitors organized into the Orwellian-named Truthful Labeling Coalition (including Perdue Farms Inc., Sanderson Farms Inc. and Livingston, California-based Foster Farms). Perdue and Sanderson had sued over a label they considered “clearly false and misleading,” and a federal judge agreed, ordering Tyson to stop them from running any advertisements with the claim last month. Now, belatedly, the USDA is acting.

The entire post is worth reading, as is Effect Measure (which is running on a “summer schedule” and therefore posting only once a day, thus making us all look bad. Hmm, perhaps a Public Health Blog Truthful Labeling Coalition is in order…)

One more on MRSA in meat

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It turns out that European governments — in contrast to the United States — are taking very seriously the emergence of MRSA in food animals and its potential for transfer to humans. (For background, posts here, here, here and here.)

How seriously? They’re doing a sampling survey of pigs on farms across the European Union, at a cost of about $3 million in EC funds, with matching funds expected from each government.

The MRSA survey piggybacks (sorry) on a year-long survey of Salmonella incidence that the EC called for in September 2007. But in December, following publication of several significant papers about the ST 398 MRSA strain in pigs and pig farmers, the EC Directorate-General for Health and Consumer Protection pushed for an addition to the Salmonella study: a same-time sampling for the presence of MRSA strains in pig operations across 29 countries.

The sampling is taking place from January to December of this year, with results mandated by mid-2009, though individual country authorities may release data earlier if they choose. (In the wake of the finding of three ST 398 cases apparently caused by retail meat in the UK, the Soil Association has called on the British government to release whatever data it has ASAP. Before the EC decision, the UK government had refused to test its pigs; cf. these House of Lords minutes.)

Of note: The Soil Association is pressing the argument that ST 398 has developed in the setting of widespread use of antibiotics in food animals, and contends the strain’s arising in the Netherlands is especially alarming because they have some of the lowest animal-antibiotic use rates in the EC it illustrates the difficulties that even a society conscientious about antibiotic overuse can have keeping track of veterinary applications. The Netherlands has been successful limiting overuse in humans, but has found controlling veterinary use much more of a struggle. (Thanks to the Soil Association for correcting my misunderstanding!)