MRSA infection more prevalent in the US than in the UK….or is it?

The antibiotic resistant strain of bacteria known as MRSA has been front page news for years in the United Kingdom, but recent reports are suggesting that Americans should be a little more concerned as well. A new study, published in the medical journal Clinical Infectious Diseases1, examined the incidence of MRSA in both England and the United States between 2006 and 2007. This work, representing a collaboration between the US Centers for Disease Control and Prevention (CDC) and the UK Health Protection Agency, looked specifically at bloodstream infections and further categorized these as either “hospital-onset” or “community-onset” based on when the infection was detected. Surprisingly, it was found that the national incidence of community-onset MRSA bloodstream infection was much higher in the US than in England (21.9 cases per 100,000 vs. 3.5 cases per 100,000). In contrast, the national incidence of hospital-onset MRSA bloodstream infection was virtually identical at 7.4 cases per 100,000 for the US and 7.8 per 100,000 for the UK. During the study period a total of 9,324 MRSA bloodstream infections were recorded in the US as compared to 11,431 in the UK, and in the UK a much higher proportion of the total were hospital-onset (69% vs. only 24% in the UK). In the US, those developing community-onset MRSA cases were more likely to have had an established risk factor such as diabetes, dialysis, or prior intravenous drug use.

So what does this all mean? Well first of all you’re probably looking at the numbers and noting that there were actually more total infections reported in the UK than the US over the same time period. That’s actually due to the fact that the UK numbers reflect the total population (about 51 million), while the US numbers are drawn from a surveillance program that only looks at hospitals in 8 major cities and one state (total population of about 16 million). Thus, if you accept that the sample area in the US actually represents the entire country, the total number of MRSA bloodstream infections would be much higher in the US. Some quick math shows that if you extrapolated the data to a US sample size of 51 million, there would have been about 30,000 MRSA infections in that group – almost triple the total number reported in the UK. The problem with this reasoning is that the US surveillance system relies heavily on sampling from urban centers, where population density is higher and risk factors are more prevalent. It is extremely unlikely that rural Nebraska faces the same MRSA issues as downtown Chicago. For exactly this reason, the calculated US incidence numbers may be artificially high in this study. Furthermore, the fact that the major difference in MRSA incidence between the two countries comes from the difference in community-onset infections suggests that these are more prevalent in urban centers more than it suggests that the US has a higher community-onset MRSA infection rate overall.

Secondly, it’s important to understand that this study only looked at a subset of MRSA infections – those that have entered the bloodstream and become systemic. These are generally the most serious of cases, and also represent the most progressive since usually an infection will start out locally before spreading to other areas via the bloodstream. While bloodstream infections are the most serious, examining those incidence rates alone misses a large number of MRSA infections that remain local (e.g. on the skin boil or in a cut). Localized infections, especially when they recur over long periods in unfortunate individuals, are also very problematic and represent a major public health concern.

Finally, the authors of the study acknowledged that the structure for monitoring and reporting MRSA infections is very different between the US and England. While the US system employs dedicated staff to actively collect infection data from select hospitals for these types of studies, England relies on a nationwide hospital-integrated process whereby the accuracy of reporting depends on the hospital personnel themselves. The latter approach makes it more difficult to validate results for a study such as this and may have skewed the UK infection numbers to lower than they actually were.

Due to the limitations of this study, whether the data actually proves that MRSA is a bigger problem in the US than the UK remains debatable. What is not debatable is that the incidence of serious MRSA infections remains high in both countries, and upwards of 19,000 Americans are dying every year as a direct result. Understanding the incidence numbers and what they mean is a critical step in determining where MRSA is coming from and how it should be addressed. This study does point to the community as a major reservoir for MRSA in US urban centers, and even to specific subsets of the population that are more susceptible to contracting an infection. Hopefully continued funding will be available to translate these numbers into effective interventions to limit MRSA infection nationwide in the future.

The abstract and full study can be found on the journal’s website at:

1 Lessa et. al (2010) Clinical Infectious Diseases 51: 925-928

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One Response to “MRSA infection more prevalent in the US than in the UK….or is it?”

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