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The Risk of Infection after Nasal Staphylococcus aureus
Abstract & Commentary
By Joseph H. John, MD, FACP, FIDSA, FSHEA, Associate Chief of Staff for Education, Ralph H. Johnson Veterans Administration Medical Center; Professor of Medicine, Medical University of South Carolina, Charleston, SC, is Associate Editor for Infectious Disease Alert.
Dr. John is a consultant for Cubist, Genzyme, and bioMerieux, and is on the speaker's bureau for Cubist, GSK, Merck, Bayer, and Wyeth.
Synopsis: Another study confirms that MRSA nasal colonization is associated with a higher rate of infection than MRSA colonization. Separately, it is reported that individuals with perennial allergic rhinitis have an increased prevalence of nasal colonization with S. aureus, but that nasal mupirocin treatment does not reduce the frequency of nasal symptoms.
Sources: Safdar N, Bradley EA. The risk of infection after nasal colonization with Staphylococcus aureus. Am J Med. 2008;121:310-315; Zeldin Y, et al. Efficacy of nasal Staphylococcus aureus eradication by topical nasal mupirocin in patients with perennial allergic rhinitis. Ann Allergy Asthma Immunol. 2008;100:608-611.
Recently, authors from the University of Wisconsin Medical School asked what is the risk of infection associated with nasal colonization with Staphylococcus aureus. Using some statistical methods that only statisticians can understand, the authors queried 528 studies, and found 113 potentially relevant. They wanted to review data of both methicillin-susceptible and methicillin-resistant Staphylococcus aureus. So, they ended up with only 31 studies which included both MSSA and MRSA; from those, only 10 met the authors' inclusion criteria. From a group of 3381 patients who were screened for colonization, the studies included 791 patients colonized by MSSA and 379 by MRSA. Rectal swabs were used with nasal swabs in two studies.
The findings were surprising: MRSA colonization was associated with a four-fold higher risk of infection following the colonization, significant at a p = 0.007. The authors conducted a sensitivity analysis to test heterogeneity in colonization and infection, and showed that no one study biased the analysis.
The focus of the second study, by Zeldin et al from Israel, was to see if nasal mupirocin reduced the frequency of attacks due to perennial allergic rhinitis. There were 60 patients, ages 5-60 years of age, in the treatment group and 55 in the control group. Sensitivity to dust mites was much higher (100% vs 13%) in the rhinitis than in the control group.
The results showed S. aureus carriage in 38% of the rhinitis group compared to 15% of the control group (p = .004). No distinction was made between MRSA and MSSA. Eradication of S. aureus carriage worked well with self-administered mupirocin in both groups. The allergist investigators were most focused on whether eradication of S. aureus carriage was associated with reduced frequency of perennial symptoms of rhinitis or if it was not. Changes in rhinitis symptom scores, measured at five weeks, compared to baseline varied very little in the carrier and non-carrier groups.
Well, there you have it: Infection is more likely to follow MRSA carriage than MSSA carriage. For old timers who have followed staphylococcal infection for decades, this may come as a surprise. Certainly we always have had plenty of infection due to S. aureus, and infection is generally related to the carrier state. The increased intensity of focus over the past decade on MRSA has been, in my opinion, overdone in relation to the relative preoccupation with MSSA. Yet, when a good review analysis, like the one by Safdar and Bradley, considers only the studies that truly studied the link between MSSA/MRSA carriage, and subsequent infection, MRSA wins.
Yes, we do need to emphasize that focus on MRSA carriage, and those organizations both abroad and in the United States that are trying to limit carriage through surveillance studies and other means are well justified to do so. The Dutch enterprise "Search and Destroy" to make MRSA a nearly national enemy has resulted in extremely small rates of MRSA in Dutch hospitals like the large facility at Erasmus University in Rotterdam.
There may be subsets of patients who are more likely than not to carry MRSA. There have been a few studies in HIV patients that suggest they are at greater risk. Now we see in the study by Zeldin et al that one subgroup, those with perennial allergic rhinitis, have a higher rate of S. aureus carriage when compared to controls. The Zeldin study also found that eradication carriage of S. aureus carriage did result in fewer rhinitis attacks. Yet, the implications for staphylococcal infection pathogenesis may be more interesting, namely, that the rhinitis group had a higher carriage rate of S. aureus than the control group, a fact that may enter into future studies concerned with identifying risk factors in subgroups for S. aureus carriage.