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Electronic letters published:
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Reply to Katz et al.
Ascertainment bias and incidence of MDR CA-MRSA clone USA 300 infections in San Francisco
Other Communities at Risk as well
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Binh An Diep, PhD San Francisco General Hospital, University of California, San Francisco, Henry F. Chambers, MD
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bdiep{at}epi-center.ucsf.edu Binh An Diep, et al.
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We thank Dr. Katz and colleagues for their concern regarding ascertainment bias of multidrug resistant USA300 cases in our population- based survey [1]. We reason that ascertainment bias would affect similarly the incidence estimates of multidrug-resistant USA300 and non- multidrug-resistant USA300 as these infections are indistinguishable to patients and clinicians. When compared to our previously published spatial distribution of multidrug resistant USA300 according San Francisco ZIP codes [1], non-multidrug-resistant USA300 exhibited a markedly different distribution not clustering about the 8 ZIP codes with higher proportions of male same-sex couples (supplementary figure). In fact, the incidence of non-multidrug-resistant USA300 was 339 cases per 100000 persons in ZIP codes with higher proportions of male same-sex couples, compared with 192 cases per 100000 persons in the other 18 ZIP codes (relative risk, 1.8 [CI, 1.6 to 1.9]). In contrast, the incidence of multidrug resistant USA300 of 59 cases per 100000 persons in the 8 ZIP codes with a higher proportion of male same sex couples, compared with 4 cases per 100000 persons in the other 18 ZIP codes (relative risk, 16.1 [CI, 9.8 to 26.5]). The Castro District (ZIP code 94114) had a disproportionate burden of multidrug resistant USA300 of 170 cases per 100 000 persons when compared with the citywide incidence of 26 cases per 100000 persons [1]. A correspondingly disproportionate burden of non-multidrug resistant USA300 was not observed when comparing the incidence of non-multidrug resistant USA300 in the Castro District of 571 cases per 100000 persons to the citywide incidence of 251 cases per 100000 persons (supplementary figure). Taken together, ascertainment bias is unlikely to explain the clustering of multidrug resistant USA300 in ZIP codes with higher proportions of male same-sex couples. More important, our cross-sectional studies from San Francisco and Boston also indicated that men who have sex with men have an increased risk of developing infections with multidrug resistant USA300. Binh An Diep, PhD; Henry F. Chambers, MD; San Francisco General Hospital University of California, San Francisco San Francisco, California 94110 Conflict of Interest:None declared |
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Kenneth A. Katz, MD, MSc, MSCE Centers for Disease Control and Prevention and San Francisco Department of Public Health, Kyle T. Bernstein, Jeffrey D. Klausner
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kenneth.katz{at}sfdph.org Kenneth A. Katz, et al.
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In their population-based survey (1), Diep and colleagues report a higher incidence of a multidrug-resistant (MDR), community-associated (CA), methicillin-resistant Staphylococcus aureus (MRSA) clone USA300 infections in the ZIP code that includes the Castro district, compared with San Francisco overall (170 versus 26 cases/100,000 persons, respectively) during 20042005. We are concerned that this finding reflects, at least in part, ascertainment bias rather than a true difference in disease incidence. Diep and colleagues defined cases as positive MDR CA-MRSA USA300 clinical cultures other than nasal swabs. Clinical considerations determined whether cultures were performed. Because all infections might not have been cultured, Diep and colleagues state that they might have underestimated the true incidence. The magnitude of underestimation probably differed importantly among groups as a result of ascertainment bias. Bias likely stemmed from greater awareness of MRSA infections among gay men and other men who have sex with men (G/MSM) and their health-care providers. In February 2003, articles in two widely read local newspapers the San Francisco Chronicle and the gay-oriented Bay Area Reporter (BAR) highlighted MRSA infections among G/MSM in San Francisco. In the San Francisco Chronicle, a prominent physician recommended culturing all infections; the front-page BAR article encouraged persons with suspected infections to seek treatment early. The San Francisco Department of Public Health issued frequently asked questions for the public and a Health Advisory for clinicians and, with community partners, hosted a forum, all focusing on MRSA infections among G/MSM. Additionally, journal articles published during 20032005 linked MRSA infections to G/MSM (2), including HIV-infected G/MSM (3). All these factors increased the likelihood both that G/MSM sought health care for suspected MRSA infections and that clinicians cultured suspected MRSA infections among G/MSM San Franciscans. That bias, plus the fact that G/MSM in San Francisco (including HIV-infected G/MSM, who comprise a majority of HIV-infected city residents) are a large, geographically concentrated group (4), likely resulted in a relative overestimation of the incidence of infections in the Castro district, compared with other ZIP codes in San Francisco. Additionally, Diep and colleagues provide evidence that among patients of a San Francisco HIV clinic who had an MRSA USA 300 infection, G/MSM were more likely to have an MDR strain of USA300 compared with non- G/MSM. However, G/MSM at that clinic are unlikely to be representative of G/MSM throughout San Francisco. Kenneth A. Katz, MD, MSc, MSCE Epidemic Intelligence Service, Centers for Disease Control and Prevention Atlanta, GA 30333 San Francisco Department of Public Health San Francisco, CA 94103 Kyle T. Bernstein, PhD, ScM San Francisco Department of Public Health San Francisco, CA 94103 Jeffrey D. Klausner, MD, MPH San Francisco Department of Public Health San Francisco, CA 94103 Note: The findings and conclusions in this report are those of the author(s) and do not necessarily represent the views of the Centers for Disease Control and Prevention. 1. Diep BA, Chambers HF, Graber CJ, Szumowski JD, Miller LG, Han LL, et al. Emergence of multidrug-resistant, community-associated, methicillin -resistant Staphylococcus aureus clone USA300 in men who have sex with men. Ann Intern Med. 2008. In Press. 2. Weber JT. Community-associated methicillin-resistant Staphylococcus aureus. Clin Infect Dis. 2005;41(Suppl 4):S269-72. 3. Lee NE, Taylor MM, Bancroft E, Ruane PJ, Morgan M, McCoy L, et al. Risk factors for community-associated methicillin-resistant Staphylococcus aureus skin infections among HIV-positive men who have sex with men. Clin Infect Dis. 2005;40:1529-34. 4. HIV/AIDS Statistics, Epidemiology, and Intervention Research Section; San Francisco Department of Public Health. HIV/AIDS Epidemiology Annual Report. San Francisco: San Francisco Department of Public Health; 2006. Conflict of Interest:None declared |
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Arlen J Peterson, Social Services
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arlen-j{at}hotmail.com Arlen J Peterson
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First, let me thank the dedication and development of understanding MRSA clone (USA300) from the medical community to the public. I work for a sexual health centre, which includes providing extensive education and treatment of STI's to sexually active individuals. I understand the relation between how the community of men who have sex with men increases the risk of MRSA infection (risky behaviors, more sexual partners, drugs, etc), Annals article highlights the risk is associated with skin-to-skin contact primarily by unprotected anal intercourse. My concern is the community of men who have sex with men are the only population emphasized in the article when anal intercourse is practiced fluently in men who have sex with women. Men who have anal intercourse with women do so for reasons mainly of pleasure and a form of birth control, usually unprotected for the latter. So, if an average person were to read a synopsized version in the news based on this article, particularly the young, they might get a message of: It's a risk for men who have sex with men, I am not of this population, therefore I am not affected. Can this article emphasize that it is the unprotected anal intercourse causing the risk of MRSA infection and that is not limited to men who have sex with men? I appreciate it and thank you for your time. Conflict of Interest:None declared |
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