RESPONSE LETTER TO DR. MORROW ET AL.

To the Editor: Dr. Morrow et al. have several concerns regarding our review of a nursing home outbreak of enterohemorrhagic Escherichia coli (EHEC)-associated diarrhea. First, they are concerned that our case definition does not adequately describe the outbreak. Second, they fear that we overstate the association between spinach and the epidemic. Finally, they wish to clarify that the lack of reports of similar outbreaks does not mean that they did not occur. The primary difference between our report and the San Mateo County Health Department (SMCHD) epidemiological investigation stems from the strictness of the case definition used. A case definition of two or more loose stools within a 24-hour period was used in our report. As Dr. Morrow et al. point out, this should instead read, “any resident or employee of the retirement community experiencing two or more episodes of diarrhea within a 24-hour period between September 21, 2003, and November 3, 2003.” We stand corrected. The stricter definition later adopted by SMCHD required the following criteria: (1) laboratory confirmation of E. coli O157:H7 infection or (2) bloody diarrhea, hemolytic uremic syndrome (HUS), or thrombotic thrombocytopenic purpura (TTP). This definition, although more specific, is not at all sensitive. Our goal as clinicians is to describe the outbreak in terms useful to the physicians who will treat these fragile patients. As our review of the literature reveals, most affected geriatric patients do not have positive blood cultures, bloody diarrhea, and HUS or TTP. Although a strict definition may be appropriate for the epidemiologist, it would not be useful for the clinician. Many cases would be missed; many patients would die. Although it is true that diarrhea is indeed frequent in a geriatric population, this does not discount the presence of epidemics or even make them particularly difficult to recognize. Our authors are intimately familiar with the patients involved; indeed, one was at that time the primary physician for the involved facility. The diarrhea occurring during the period from September 21, 2003, to November 3, 2003, did not represent the status quo. Finally, the San Mateo epidemiologists point out that using this case definition makes it difficult to determine the incubation period for cases. This is a valuable point, and we defer to their outbreak assessment, as cited in our original article. Their second concern is with the implication of spinach as a cause of the outbreak. They agree that spinach is a plausible culprit in the initial outbreak, based on a case-controlled analysis, but point out that we overstate this finding by applying it to our looser case definition, because these cases were not included in their analysis. We appreciate this being pointed out, because this may well weaken the statistical association. Additionally, they note that there may be person-to-person spread. We agree. Indeed, a “second-wave” phenomenon is commonly seen in EHEC outbreaks.1,2 The relationship between spinach and this outbreak should be viewed as a statistical association, with a plausible mechanism based on laboratory research, not as a confirmed causal relationship. Finally, they point out that the limited number of prior reports should not imply that they did not occur. We could not agree more and would like to point out that much of the literature that is available comes from Canada, perhaps representing a more-permissive environment for the public reporting of these deadly outbreaks.3–6 We appreciate the input from SMCHD. We encourage those interested in EHEC O157:H7 to read our report from a clinical perspective and SMCHD's epidemiological assessment of this outbreak. This letter was authored equally by Dr. Reiss and Dr. Keeffe, neither of whom received any financial support or sponsorship.

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