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Shiga toxin-producing Escherichia coli (STEC) infection causes diarrhea that is often bloody and can result in potentially life-threatening hemolytic uremic syndrome (HUS).1Escherichia coli O157:H7 is the most common cause of STEC infection in the United States, producing 73,000 illnesses annually, according to the last estimate in 1999.2 Unlike O157, however, little is known about the incidence of non-O157 strains. Because STEC other than O157 are not commonly identified, the incidence, trends, and epidemiology of non-O157 STEC are not well understood. To assess trends in Shiga toxin enzyme immunoassay (Stx EIA) testing by local clinical laboratories, the Connecticut Department of Public Health (CTDPH) analyzed results of confirmatory testing conducted in the state laboratory during 2000-2005. The findings indicated that a total of 403 STEC infections were reported by clinical laboratories in Connecticut, including 207 identified as STEC by Stx EIA testing alone, and that the use of Stx EIA increased from 2000 to 2005. Use of Stx EIA without prompt culture confirmation can delay or prevent serotyping and subtyping of isolates and detection of both O157 and non-O157 STEC outbreaks. Public health authorities in all states should ensure that clinical laboratories forward Stx EIA-positive specimens to the state laboratory for isolation and identification of STEC, as recommended by the Association of Public Health Laboratories* and CDC.3
Clinical laboratories typically use sorbitol-MacConkey (SMAC) agar, a culture method, to identify STEC O157, which cannot ferment sorbitol and therefore forms colorless colonies. Like other intestinal flora, most non-O157 STEC strains ferment sorbitol and form pink colonies; therefore, SMAC agar cannot be used to readily differentiate between sorbitol-fermenting non-O157 STEC strains and other sorbitol-fermenting intestinal flora growing on the plate. Rapid diagnostic EIAs capable of detecting Stx in stool specimens or culture broths are commercially available and used increasingly by clinical laboratories. These nonculture methods are capable of detecting both O157 and non-O157 STEC strains; however, these methods should not be considered as substitutes for culture.
Clinical laboratories in Connecticut have been required to report culture-confirmed STEC O157 infections to state public health officials since 1992 and Stx EIA-positive infections since 2000.4 During 2000-2005, the number of clinical laboratories in Connecticut conducting Stx EIA testing increased from four (11%) of 35 laboratories to 10 (31%) of 32 laboratories. Because not all Stx EIA tests at these laboratories are confirmed by culture, clinical laboratories performing Stx EIA without culture confirmation have been required to submit the enrichment broth from all Stx-positive stool specimens to the CTDPH state laboratory since 2000.
At the CTDPH state laboratory, Stx-positive broths are plated on SMAC agar and SMAC agar enriched with cefixime-tellurite (CT-SMAC). Sorbitol-negative colonies are screened for the O157 antigen using a latex agglutination test and, if positive, are tested for the H7 antigen. If the sorbitol-negative colonies are O157-negative, both sorbitol-positive and sorbitol-negative colonies are tested for Stx using EIA. In November 2002, the CTDPH state laboratory instituted the additional step of screening Stx-positive colonies for the six most common non-O157 STEC serogroups in the United States (O26, O45, O103, O111, O121, and O145), using commercial antisera. All non-O157 STEC isolates are forwarded to CDC for further characterization. To allow examination of the epidemiology of non-O157 STEC, in April 2004, CTDPH also began interviewing all patients with confirmed STEC cases using a standardized questionnaire that collects clinical and exposure information.
During 2000-2005, a total of 403 laboratory-confirmed STEC infections were reported in Connecticut. Of these, 196 (49%) were identified as STEC O157 at clinical laboratories using culture; the remaining 207 (51%) were identified as STEC at clinical laboratories using Stx EIA with no culture confirmation. The percentage of STEC isolates identified initially by Stx EIA testing increased significantly (p<0.001) from 33% in 2000 to 59% in 2005. Similarly, the percentage of STEC O157 isolates identified as STEC initially by Stx EIA testing increased significantly (p<0.01) from 23% in 2000 to 40% in 2005. Among the Stx EIA-positive broths submitted to the CTDPH state laboratory, 82 (40%) yielded STEC O157 and 125 (60%) yielded non-O157 STEC. The percentage identified as non-O157 STEC has remained higher than 50% since 2001. Four serogroups accounted for 88 (70%) of the STEC non-O157 isolates: O103, 26 (21%) isolates; O111, 26 (21%) isolates; O26, 18 (14%) isolates; and O45, 18 (14%) isolates. The remaining 37 (30%) belonged to 15 other serogroups. During 2000-2005, the incidence of identified non-O157 STEC infections increased 50%, from 0.4 to 0.6 per 100,000 population.
Patients with non-O157 STEC infection were less likely than those with STEC O157 infection to have had bloody diarrhea (56% versus 90%, p<0.001), have been hospitalized (12% versus 45%, p<0.001), have developed HUS (zero versus 9%, p<0.001), or have eaten at a restaurant in the 7 days preceding illness onset (59% versus 88%, p=0.01). No differences were found in the proportion of patients who had eaten ground beef, had contact with farm animals, or visited a petting zoo in the 7 days before illness onset.
Q Phan, MPH, P Mshar, MPH, T Rabatsky-Ehr, MPH, C Welles, R Howard, MS, J Hadler, MD, Connecticut Dept of Public Health; S Hurd, MPH, P Clogher, MPH, R Marcus, MPH, Dept of Epidemiology and Public Health, Emerging Infections Program, Yale Univ School of Medicine, New Haven, Connecticut. L Demma, PhD, Div of Foodborne, Bacterial, and Mycotic Diseases, National Center for Zoonotic, Vector-Borne, and Enteric Diseases (proposed), CDC.
Non-O157 STEC infections represent a substantial portion of laboratory-confirmed STEC cases in Connecticut, consistent with findings from studies in other states.5,6 The number of clinical laboratories in Connecticut conducting Stx EIA testing has been increasing, thus the identified increase in the incidence of non-O157 STEC infections likely is a reflection of increased Stx EIA testing in the state and subsequent required submission of Stx-positive broths to the state laboratory for further characterization. However, because only 31% of clinical laboratories tested for non-O157 STEC in 2005, the number of detected cases likely represents the minimum annual incidence in Connecticut for that year.
Overall, infections caused by non-O157 STEC were less severe than those caused by STEC O157. However, the severity of disease caused by STEC is related to the virulence profile of the infecting strain, and some non-O157 serotypes cause illness as severe as that caused by STEC O157.7,8
The sources of non-O157 STEC infections are not well described, although outbreak investigations indicate that some sources are similar to those of STEC O157 infections.9,10 Furthermore, the similar exposures of patients with STEC O157 and non-O157 STEC cases in Connecticut described in this report suggest that many of the routes of transmission are similar.
The findings in this report are subject to at least three limitations. First, most clinical laboratories in Connecticut do not conduct Stx EIA testing; 22 (69%) of 32 laboratories use culture methods. As a result, the true number of non-O157 STEC infections remains undefined. Second, lack of uniformity exists among clinical laboratories regarding types of stool specimens that are cultured for STEC O157 or tested for Stx. Some laboratories culture or test all stool specimens, others only bloody stools, and others only on physician request. Finally, the numbers of each non-O157 STEC serogroup were too small to permit serogroup-specific analysis of disease severity and epidemiology.
In Connecticut, Stx EIA testing increasingly is replacing direct culture for STEC O157 in clinical laboratories. Connecticut has taken steps to ensure that all STEC isolates are further characterized, which can enable evaluation of the incidence and epidemiology of non-O157 STEC. Clinical laboratories in all states should forward Stx EIA-positive specimens to the public health laboratory for confirmation and characterization by culture methods to rule out false-positive EIA results and ensure accurate STEC surveillance.3
*Association of Public Health Laboratories. Guidelines for isolation and identification of Shiga toxin-producing E. coli, 2006. Available at http://www.aphl.org/programs/food_safety.
Laboratory-Confirmed Non-O157 Shiga Toxin-Producing. JAMA. 2007;297(9):945-946. doi:10.1001/jama.297.9.945