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University Outbreak of Calicivirus Infection Mistakenly Attributed to Shiga Toxin-Producing Escherichia coli O157:H7 --- Virginia, 2000

On February 21--22, 2000, the Virginia Department of Health (VDH) was notified by a university student health center of two suspected cases of Escherichia coli O157:H7. At a local hospital laboratory, stool specimens from the two ill students tested positive for Shiga toxinproducing E. coli (STEC) using a commercially available enzyme immunoassay (EIA) kit. Further investigation revealed that the outbreak of gastrointestinal illness was caused by a Norwalk-like virus (NLV), a member of the calicivirus family. This report summarizes the outbreak investigation and laboratory findings used to identify the causative agent, and highlights the need for follow-up cultures on all specimens testing positive for STEC by EIA and for submission of isolates to state laboratories so that public health agencies can respond appropriately in identifying common source outbreaks.

Three staff members from Virginia's epidemiology office were sent to assist the local health department with the epidemiologic and environmental investigations. VDH staff interviewed 12 students who had sought care for gastrointestinal symptoms at the student health center during the previous week. Most students reported illnesses that appeared more likely to be caused by a virus than by STEC (i.e., vomiting and/or diarrhea lasting 1--2 days that occurred approximately 24--48 hours after eating at an area restaurant [restaurant A]). Other restaurant patrons were located by questioning ill students about persons they knew or recognized at restaurant A on February 18. A case of illness was defined as vomiting or diarrhea occurring within 72 hours of eating at restaurant A. A survey was conducted of 36 ill and 32 well restaurant A patrons. The median incubation period was 31.3 hours (range: 2.5--49.0 hours). Symptoms included nausea (97%), vomiting (97%), abdominal cramps (86%), chills (78%), muscle aches (67%), fever (64%), headache (61%), and diarrhea (58%). The median illness duration was 26.5 hours (range: 6--120 hours). One ill person was hospitalized and 10 others sought medical care. Eating a sandwich or "sub" (76%) was associated highly with illness (relative risk=14.5; 95% confidence interval=2.1--98.1). No other food item was associated with illness.

The two stool specimens that had tested positive for Shiga toxin at the local hospital laboratory did not yield E. coli O157:H7 or other STEC when tested on February 29 at the Virginia Division of Consolidated Laboratory Services (DCLS) using standard biochemical and EIA analysis. Additional stool specimens obtained from ill persons and submitted to DCLS also did not yield Shiga toxin-producing organisms. On subsequent testing by reverse transcriptase-polymerase chain reaction, four of eight specimens were positive for NLV. These results were consistent with the patients' clinical presentation.

Reported by: X Jiang, PhD, N Wilton, E Jing, W Zhong, Center for Pediatric Research, Eastern Virginia Medical School, Hampton Roads, Virginia. D Warren, MD, L Rose, L Nycum, Peninsula Health District; D Jordan, G Gaines, J Beyer, C Puckett, Environmental Health Svcs; ST Kelly, MY Mismas, D Patel, S Henderson, DM Toney, PhD, JL Pearson, DrPH, Div of Consolidated Laboratory Svcs; E Barrett, DMD, MJ Linn, Office of Epidemiology, Virginia Dept of Health. Foodborne and Diarrheal Diseases Br, Div of Bacterial and Mycotic Diseases; Viral Gastroenteritis Section, Respiratory and Enteric Virus Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; and EIS officers, CDC.

Editorial Note:

In 1995, rapid assays for Shiga toxin first became commercially available. These nonculture assays can detect E. coli O157:H7 and other Shiga toxinproducing strains in stool specimens and culture broth (1). However, as the findings in this report illustrate, these nonculture rapid assays are subject to false positives, which can result in unnecessary public concern and expenditure of public health resources. Follow-up cultures are needed to confirm the presence of STEC and to obtain isolates for subtyping by pulsed-field gel electrophoresis at state public health laboratories.

Although subtyping is of limited value to the individual patient, it is a useful tool for identifying and responding to common source outbreaks caused by E. coli O157:H7 (2). Several states require clinical laboratories to submit E. coli O157:H7 isolates for this purpose. Routine submission of all STEC to state public health laboratories also allows enhanced surveillance for illness caused by non-O157 STEC. In 2000, the Council of State and Territorial Epidemiologists adopted a position supporting culture confirmation of positive results from rapid assay tests for pathogens of public health importance (3).

Because the clinical signs and symptoms of NLV infection are nonspecific and overlap with other causes of foodborne disease, criteria were developed to aid health-care providers in identifying NLV-associated infection (4,5). These criteria include 1) an illness of 12--60 hours duration, 2) an incubation period of 12--36 hours, and 3) an illness characterized by acute onset of nausea, vomiting, diarrhea, abdominal cramping, and, in some cases, fever and malaise (4,6). Diarrhea is usually more common among adults and vomiting is usually more common among children (4). Additional information on NLV is available from CDC's National Center for Infectious Diseases, Division of Viral and Rickettsial Diseases, Respiratory and Enteric Viruses Branch, Viral Gastroenteritis Section at


  1. Kehl KS, Havens P, Behnke CE, Acheson DWK. Evaluation of the premier EHEC assay for detection of Shiga toxin-producing Escherichia coli. J Clin Microbiol 1997;35:2051--4.
  2. Bender JB, Hedberg CW, Besser JM, Boxrud DJ, MacDonald KL, Osterholm MT. Surveillance by molecular subtype for Escherichia coli O157:H7 infections in Minnesota by molecular subtyping. N Engl J Med 1997;337:388--94.
  3. Council of State and Territorial Epidemiologists. Position statement 2000-ID-04. Available at Accessed June 2001.
  4. Kaplan JE, Feldman R, Cambell DS, Lookabaugh C, Gary GW. The frequency of a Norwalk-like pattern of illness in outbreaks of acute gastroenteritis. Am J Public Health 1982;72:1329--32.
  5. Deneen VC, Hunt JM, Paule CR, et al. The impact of foodborne calicivirus diseases: the Minnesota experience. J Infect Dis 2000;181:1--3.
  6. CDC. Norwalk-like viruses: public health consequences and outbreak management. MMWR 2001;50(no. RR-9).

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