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Viral Gastroenteritis Associated with Consumption of Raw Oysters -- Florida, 1993

During November 20-30, 1993, four county public health units (CPHUs) of the Florida Department of Health and Rehabilitative Services (HRS) in northwestern Florida conducted preliminary investigations of seven separate outbreaks of foodborne illness following consumption of raw oysters. On December 1, the HRS State Health Office initiated an investigation to characterize the illness, examine risk factors for oyster-associated gastroenteritis, and quantify the dose-response relation. This report presents the findings of these two investigations. Preliminary Investigations by the HRS CPHUs

In November 1993, private physicians notified the CPHUs of 20 persons with possible foodborne illness. These 20 ill persons identified seven well meal companions. Raw oysters were the only common food item eaten by all ill persons; no well meal companions had eaten oysters. At the request of the HRS State Health Office, CPHUs initiated active surveillance for cases of raw oyster-associated gastroenteritis among patients of hospital emergency departments, urgent-care centers, and private physicians in northwestern Florida. A case was defined as sudden onset of nausea, vomiting, diarrhea, or abdominal cramps within 72 hours of eating raw oysters. Twenty-five additional cases of gastroenteritis associated with eating raw oysters were detected.

Traceback of implicated oysters by the CPHUs and the Florida Department of Environmental Quality indicated the oysters had been harvested from Apalachicola Bay in northwestern Florida during November 15-23. Epidemiologic Investigation by the HRS State Health Office

The 45 persons with raw oyster-associated gastroenteritis reported by the CPHUs identified 26 well meal companions who had eaten oysters during the same meal as ill persons, but did not become ill. Of 44 ill persons for whom data were available, 36 (82%) had developed diarrhea; 34 (77%), nausea; 33 (75%), abdominal cramps; 25 (57%), vomiting; 17 (39%), fever; 15 (34%), headache; and 14 (32%), myalgia. The attack rate was 63%. Of the 45 ill persons, 10 were hospitalized for 24 hours or longer. For 30 persons for whom data were available, the median incubation period was 31 hours (range: 2-69 hours). For 26 persons for whom data were available, the median duration of illness was 48 hours (range: 10 hours-7 days); for 13 persons, duration of illness was more than 3 days. No household contacts of ill persons developed gastroenteritis.

No differences were identified between persons who became ill and well meal companions in preexisting medical conditions or medications. Consumption of alcohol or food (e.g., crackers and hot sauce) with the oysters was not associated with risk for illness. Based on the 33 cases for which data were available, a dose-response relation was observed between illness and number of raw oysters eaten (chi square for trend=3.98; p=0.05). The attack rate was highest among raw-oyster eaters who had consumed more than 5 dozen oysters (91%) and lowest among those who had consumed less than 1 dozen oysters (46%).

Paired serum specimens from 10 patients were tested for antibody to Norwalk-like virus by enzyme immunoassay (1); three pairs demonstrated a fourfold or greater rise in titer. Seven stool specimens were examined by electron microscopy (EM) and reverse transcription-polymerase chain reaction (RT-PCR). In four specimens, small round-structured viruses were detected by EM; in one specimen, a Norwalk-like genome was confirmed by RT-PCR (2,3). This Norwalk-like virus strain had a nucleotide sequence distinct from similar viruses in nearly simultaneous outbreaks associated with consumption of oysters harvested along the Louisiana coast (4).

No confirmed evidence of improper handling (e.g., inadequate refrigeration time or temperature) of the implicated oysters was detected. However, three ill persons had purchased oysters from retail establishments that were not licensed seafood dealers.

The National Shellfish Sanitation Program (NSSP) requires fecal coliform testing at least once each month. Fecal coliform testing of water drawn from 39 monitoring sites in Apalachicola Bay on October 3, November 21, and November 24 indicated that water quality in the bay met the criteria of the NSSP (5). No environmental source of pollution was identified. Sanitation procedures at the oyster-processing facilities where seafood dealers purchased oysters met standards set by the Florida Department of Environmental Protection (FDEP). However, based on the epidemiologic evidence of illness associated with oysters harvested from those waters, FDEP temporarily closed the shellfish-harvesting area of Apalachicola Bay during December 1-7. No cases of gastroenteritis related to consumption of oysters harvested after December 7 have been reported.

Reported by: C Davis, A Smith, MD, R Walden, Bay County Public Health Unit, Panama City; G Bower, K Cummings, B Dean, J Rigsby, Jackson County Public Health Unit, Marianna; P Justice, C Anderson, N Brown, J Minor, Washington County Public Health Unit, Chipley; EF Geiger, MD, V Laxton, District 1 Health Office, Pensacola; L Crockett, MD, W McDougal, District 2 Health Office, Tallahassee; WG Hlady, MD, RS Hopkins, MD, State Epidemiologist, State Health Office, Florida Dept of Health and Rehabilitative Svcs. Food and Drug Administration. Viral Gastroenteritis Section, Respiratory and Enterovirus Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; Div of Field Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: This report documents outbreaks of viral gastroenteritis in Florida linked to consumption of raw oysters from waters that apparently met the standards for shellfish sanitation. Clinical and epidemiologic features of the outbreaks are similar to recently reported multistate outbreaks of viral gastroenteritis associated with eating oysters harvested in Louisiana (4). RT-PCR with sequencing identified different strains of the virus in the multistate outbreak and the Florida outbreak, suggesting independent sources of oyster contamination.

Although infection with the oysterborne Norwalk-like virus caused no fatalities in this outbreak, raw oyster consumption has been linked in Florida to 30 fatal cases of infection with Vibrio vulnificus during 1981-1992 among persons with preexisting liver disease (6). V. vulnificus is a ubiquitous organism found in seawater. In Florida, consumer information statements (required as labels on bags of oysters and in restaurants) emphasize the risk for Vibrio infection among persons with underlying liver disease and other preexisting illnesses (6). In addition, these statements suggest that such persons eat oysters fully cooked and consult with their physician if uncertain about whether they are at risk.

States conduct monitoring programs to assure clean oyster beds, legal harvesting, and proper handling of oysters. However, at both the Louisiana and Florida oyster harvest sites, routine fecal coliform water-quality monitoring conducted once each month did not detect oyster-bed contamination. Furthermore, the outbreak reported in Florida was identified in part because of publicity about the larger outbreaks associated with oysters harvested in Louisiana. These findings suggest that monitoring waters for fecal coliforms may be insufficient to indicate the presence of viruses (e.g., Norwalk-like virus). Continued surveillance for outbreaks of gastroenteritis associated with consumption of raw oysters is needed to assess efficacy of the NSSP in preventing human illness. Public health officials should consider raw oyster consumption as a possible source of infection during the evaluation of gastroenteritis outbreaks.


  1. Monroe SS, Stine SE, Jiang XI, Estes MK, Glass RI. Detection of antibody to recombinant Norwalk virus antigen in specimens from outbreaks of gastroenteritis. J Clin Microbiol 1993; 31:2866-72.

  2. Moe CL, Gentsch J, Ando T, et al. Application of PCR to detect Norwalk virus in fecal specimens from outbreaks of gastroenteritis. J Clin Microbiol 1994;32:642-8.

  3. Ando T, Mulders MN, Lewis DC, Estes MK, Monroe SS, Glass RI. Comparison of the polymerase region of small round structured virus strains previously classified in three antigenic types by solid-phase immune electron microscopy. Arch Virol 1994;135:217-26.

  4. CDC. Multistate outbreak of viral gastroenteritis related to consumption of oysters -- Louisiana, Maryland, Mississippi, and North Carolina, 1993. MMWR 1993;42:945-8.

  5. Office of Seafood, Shellfish Sanitation Branch, Food and Drug Administration. Sanitation of shellfish growing areas, part 1. {Section C.3.c}. In: National Shellfish Sanitation Program manual of operations. Washington, DC: US Department of Health and Human Services, Public Health Service, 1992:C8-C9.

  6. Hlady WG, Mullen RC, Hopkins RS. Vibrio vulnificus from raw oysters: leading cause of reported deaths from foodborne illness in Florida. J Fla Med Assoc 1993;80:536-8.

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