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Isolation of Vibrio cholerae O1 from Oysters -- Mobile Bay, 1991- 1992

On July 2, 1991, during routine monitoring, the Food and Drug Administration (FDA) isolated toxigenic Vibrio cholerae O1, serotype Inaba, biotype El Tor from oysters and intestinal contents of an oyster-eating fish taken from closed oyster beds in Mobile Bay (1). This isolate was indistinguishable from the Latin American epidemic strain and differed from the strain of V. cholerae O1 that is endemic to the Gulf Coast. This report summarizes the public health response to this isolation of V. cholerae O1.

On July 18, Gulf Coast residents were advised by the Mobile County Health Department and the Alabama Department of Public Health (ADPH) to wash their hands after handling raw seafood and to eat seafood well cooked. FDA and ADPH initiated biweekly sampling of oysters from Mobile Bay, and on July 22 and September 16, 1991, the Latin American strain was again isolated from oysters. The Mobile Bay oyster beds, initially closed on May 31, 1991, remained closed to harvesting until November 4, 1991. On June 15, 1992, toxigenic V. cholerae O1 was again isolated from a sample of oysters from a restricted shellfish-growing area, and the adjacent growing areas were closed to harvesting. On August 19, 1992, the oyster beds were reopened after samples were repeatedly negative. No toxigenic vibrios have been isolated since June 1992.

Toxigenic vibrios have not been isolated from Moore swabs that were placed in effluent from sewage treatment plants in the Mobile Bay area after each isolation of V. cholerae O1 from oysters. FDA and ADPH continue monitoring of shellfish obtained or harvested from the Mobile Bay area, and ADPH maintains surveillance for cases of cholera.

Reported by: BH Eichold, II, MD, JR Williamson, MPH, Mobile County Health Dept, Mobile; CH Woernle, MD, State Epidemiologist, Alabama Dept of Public Health. RM McPhearson, ScD, Food and Drug Administration. Enteric Diseases Br, Div of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: No cases of cholera have been identified in Alabama in recent decades. Surveillance for clinical cases has increased since the beginning of the Latin American outbreak in 1991, and many clinical laboratories now routinely culture diarrheal stool specimens on culture media appropriate for isolation of V. cholerae. The strain responsible for the epidemic in Latin America can be distinguished in the laboratory from the endemic V. cholerae O1 strain that is unique to the U.S. Gulf Coast (2).

The isolation of the Latin American strain of V. cholerae O1 from Gulf Coast oysters during two successive summers illustrates the potential for this organism to be repeatedly introduced or to persist in the environment at least transiently after a single introduction. However, there have been no recognized cases of cholera in the United States caused by the Latin American strain as a result of consumption of seafood harvested from the Gulf of Mexico. It is unknown how the Latin American strain was introduced into Mobile Bay. Surveillance using Moore swabs would have detected clinical cases and asymptomatically infected shedders of V. cholerae O1 (3). However, repeatedly negative Moore swabs indicate that municipal sewage was probably not the source of the strain.

Introduction of toxigenic vibrios into Mobile Bay may have resulted from discharge of contaminated ballast water from freighter vessels. To control buoyancy, ships take on large volumes of ballast water in a harbor and discharge it in other locations. This process may have been responsible for the introduction of other harmful species such as the zebra mussel in the Great Lakes (4). In 1991, the FDA isolated toxigenic V. cholerae O1 from the ballast tanks of ships that had originated from Latin American ports and arrived at Mobile Bay (5). To reduce the risk of introducing harmful organisms through contaminated freighter ballast water, the International Maritime Organization has recommended that freighters empty and refill their ballast water tanks twice on each voyage while in international waters (6). The efficacy of ballast water exchanges in reducing the level of contamination of ballast water has not been assessed. Although ballast water exchanges may decrease the risk of introduction of V. cholerae O1 from other ports into U.S. harbors, this approach would not eliminate the strain already endemic in U.S. Gulf Coast waters.

Since 1973, 91 cases of cholera have occurred in the United States that were unrelated to international travel. Most of these followed consumption of raw or undercooked seafood harvested from the U.S. Gulf Coast contaminated with the Gulf Coast strain of V. cholerae O1. The risk for transmission of cholera can be reduced by avoiding consumption of raw or undercooked seafood.


  1. DePaola A, Capers GM, Motes ML, et al. Isolation of Latin American epidemic strain of Vibrio cholerae O1 from U.S. Gulf Coast {Letter}. Lancet 1992;339:624.

  2. Wachsmuth IK, Bopp CA, Fields PA. Difference between toxigenic Vibrio cholerae O1 from South America and U.S. Gulf Coast {Letter}. Lancet 1991;337:1097-8.

  3. Barrett TJ, Blake PA, Morris GA, et al. Use of Moore swabs for isolating Vibrio cholerae from sewage. J Clin Microbiol 1980;11:385-8.

  4. Roberts L. Zebra mussel invasion threatens U.S. waters. Science 1990;249:1370-2.

  5. McCarthy SA, McPhearson RM, Guarino AM, Gaines JL. Toxigenic Vibrio cholerae O1 and cargo ships entering Gulf of Mexico {Letter}. Lancet 1992;339:624-5.

  6. Coast Guard, US Department of Transportation. International Maritime Organization ballast water control guidelines. Federal Register 1991;56:6483.

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