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Over a three-year period, the total Vibrio vulnificus level in oysters harvested from a commercial site in Apalachicola Bay, FL, was measured at monthly intervals for three years by both direct-plating and most probable number procedures. The concentration of V. vulnificus varied from less than 1 bacterium/g during most autumn and winter months to 105/g during mid-summer. Eight cases of disease traced to oysters from this location occurred during the May-September months when the V. vulnificus levels in oysters reached or exceeded 103/g (range, 103/g-105/g).
The blood isolates from three patients who died within 24-48 hours after eating raw oysters were studied by pulsed-field gel eletrophoresis (PFGE). PFGE was performed for at least 50 isolates from each blood specimen. The profile for all isolates was the same and unique for each patient. In one case in which implicated oysters were available for study, eight different V. vulnificus PFGE profiles were identified, and only the one that was identical to the blood isolate was virulent in the iron-overloaded mouse model. In the only other case in which oysters were available, 18 different PFGEprofiles were identified, all different from the blood isolate.
V. vulnificus thrives in the warm water and shellfish of the Gulf of Mexico, but it has also been detected along the East coast up to Cape Cod. It is probably the most common cause of serious morbidity and death from Vibrionaceae in the United States. Persons with underlying immunodeficiency (usually alcohol-related liver disease) are those primarily at risk, and the mortality rate from septicemia and wound infections among that population is about 50%.
Almost 60% of infections follow ingestion of raw oysters. Most of these occur during the summer months when the concentration of V. vulnificus in oysters is high, but the actual infecting dose was previously unknown. Also, although studies using PFGE had established that individual oysters are populated by numerous strains of V. vulnificus (a dozen raw oysters can contain hundreds of strains), it was not known whether disease is caused by single or multiple strains.
This study demonstrates that high-risk individuals are susceptible to lethal septicemic infection after ingesting raw oysters containing contentrations of V. vulnificus as low as only 2 ´ 103/g. Current regulatory standards do not monitor for the presence of V. vulnificus in oyster harvest waters, and, unfortunately, there is no correlation between V. vulnificus and fecal coliform levels. However, the information gleaned from this study plus further investigations into the methods used to handle oysters after harvest vis-à-vis the possible selective proliferation of pathogenic strains might eventually lead to standards.
The PFGE results proved that V. vulnificus infection results in proliferation of a single pathogenic strain from among many strains ingested. That the single infective strains from each of the three individuals did not differ by PFGE results supports the single-organism hypothesis for bacterial infections (e.g., that within mixed populations "each bacterium acts alone or is independently capable of introducing infection").
Interestingly, only 5-10 V. vulnificus infections are reported in Florida each year, although it is estimated that more than 70,000 persons with advanced liver disease eat raw oysters there. Presumably, differences in both host susceptibility and strain virulence contribute to such a low incidence of disease. Regardless of the small number of cases, it seems prudent to warn cirrhotic and other highly immunosuppressed persons that eating even a single raw Gulf oyster in mid-summer could be fatal. In Los Angeles, the Department of Health Services has developed a warning system including posted warning signs for certain populations to avoid Gulf Coast oysters (Mouzin E, et al. JAMA 1997;278:576-578). Unfortunately, signs are evidently not posted appropriately, as evidenced by recent cases in target populations in Los Angeles. More effective strategies will be needed in Los Angeles and perhaps other locations in the United States. For the time being, susceptible oyster lovers who can’t resist their shellfish should either wait until winter or only eat oysters from northern waters. (Dr. Ackley is Professor of Clinical Medicine, Robert Wood Johnson Medical School.)