In response to four separate outbreaks that occurred in 1997 and 1998 in the United States (21, 22), FDA conducted a risk assessment (RA) on the public health impact of Vibrio parahaemolyticus transmitted by raw oysters. Initiated in January 1999, this risk assessment focused specifically on oysters, since this was the food predominantly linked to the outbreaks.
In May 1999, FDA announced its intent to conduct a risk assessment of the public health impact of V. parahaemolyticus in raw molluscan shellfish in the Federal Register (43). At that time, the public was invited to comment on the planned assessment and submit scientific data and information for use in the assessment. The advice and recommendations of the National Advisory Committee on Microbiological Criteria for Foods (NACMCF) were sought in May and September of 1999, on the assumptions and the risk assessment model structure to be used. During the conduct of this risk assessment, FDA solicited the technical advice and opinions of several scientific and state shellfish experts. In addition, critical review of this risk assessment model was solicited and received in December 1999 and May 2000, from members of the Interagency Risk Assessment Consortium, other government employees, and special government experts (SGE).
A chronology of the technical and scientific review involved in the development of this risk assessment is provided in Appendix I.
Over 700 cases of illness caused by V. parahaemolyticus (21, 22) resulted from the 1997 and 1998 outbreaks that occurred in three regions of the country, one in the Gulf Coast, two in the Pacific Northwest, and one in the Northeast. One particular serotype (O3:K6) of V. parahaemolyticus, previously associated only with illnesses in Asia (109), was predominantly isolated from patients as the principal cause of the outbreaks occurring in the Northeast and Gulf Coast regions. It has been suggested that the 1998 Gulf Coast outbreak may have been a result of elevated water temperatures (34). The influences exerted by these and other factors remain uncertain. Whatever factors were involved in these four incidents, the recurrence of V. parahaemolyticus outbreaks in North America and the large number of individuals affected have renewed concerns for this pathogen in raw oysters as a microbial food safety problem in the United States.
First isolated and implicated in an outbreak of food poisoning in Japan in 1950 (48), V. parahaemolyticus has been associated with outbreaks and individual cases of illness in the United States since 1969 (11, 32, 91, 107). However, since 1981 and until 1997 the only V. parahaemolyticus illnesses transmitted by raw molluscan shellfish in the United States have been reports of intermittent, sporadic cases, and relatively few of these occurred. The recurrence of outbreaks caused by this organism was unexpected, and the wide-ranging geography and total number of illnesses recorded were surprising. Consequently, a systematic evaluation of factors affecting V. parahaemolyticus in oysters and the sequence of events leading to consumer illnesses was undertaken in order to gain a fuller understanding of the risks posed by this bacterial pathogen.
The levels of V. parahaemolyticus in oysters at the time of consumption can be influenced substantially by the methods chosen for harvesting and handling oysters after harvest, and these practices may vary considerably in different geographic areas and at different times of year. For example, the period of unrefrigerated storage may vary from just a few hours to more than a day, and perhaps longer in some cases. In general, the longer oysters remain unrefrigerated, the higher the level of V. parahaemolyticus in those oysters will become, and the higher the environmental temperature, the faster V. parahaemolyticus will grow. Thus, the levels of V. parahaemolyticus in oysters kept alive until consumption can range widely. This seemed likely to be an important element in estimating the level of risk to consumers.
The infectious dose of V. parahaemolyticus is not known. Based on data from volunteer studies conducted more than 25 years ago, along with data from earlier United Staets outbreaks caused predominantly by cross-contamination of cooked crabs, FDA had previously indicated that V. parahaemolyticus in shellfish should not exceed a level of 10,000 viable cells per gram (64). However, data obtained during the recent outbreaks indicated that fewer than 10,000 V. parahaemolyticus per gram were present in oysters obtained from implicated harvest areas during the same period (21). In fact, the overall levels of V. parahaemolyticus found in some oysters from implicated harvest sites were as low as 100 and often less than 1,000 cells per gram. In view of this, and the fact that not all V. parahaemolyticus strains are pathogenic, it seemed possible that the earlier guidance alone may not be sufficient to protect consumers from V. parahaemolyticus illness associated with raw oysters.
The risk assessment has focused on raw oysters because these were the predominant seafood implicated in the 1997 and 1998 outbreaks (5, 21, 22). This endeavor investigated the prevalence of V. parahaemolyticus, the extent of consumer exposure to the organism, the resulting public health impact, and the effects of several post harvest treatments to control exposure to this hazard in raw oysters. A systematic evaluation of the available scientific information was conducted to assist public health officials in assessing different potential control measures and developing food safety guidance and policies. The risk assessment had two main objectives: (1) create a mathematical model and assess the current risk of becoming ill due to the consumption of pathogenic V. parahaemolyticus in raw oysters; and (2) develop a comprehensive and current scientific framework, which will assist the agency with the review of current programs relating to the regulation of V. parahaemolyticus in raw molluscan shellfish to ensure that such programs protect the public health.
Exposure is a function of the prevalence of V. parahaemolyticus in the oysters consumed and the consumption patterns of the population. Thus, the number of pathogenic V. parahaemolyticus in oysters at consumption, and what leads to this number, is critical exposure information. Accordingly, the scope of the risk assessment specifically attempted to address the following questions:
Thus, the risk assessment encompassed the relationships between oysters, V. parahaemolyticus, and illnesses, and exposure to pathogenic V. parahaemolyticus. The risk assessment attempted to evaluate the following factors:
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