CONTROL (mollusca)

Summary

Signiﬁcant hazards a r Shellﬁsh toxins. r Virus.

r Enteric bacterial pathogens. r Vibrio spp.

Control measures

Initial level (H 0 )

r Avoid harvesting from contaminated areas.

Reduction (Σ R)

r Depuration (partially effective). r Cooking.

Increase (Σ I)

r Keep animal alive.

Testing

r Surveying water quality for algal blooms and preferably for enteric virus. r Animals tested for Salmonella.

Spoilage

r Spoilage does not occur if animal is kept alive.

a In particular circumstances, other hazards may need to be considered.

Hazards to be considered. Live molluscs to be consumed raw (e.g. oysters) pose several severe risks to the consumer. The ﬁlter feeding nature of the animal allows it to concentrate pathogenic organisms that pose a very low risk when present in low numbers. Virus, shellﬁsh toxins, Vibrios and enteric bacteria are all major hazards in this product.

Control measures. Assuring the safety of molluscs for the consumer lies in actions taken at two levels: (1) control of the initial level of the pathogenic agents (e.g. controlling H 0 ), and (2) reducing levels (ΣR), e.g. by a heating step.

Initial level of hazard (H 0 ). Although H 0 is controlled by growth conditions, and control of harvest and distribution, the reducing step will typically be part of the ﬁnal preparation. None of the control measures that can be installed to “control” H 0 will be able to guarantee a completely safe, raw product. Thus, the product will always pose some level of risk to the consumer. Health warning labels have been used in retail establishments in some states in the United States to caution at-risk individuals concerning the hazards associated with the consumption of raw shellﬁsh.

Reduction of hazard (Σ R). The consumer can greatly influence the health risk from eating molluscs by introducing a reduction step (ΣR). A decision to eat shellfish only if adequately cooked reduces dramatically the risks associated with the consumption of these foods (IOM, 1991). This can also have an impact on non-bacterial concerns. For example, the kinetics of heat inactivation of paralytic shellfish poison in soft-shell clams was found to be similar to those of most bacteria (Gill et al., 1985).

The shellfish should be harvested from areas free from contaminating sewage or other undesirable land run-off (e.g. animal waste), and the water should not contain sufficient toxic microalgae that would render the shellfish hazardous. The approach used by the U.S. National Shellfish Sanitation Program Manual of Operations Parts I & II (FDA, 1989a,b) has been found to be very effective when diligently applied. Most molluscs are harvested from sediments and should be washed immediately after capture

207 with clean seawater to remove mud and debris. However, even with thorough washing, molluscs, as

FISH AND FISH PRODUCTS

filter feeders, can be assumed to contain a high level of bacteria in their digestive tract. This can be reduced somewhat by providing the live animal with a source of clean seawater with low bacterial counts, and allowing it to purge itself. The most widely used methods are depuration and relaying. Each relies on the mollusc’s inherent ability to clean itself of contamination when supplied with a source of pathogen-free water. Depuration involves shellfish being placed in a controlled flow of disinfected water for a relatively short period of time (usually 24–48 h). With relaying, the shellfish are transferred to new beds that have naturally clean seawater. They are kept there for periods up to several weeks and then re-harvested. Even following such a depuration, pathogenic agents may persist in the animals. Although these methods are reasonably effective in eliminating or reducing greatly Enterobacteriaceae within the mollusc, they are substantially less effective for viruses and toxins, and have little effect on vibrios, including V. cholerae O1 (Richards, 1988; Croci et al., 1994; Murphree and Tamplin, 1995) and V. vulnificus. Reducing the storage temperature of the live animals after harvest, while awaiting processing, or during distribution if marketed live, helps reduce the potential for growth of mesophilic spoilage organisms and pathogens.

The presence of naturally occurring, potentially pathogenic, vibrios in shellfish at harvest is difficult to deal with because in most cases it is a question of both numbers of particular vibrios and consumer susceptibility. Where cholera is endemic or epidemic, the safest procedure is to warn consumers against eating raw shellfish and to close harvesting on shellfish beds where V. cholerae O1 is regularly isolated from water. This requires water samples to be tested specifically for the microorganism. The evidence concerning V. vulnificus, suggests that risk is partly related to numbers of the species present in the shellfish which, in turn, usually relates to water temperature (Ruple and Cook, 1992). It has been suggested that harvesting be restricted when water temperatures exceed a particular value associated with the development of high V. vulnificus populations in oysters (NACMCF, 1992). Presently, some producers in high temperature water areas are labeling their packaged oysters to warn susceptible consumers of the potential hazard of eating oysters raw. Chilling oysters immediately after harvest may significantly reduce levels of V. vulnificus. (Cook and Ruple, 1989; Ruple and Cook, 1992). Depuration also appears to help reduce V. vulnificus levels in oysters (Groubert and Oliver, 1994). A range of technologies, such as mild heating, freezing, or high-pressure treatment may also assist in reducing levels of pathogenic bacteria.

If oysters are schucked, cross-contamination during shucking must be avoided, and adequate cooling applied to prevent growth of small numbers of pathogens to more infective numbers prior to eating (Cook, 1994; Bouchriti et al., 1995). These two factors can give rise to a number of controls that are similar to those discussed earlier for ﬁnﬁsh and crustaceans.

Increase of hazard (Σ I). Animals must be kept alive and at low temperature to avoid increase in numbers of pathogenic bacteria.

Testing. The control of shellfish sources requires ongoing assessment of the quality of the growing waters, although it must be emphasized that testing and evaluating water quality cannot guarantee that the animals are free of pathogens. Procedures involved include establishing the suitability of a water area by examining water drainage access, bacteriological, and viral analysis of the waters and shellfish to provide baseline data, and regular surveillance of the areas thereafter. Surveillance includes water analysis and analysis of indicator molluscs for algal toxins when deemed necessary. Shellfish beds are opened or closed to harvest on the basis of analytical results and there are provisions for the identification of shellfish according to area of harvest throughout shipment. Also, the EU legislation (EEC, 1991a) has special requirements for the growing waters of molluscs and classifies waters as category A, B, or

C depending on levels of e.g. fecal coliforms (EEC, 1991a; Lees, 2001).

MICROORGANISMS IN FOODS 6

Typically, water surveillance involves periodic examination for the presence of indicators of faecal contamination, such as members of the Enterobacteriaceae. While these have generally been useful, they have limited efficacy as index organisms for the presence of specific enteric pathogens (Hood et al, 1983; Martinez-Manzanares et al., 1992). Further, there is little relationship between enteric indicators in growing waters and the presence of Vibrio spp., viruses, or algal toxins in molluscs. As previously mentioned, algal toxins require identification of the toxic species. Similarly, viruses require specialized analyses.

The periodic examination of shucked shellﬁsh for microbiological indicators, such as APC or

E. coli counts, can be used to verify the overall effectiveness of process control programmes over time. However, their use in the routine examination of production lots is of limited value for assuring either the microbiological safety or quality of raw molluscan shellﬁsh.

Realizing that surveillance of water quality cannot guarantee a safe product, several agencies have set microbiological limits e.g. for E. coli or salmonellae, for mollusc (EEC, 1991a, 1993). However, such end product controls can never eliminate a certain level of risk (ICMSF, 2001) and this should be made clear to the consumer.

Spoilage. Spoilage is not an issue for molluscs destined for live consumption, as the immune system of the animal will prevent degradation of the meat. For shucked, chilled oysters, sensory evaluation is the best way of assessing spoilage, and low-temperature storage (0–2 ◦

C) should be maintained to retard spoilage.

CONTROL (mollusca)