Shellfish species

Seafood Toxins. Vktually scores of fish and shellfish species have been reported to have toxic manifestations. Most of these toxicities have been shown to be microbiological ki origin. There are a few, however, that are natural components of seafoods. 

One of the first applications of the HPLC method was the investigation of differences in toxin profiles between shellfish species from various localities ( ). It became apparent immediately that there were vast differences in these toxin profiles even among shellfish from the same beach. There were subtle differences between the various shellfish species, and butter clams had a completely different suite of toxins than the other clams and mussels. It was presumed that all of the shellfish fed on the same dinoflagellate population, so there must have been other factors influencing toxin profiles such as differences in toxin uptake, release, or metabolism. These presumptions were strengthened when toxin profiles in the littleneck clam (Prototheca Staminea) were examined. It was found that, in this species, none of the toxin peaks in the HPLC chromatogram had retention times that matched the normal PSP toxins. It was evident that some alteration in toxin structure had occurred that was unique in this particular shellfish species. 

Palmer, J.B. and G.M. Rand. 1977. Trace metal concentrations in two shellfish species of commercial importance. Bull. Environ. Contam. Toxicol. 18 512-520. 

In summary, the toxin composition of shellfish, while not random, can be expected to vary greatly with location, time, and shellfish species. 

Molluscan shellfish play an important role in human nutrition and the world economy (Wild and Lehrer, 2005). Table 4.2 provides data on the worldwide production/catch of various molluscan shellfish species for 2005. The most widely available species are oyster, squid, clam, mussel, and scallop. Aquaculture has become an important contributor to the production of molluscan shellfish with the exception of the cephalopods. However, the popularity and frequency of consumption of various molluscan shellfish varies widely across various countries and cultures. Accurate information on comparative consumption patterns for molluscan shellfish in various countries does not exist. Molluscan shellfish are consumed as freshly cooked or even raw seafood items particularly in coastal communities. But mollusks also are consumed as processed foods in a variety of forms. 

TABLE 4.4 Tropomyosin allergens from molluscan shellfish species... 

Clearly, some individuals with molluscan shellfish allergy are reactive to all species of molluscan shellfish. Cross-reactivity has been established by clinical history, challenge trials (in a few instances), skin prick testing, and IgE-binding studies. Most clinical studies of cross-reactivity have been limited to a few species often within one class of molluscan shellfish. However, the totality of the evidence indicates that individuals with documented reactivity to one molluscan species and evidence of IgE against that species should be counseled to avoid other molluscan shellfish species. This recommendation is especially prudent for the individual classes of molluscan shellfish gastropods, bivalves, and cephalopods. 

Cross-reactivity between molluscan and crustacean shellfish species also occurs rafher frequently. Since tropomyosin is the major allergen in both molluscan and crustacean shellfish, fhe frequency of cross-reactions is not surprising. Allergy to crustacean shellfish is more frequenfly diagnosed than molluscan shellfish allergy (Hefle et ah, 2007). Many of fhese individuals may be at risk of reactions fo molluscan shellfish also. Appropriately, most individuals with either molluscan or crustacean shellfish allergy are advised to avoid all shellfish. 

The tropomyosins of mite and insect species show some sequence identity (63-65%) with snail tropomyosin and share similar epitopes (EFSA, 2006 Fig. 4.1). Still, tropomyosin appears to play a minor role in the crossreactivity of dust mites and snails (Asturias et ah, 2002 Guilloux et ah, 1998 Van Ree et ah, 1996a). Other non-tropomyosin allergens are likely to be involved including Der p 4 (amylase), Der p 5, Der p 7, and hemocyanin (Martins et ah, 2005 Mistrello et ah, 1992 Van Ree et ah, 1996). While snail is the main molluscan shellfish species involved in cross-reactions with dust mites, some patients allergic to dust mites and snails are also sensitized to mussels (DeMaat-Bleeker et ah, 1995 Van Ree et ah, 1996b). In their study of 70 patients sensitized to molluscan shellfish, Wu and Williams (2004) noted that 90% were also sensitized to dust mites. However, the clinical significance of this sensitization was not documented. 

Since diazinon is not a priority pollutant and has not been considered to pose serious threats from bioconcentration or bioaccumulation in fish and shellfish species, it has attracted far less attention in the United States than persistent organochlorines like DDT or chlordane in routine surface water monitoring networks. Carey and Kutz (1985) reported that the maximum diazinon residue collected in a national surface water monitoring program conducted from 1976 to 1980 was 2.38 ppb and that diazinon was detected in only 1.2% of the samples collected. More recently, Pereira and Hostettler (1993) conducted a study of the Mississippi River and its tributaries during 1991 and 1992. These authors reported that... 

We have general population data only. We do not have details on variations in consumption patterns. We do not have data on variability within and across fish/shellfish species. 

Level of uncertainty High Estimation of the concentration of PBLx is limited to a single value that is dependent on many parameters (e.g. fish/shellfish species and fish/shellfish size) because of limited sampling. 

Figure 9.3. The two main pathways for metabolism of PTX-2 in shellfish. The oxidative pathway has so far been confirmed only in P. yessoensis. The hydrolytic pathway appears to occur in all other shellfish species studied, including mussels, clams, and other species of scallop.

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