The rate of DSP toxin loss varies with the season. Low water temperatures apparently retard toxin loss; however, the degree to which temperature affects the uptake and release of toxins is unknown. The rate of detoxification is highly dependent on the site of toxin storage - that is toxins in the gastrointestinal tract (e.g. Mytilus) are eliminated much more readily than toxins bound in tissues. Information concerning bivalve molluscs reared in aquaculture showed that retention time of the toxin in Mytilus edulis varied from one week to six months. Studies with mussels reared in an aquaculture pond and in the laboratory showed that a highly toxic (three MU) level of DSP toxins dropped to acceptable levels more quickly in the aquaculture pond than in the laboratory. It was suggested that the quality of food available to the mussels during detoxification may affect the rate at which toxins are eliminated (Hallegraeff et al., 1995).
The rate of removal of DSP toxin from shellfish (depuration rate) most likely depends upon the species and may be affected by such interrelated factors as feeding or pumping of the shellfish, temperature, salinity and the level of non-toxic algae and particulates. In Japan, DSP toxins decreased from 4.4 to 2.5 MU/g (by mouse bioassay) in one week and then to 0.5 MU/g by the next week. In the Netherlands, toxicity in mussels was no longer detectable by rat bioassay after four weeks at water temperatures of 14 to 15 °C (Hungerford and Wekell, 1992). At the coast of Sweden (water temperatures 1.4 to 3 °C) after the bloom had subsided, OA levels in mussels decreased in one week from 7.2 to 1.8 mg/g hepatopancreas as measured by LC with fluorescence detection (Edebo et al., 1988b). Except for a method to reduce PSP levels in Mediterranean cockles, there are currently no useful methods available for effectively reducing phycotoxins in contaminated shellfish. All methods tested until now (generally tested for reducing PSP toxins such as transfer of shellfish to waters free of toxic organisms for self-depuration, vertical displacement of mussels in the water column as a means of minimizing toxin accumulation, ozone treatment of the water, temperature or salinity stress, electric shock treatments, reduced pH or chlorination, cooking) appeared to be unsafe, too slow, economically unfeasible or yielded products unacceptable in appearance and taste (Hallegraeff et al., 1995). Only after very rigorous boiling (163 minutes at 100 °C) toxin denaturation occurs (Scoging, 1991).
Mussels (M. galloprovincialis) from Galicia in northwestern Spain contaminated with DSP toxins were transplanted to several uncontaminated sites having different environmental conditions (salinity, temperature, fluorescence, light transmission). The depuration kinetics of OA in each batch was monitored during a 70 day period. Fluorescence and light transmission appeared to have the most prominent effect on depuration. In most cases, there was an inverse relation between depuration and body weight. It could not be clearly concluded whether the DSP depuration evolves following 1- or 2-compartment kinetics (Blanco et al., 1999).
González et al. (2002) reported preliminary results about the instability of free okadaic acid in a supercritical atmosphere of carbon dioxide with acetic acid. Most of the toxin (up to 90 percent) was eliminated and the biological activity against phosphatase was also severely affected (up to 70 percent reduction). Detoxification of contaminated shellfish required a partial dehydration and the detoxification yield was lower than that obtained with the free toxin. Toxin content of partially freeze-dried mussel hepatopancreas containing 1 µg of OA/g was reduced to 51 to 57 percent after 190 minutes of exposure to the supercritical mixture.
3.6.2 Preventive measures
The prevention of shellfish-borne diseases requires monitoring of the marine environment and shellfish flesh. Frequent inspection of seawater around aquaculture facilities or shellfish farms for the presence of toxin producing strains of phytoplankton is an approach that is gaining support in several countries, and has received considerable impetus following the discovery that toxin-producing algae have been transferred in the ballast water of ships to completely new marine locations around the globe (Wright, 1995).
Data on the occurrence, type and concentrations of toxic algal species may indicate which toxins may be expected during periods of algal blooms and which seafood products should be considered for analytical monitoring. One problem is that certain algal species, which have never occurred in a certain area, may suddenly appear and then rapidly cause problems. Nevertheless, several countries have monitoring programmes to check for the occurrence of (toxic) phytoplankton species in areas where shellfish are grown. Some countries monitor the presence of only one or two algal species, while others check for a long list of species. In some countries, the shellfish areas are closed when the number of cells of certain algal species exceeds certain concentrations according to the type of species. Other countries close their harvest areas only when the toxins have been detected in the shellfish. Closure of harvesting areas in Italy occurs when the presence of toxic algae in water and toxins in mussels are observed simultaneously (Hallegraeff et al., 1995).
The principal strategy to prevent DSP intoxication is effective monitoring of mussels with respect to DSP toxins so that contaminated products do not reach the market. However, the presentation above shows that weekly sampling may be insufficient for maximum protection of human health in endemic areas. A reliable sample plan is required in addition to efficient means of detection. However, several factors complicate efficient monitoring (Aune and Yndestad, 1993) including:
cell numbers of toxin producing algae needed to produce toxic mussels vary considerably;
time period of toxicity varies with region and episode;
implicated toxins and algae may be different in different regions;
simultaneous presence of DSP and PSP toxins complicates monitoring;
toxicity of contaminated mussels can vary several fold with different depths even at the same sampling location;
other seafood, like oysters, might also be contaminated, although at a lower level.