291
5.3.1 Paralytic Shellfish Poisoning (PSP)
PSP is caused by eating bivalves that contain saxitoxin (STX) and its derivatives.
STX and its analogues are synthesized by Alexandrium tamarense and Alexandrium
cantenella dinoflagellates in both marine and freshwater environments (Faber
2012). The toxins, a group of about 20, are bacterial catabolites in dinoflagellate
cells of the genera, Alexandrium, Gymnodinium and Pyrodinium. The toxins produced by these organisms are the most common and widespread of the shellfish
toxins and are among the most potent neurotoxins known. They act by blocking the
passage of sodium ions through cell membranes, thus inhibiting nerve impulse
transmission (Gosling 2015). The basic effective toxin is saxitoxin (Pomati et al.
2006). Saxitoxin is an inhibitory neurotoxin that induces flaccid paralysis by
actively suppressing excitation of neuronal impulses. In humans, PSP may develop
after consuming contaminated raw or cooked shellfish containing high concentrations of STX. Since Saxitoxin is a heat resistant and very strong neurotoxin, it can
cause adverse effects in humans even at low doses. There is no antidote and detoxification pathway either (Faber 2012).
Bivalve shellfish such as mussels, clams, oysters, and scallops pose a particularly
high risk as they feed by filtration providing toxin accumulation (Turnbull et al.
2013). During the process of filtration, the dinoflagellate cells and cysts are transported to the oesophagus and the stomach of the bivalve molluscs. The digestion
takes place in the stomach and the diverticula whereby the PSP toxins are released
and enter the digestive organs. The particular toxin mixture retained in soft tissues
of the shellfish varies in concentration and over time and is determined by the species and strains of the dinoflagellates and shellfish as well as by other factors like
environmental conditions (FAO 2004). While the toxin is found quite intensely in
the digestive glands in clams, its concentration in the gills, ovaries and adductor
muscles is less than 80 μg/100 g. Since toxin is not easily found in the adductor
muscles in oysters, even if they are exposed to toxic algae, these parts are not dangerous for public health (Cambella et al. 1993). In a study with mussels, Kwong
et al. (2006) listed the accumulation of toxins in organs as hepatopancreas, internal
organs, gills, legs, and adductor muscles. It has been reported that different types of
shellfish accumulate toxins at different rate, even in individuals of the same species
and even under the same environmental conditions, it is thought that these differences may result from the difference in shell size (Setala et al. 2014; Morono
et al. 2001).
Symptoms of PSP can include tingling, numbness, or burning in the extremities
or mouth; a lack of coordination; drowsiness; fever; and rash. Individuals may also
experience nausea, vomiting and diarrhoea. In severe cases, PSP can result in respiratory arrest and death.
Saxitoxin cannot be removed from shellfish by cooking, freezing, or other postharvesting processes. Toxins do not alter the taste, smell, or appearance of shellfish.
The most obvious detoxifying method for shellfish contaminated with PSP is to
transfer the shellfish to waters free from toxic organisms. Although this is a suitable
5.3 Biotoxins
5.3.1 Paralytic Shellfish Poisoning (PSP)
PSP is caused by eating bivalves that contain saxitoxin (STX) and its derivatives.
STX and its analogues are synthesized by Alexandrium tamarense and Alexandrium
cantenella dinoflagellates in both marine and freshwater environments (Faber
2012). The toxins, a group of about 20, are bacterial catabolites in dinoflagellate
cells of the genera, Alexandrium, Gymnodinium and Pyrodinium. The toxins produced by these organisms are the most common and widespread of the shellfish
toxins and are among the most potent neurotoxins known. They act by blocking the
passage of sodium ions through cell membranes, thus inhibiting nerve impulse
transmission (Gosling 2015). The basic effective toxin is saxitoxin (Pomati et al.
2006). Saxitoxin is an inhibitory neurotoxin that induces flaccid paralysis by
actively suppressing excitation of neuronal impulses. In humans, PSP may develop
after consuming contaminated raw or cooked shellfish containing high concentrations of STX. Since Saxitoxin is a heat resistant and very strong neurotoxin, it can
cause adverse effects in humans even at low doses. There is no antidote and detoxification pathway either (Faber 2012).
Bivalve shellfish such as mussels, clams, oysters, and scallops pose a particularly
high risk as they feed by filtration providing toxin accumulation (Turnbull et al.
2013). During the process of filtration, the dinoflagellate cells and cysts are transported to the oesophagus and the stomach of the bivalve molluscs. The digestion
takes place in the stomach and the diverticula whereby the PSP toxins are released
and enter the digestive organs. The particular toxin mixture retained in soft tissues
of the shellfish varies in concentration and over time and is determined by the species and strains of the dinoflagellates and shellfish as well as by other factors like
environmental conditions (FAO 2004). While the toxin is found quite intensely in
the digestive glands in clams, its concentration in the gills, ovaries and adductor
muscles is less than 80 μg/100 g. Since toxin is not easily found in the adductor
muscles in oysters, even if they are exposed to toxic algae, these parts are not dangerous for public health (Cambella et al. 1993). In a study with mussels, Kwong
et al. (2006) listed the accumulation of toxins in organs as hepatopancreas, internal
organs, gills, legs, and adductor muscles. It has been reported that different types of
shellfish accumulate toxins at different rate, even in individuals of the same species
and even under the same environmental conditions, it is thought that these differences may result from the difference in shell size (Setala et al. 2014; Morono
et al. 2001).
Symptoms of PSP can include tingling, numbness, or burning in the extremities
or mouth; a lack of coordination; drowsiness; fever; and rash. Individuals may also
experience nausea, vomiting and diarrhoea. In severe cases, PSP can result in respiratory arrest and death.
Saxitoxin cannot be removed from shellfish by cooking, freezing, or other postharvesting processes. Toxins do not alter the taste, smell, or appearance of shellfish.
The most obvious detoxifying method for shellfish contaminated with PSP is to
transfer the shellfish to waters free from toxic organisms. Although this is a suitable
5.3 Biotoxins