Fish Microbiology and Quality Assurance

6.4.1. Shellfish toxins
Filter feeding bivalve mollusks become toxic by feeding on blooms of toxic algae (dinoflagellates). Though > 4000 species of marine phytoplation are known, only about 2% produce toxins. Important shellfish associated illness are,

• Paralytic shellfish poisoning (PSP)
• Diarrhetic shellfish poisoning (DSP)
• Neurotic shellfish poisoning (NSP)
• Amnesic shellfish poisoning (ASP)

Paralytic shellfish poisoning (PSP)

• PSP is caused by eating toxic filter feeding molluscs such as clams, oysters, mussels and cockles.
• Bivalve molluscs become toxic by feeding on certain toxin dinoflagellates which from bloom (> 106 cells/L) and discolor water (red tide).
• The dinoflagellates (toxic) involved are species of Alexandrium, Gonyaulax, Gymnodinium, Pyrodinium etc.
• Toxin associated with PSP is saxitixin which is highly toxic and fatal in small doses (1-4 mg). This toxin is also resistant to heat and acid, and has no antidote. Generally gets destroyed when heated for 3-4 hours at pH 3.
• Symptoms of illness develop within 2 hours after ingestion of toxic molluscs.
• Toxin affects respiratory and cardiovascular regulating center and death occurs due to respiratory failure. Mortality rate is 1-22%.

Symptoms
Paresthesia (tingling, numbness and burning) which begins in mouth, lips, tongue and later spreads over face, scalp and neck, fingertips and toes.
Safe limits
Max allowable limit of PSP toxin in shellfish is 80 µg/ 100 g.
Prevention

• Avoiding eating toxin containing shellfishes.
• Regular monitoring of shellfish growing and harvesting water for toxic algae and toxin levels in shellfish.
• Detoxification of bivalves by natural depuration process or by keeping in clean water.

Diarrhetic shellfish poisoning (DSP)
DSP is caused by eating bivalve molluscs which have accumulated toxin by feeding on bloom of dinoflagellates such as Dinophysis and Procentrum. Toxin responsible for DSP is okadaic acid and its derivatives.
Symptoms
Acute diarrhea with vomiting and abdominal pains. Victims recover within 3-4 days. No mortalities have been reported. Tolerance level is 20 µg/100g.

Neurotoxic shellfish poisoning (NSP)
NSP is caused by consuming shellfish that have been exposed to toxic dinoflagellate bloom of Ptychodiscus breve (Gmnodinium breve). Toxin responsible for NSP is a family of brevitoxins which are llipophilic, relatively insoluble in water and soluble in non-aqueous solvents.
Symptoms
Resemble those of PSP except paralysis. It is not fatal and causes neurological symptoms.
Tolerance level: 20 mu /100g (MU: mouse units).

Amnesic shellfish poising (ASP)
ASP is caused by consuming bivalve molluscs that have accumulated toxin by feeding on toxic diatom, Nitschia. This is the only shellfish poisoning caused by diatom. Toxin responsible is domoic acid.
Symptoms
Symptoms vary from slight nausea and vomiting to loss of balance, neurological disturbance resulting in confusion and short term memory loss.
Tolerance level (as domoic acid)is 20mg/kg