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Algal Toxins in Shellfish
Microscopic algae form an important
component of the plankton diet of shellfish such as mussels, oysters and scallops. Under
favourable environmental conditions algal populations of only a few cells can quickly
multiply into dense blooms containing millions of cells per litre. Certain species of
dinoflagellates, diatoms and cyanobacteria can produce potent neurological toxins which
can find their way through shellfish to humans. An outbreaks of shellfish toxin poisoning
occurred recently in south-east Queensland via ingestion of contaminated oysters. When
humans eat shellfish contaminated with these microalgae, they may suffer a variety of
gastrointestinal and neurological illnesses. These include paralytic shellfish poisoning
(PSP) which in extreme cases can lead to death through respiratory paralysis, diarrhetic
shellfish poisoning (DSP) which causes severe gastrointestinal problems and can promote
stomach tumours, neurotoxic shellfish poisoning (NSP) which causes respiratory distress,
and amnesic shellfish poisoning (ASP) which can lead to permanent brain damage (short term
memory loss). Problems caused by cyanobacteria poisons include liver damage or PSP. Toxic
cyanobacterial blooms are primarily confined to fresh water and brackish water
environments and would therefore not normally affect marine shellfish aquaculture
operations. The clinical symptoms of varies types of shellfish poisoning are shown in the
following table.
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