Toxic Algal Blooms

Phytoplankton or microalgae are simple marine plants that form the primary food source for many marine bivalve molluscs. However, of the 5,000+ species of phytoplankton that exist worldwide about 2% are known to be harmful or toxic (Source: Wikipedia). Where these toxin producing algae occur in large numbers then they are often referred to as Harmful Algal Blooms or HABs. Certain species of phytoplankton are associated with producing biotoxins that can be the cause of shellfish poisonings in consumers. Biotoxins implicated in shellfish poisonings include the following:

• Okadaic Acid: Produced by the dinoflagellate Dinophysis, this biotoxin is responsible for causing Diarrhetic Shellfish Poisoning (DSP).
• Saxitoxin: Produced by the dinoflagellate Alexandrium, this biotoxin is responsible for causing Paralytic Shellfish Poisoning (PSP).
• Domoic Acid: Produced by the pennate diatom Pseudo-nitzschia, this biotoxin is responsible for causing Amnesic Shellfish Poisoning (ASP).

Due to the potentially serious health impacts that some of these toxin producing phytoplankton can have there is a monitoring programme carried out around England and Wales by Cefas on behalf of the Food Standards Agency (FSA). This monitoring programme consists of a weekly analysis of seawater samples for the presence of these phytoplankton. This monitoring is currently a requirement of EC Regulation 854/2004.

Where an alert level is recorded in a shellfish production area for these biotoxin producing phytoplankton then a Temporary Closure Notice and/or Warning Notices will be issued by the Local Enforcement Authority.  These will remain in place until the waters and shellfish are found to be free of both the phytoplankton and associated biotoxins.

It should be noted that the FSA state that, whilst an official monitoring programme exists, it is the responsibility of Food Business Operators (FBOs) to ensure that the shellfish placed on the market do not contain biotoxins above permitted levels.  In practical terms, for a shellfish farm there is little that can be done whilst a HAB is present other than avoiding harvesting until the harvesting water and shellfish are given the all clear.

In terms of testing, simple test kits are now available to industry which can detect the main biotoxins associated with these types of shellfish poisoning incidents. Over time FBOs involved with shellfish production will gain experience of their production areas and will build an understanding of the times of highest risk when HABs are likely to occur. With this knowledge FBOs can then increase testing at times of highest risk as part of their due diligence and in order to protect consumers.

For further information on the official monitoring programme see the following weblink: