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Food Safety Focus (70th Issue, May 2012) – Food Safety Platform

From Algae to Paralytic Shellfish Poisoning

Reported by Mr. Arthur YAU, Scientific Officer,
Risk Assessment Section,
Centre for Food Safety

This is the second article in a series of three that focus on marine toxins originating from minute organisms (e.g. planktons) which can affect seafood safety.

Algae or Shellfish?

The paralytic shellfish poisoning (PSP) toxins are actually produced not by the shellfish but by certain species of microscopic algae known as dinoflagellates. The toxins were first noticed following food poisoning after consumption of shellfish. These algae are found in the coastal and estuarine waters across broad latitudes, ranging from the tropics to the much cooler areas like Alaska. Under favourable condition, the algae can grow profusely. Such phenomena may turn the sea reddish or brownish and is called harmful algal bloom (HAB), also commonly known as red tide.

The PSP toxins are a group of 21 structurally closely related chemicals. They can accumulate in various parts of shellfish, especially in digestive and reproductive glands. When consumed by human, the toxins affect sodium channels in the nervous system, giving rise to symptoms like tingling, numbness, burning sensation near the mouth, fever, rash and staggering within minutes to hours. The toxins also give gastrointestinal symptoms. In severe cases, the symptoms may progress to muscle paralysis, respiratory arrest and even death. However, most people recover fully in a few days.

PSP producing algae (circled) can grow rapidly under favourable weather and water condition. Shellfish filter feed on them and accumulate the PSP toxins in their bodies, which may eventually cause human poisoning.
PSP producing algae (circled) can grow rapidly under favourable weather and water condition. Shellfish filter feed on them and accumulate the PSP toxins in their bodies, which may eventually cause human poisoning.

PSP is Preventable

The number of PSP poisoning cases seems to be on the rise in the past decades internationally, but it is unclear whether it is due to heightened awareness, improved surveillance, expanded shellfish consumption or due to real increase. In Hong Kong, from 2007 to 2011, the Centre for Health Protection recorded 34 suspected shellfish toxins-related food poisoning affecting 68 persons. Among them, 14 outbreaks reported in May 2010, involving 29 persons, were found to be associated with consumption of fresh scallops. An epidemiological study conducted at that time also showed that persons who consumed the gonad of the scallops had a higher risk to develop the symptoms.

The chance of PSP poisoning can be much reduced if one avoids consuming the digestive and reproductive glands where much of the toxins concentrate. All organs of bivalves should be discarded and only the adductor muscles should be consumed when possible. The cooking liquid, especially of steamed fan shells, should also be abandoned to reduce the amount of PSP toxins present. One should always buy shellfish from reliable sources, and scrub and clean the shells thoroughly before cooking. Overindulgence in shellfish should be avoided.

Algal Bloom Watch and PSP Surveillance

Like many other food authorities, Hong Kong has put in place monitoring and management programmes to safeguard shellfish safety. On a regular basis, the Centre for Food Safety (CFS) conducts surveillance in food. For harmful algae occurring in Hong Kong waters, the Government has established a Red Tide / HAB Management Operation Master Plan with a view to minimising the possible impacts of HAB on marine fish culture activities and human health. Under the framework, the Agriculture, Fisheries and Conservation Department informs various Departments including the Food and Environmental Hygiene Department when suspected harmful phytoplanktons are found in local waters. The CFS will take shellfish samples from markets near the areas where higher number of toxic or potentially toxic planktons are found for shellfish toxins analysis. From 2009 to 2011, 25 such extra shellfish samples were taken for PSP analysis and all results were satisfactory.

For the trade, when sourcing shellfish, source from countries with established monitoring programmes for toxic algae and avoid areas that have recently been affected by PSP toxins. Traders should also be mindful to meet the Food Safety Ordinance requirement regarding keeping relevant transaction records for tracing the source.

In the next issue, we shall talk about other shellfish poisoning toxins.