The risks of Domoic acid poisoning in humans.

Domoic acid (DA) is a biotoxin produced by the marine diatom from the genera Pseudo-nitzschia.  It is known to cause amnesic shellfish poisoning in humans, it has neurological and gastrointestinal effects but may also result in seizure or coma.  It is ingested through consumption of shellfish, which are known to accumulate the toxin for longer periods of time than other fish or shellfish.  Previous studies on scallops have found the toxin to be present in levels exceeding the regulatory limit of 20 μg/kg in up to 17% of tested samples within the EU. 

Although there are measures in place to limit consumption by humans it is unknown what effect low levels of this toxin may have, if it will build up over time or even if it could be passed from mothers to their unborn children as has been seen in rats.  Although the tolerable daily intake (TDI) for humans is set at 0.075 ppm there have been no apparent clinical effects found for levels of less than 1.0 ppm.  That being said, other edible marine species such as razor clams and crabs have TDI’s of 19.4 and 31.5 ppm respectively.  Should these be accidentally consumed they could have dire consequences for humans.

A team in Belgium have analysed shellfish samples and data obtained from national food agencies to try to determine the actual risks to humans from eating scallops and oysters in particular. From their sample of shellfish (Oysters, Mussels and scallops), they discovered 22% of their scallops had DA levels higher than the regulatory limits, this was the highest percentage of all shellfish.  By extrapolating this data they calculated that actually less than 1% of the Belgian population would be at risk from amnesic shellfish poisoning due to eating shellfish with levels of DA that exceeded the acute dose. 

There are fairly stringent procedures in place to ensure acute exposure is limited however the effects of long term exposure are currently unknown.  It is also not known if cooking definitely reduces concentrations of DA in all species and this is certainly an area that could be fairly easily evaluated.  The study also highlighted the fact that long term exposure could actually affect as many as 5% of the population and confirmed that certain groups of the population such as pregnant women, young children or those with any form of reduced liver function could be at greater risk of poisoning. 

Given that DA poisoning can not only effect humans but also some marine mammals and birds it is certain that research into accumulation of DA and the way it may move through the food chain is essential.  There have been fairly recent advances in ways to detect the poison using enzyme-linked immunosorbent assay (ELISA), which now means that testing is much cheaper than it has been previously.  Currently there seem to be only isolated outbreaks amnesic shellfish poisoning but going forward it will be important to discover what effects environmental changes such as global warming may have on both the diatoms themselves and also on the species that accumulate the poison.  We must also consider the fact that as availability of fish populations changes shellfish may grow in popularity which would again increase the risk of outbreaks and isolated cases.

Andjelkovic, M., Vandevijvere, S., Van Klaveren, J., Van Oyen, H., & Van Loco, J. (2012). Exposure to domoic acid through shellfish consumption in Belgium. Environment international, 49, 115-119.