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Mystery surrounds French oyster ban

September 21, 2006 By Declan Butler This article courtesy of Nature News.

Shellfish spat highlights problems with generic safety tests.

A ban on the sale of France's famous Arcachon oysters this August provoked angry demonstrations by oyster farmers, who say that there's no way of proving the shellfish were actually toxic. The controversy has thrown a spotlight on the shortcomings of current safety testing.

The oysters were pulled on 31 August when they failed the standard test: mice injected with shellfish juices died. Two people in the area have died. Although an autopsy later showed that one patient died of causes unrelated to shellfish, the other case is still unclear.

No rush of people appeared at clinics with complaints of poisoning, and no toxic algae could be detected in the water. Laurent Rosso, director of food-quality research at the French Food Safety Agency at Maisons-Alfort in the southwest suberbs of Paris, admits that they have no idea what killed the mice. So what's been going on?

Tricky assay

The problem is that it's just a global test.
Zouher Amzil
Ifremer's Phycotoxin laboratory in Nantes
The mouse assay is the current gold standard for testing for the risk of diarrhetic shellfish poisoning, an unpleasant and debilitating illness that can lead to severe dehydration.

In the test, the fatty part of the digestive glands of shellfish are extracted using solvents, the solvents are washed away, and the result is injected into three mice. If two of the three mice die within 24 hours, a ban is placed on human consumption. The ban is lifted after two consecutive tests turn up negative (as happened in this case on 14 September).

But events have highlighted problems with this bioassay. The dead mice don't indicate which toxins are present, or in what concentrations. "It's just a global test," says Zouher Amzil, head of the main phycotoxin laboratory at France's national marine research agency, Ifremer, in Nantes.

And false positives, though rare, can happen. The solvent used to pull material from the oysters can be itself fatal to mice, so if it isn't properly washed from the extract then the mice can die even when the shellfish are clean.

The specifics

For these reasons, the German Federal Institute for Risk Assessment in Berlin, for one, argues that the mouse test should be replaced with more specific chemical tests for known phycotoxins.

"Chemical assays are better than the mouse assay," agrees Russo. He thinks that mouse deaths almost always mean there is a problem, but concedes that the lack of detail can be problematic.

Chemical tests would allow experts to assess the threat to humans better. This in turn would allow authorities to weigh up whether a ban on shellfish sales was warranted.

However, such tests aren't necessarily simple and they haven't been validated for the purpose of keeping people safe from toxins, points out Rosso. "There are no tests that are sufficiently robust and reliable to be used accurately on an operational basis," he says. Some specific bioassays are being developed by a European project called BIOTOXMARIN, for instance. But, says Rosso, "they are still in the research phase".

Unknown unknowns

Other experts, including Amzil, say a mouse test would still be needed to catch any as-yet-unknown toxins.

Cases of such 'mystery' toxins are very rare. They have shown up in other places around the world, but have inevitably been traced back to a toxic plankton not typical of the region, perhaps transported in the ballast water of ships. Amzil expects that a similar explanation will be forthcoming in this case.

Researchers are now scouring the oysters and the water for any signs of toxin. "We will search for what killed the mice, and see if it was a potential risk for man," says Amzil.

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