Science Centres: Aquatic Biodiversity and Biosecurity

A New Zealand woman has died in Thailand after eating what was described as ‘toxic seaweed’. Some media reports have suggested the cause was ciguatera. What is it and what are the risks?

Is ciguatera a seaweed toxin?

Ciguatera is caused by a number of species of toxic dinoflagellates, which can attach themselves to seaweed but are generally found in higher concentrations in fish, especially predator fish.

Dinoflagellates are microscopic, single-celled algae. There are at least several thousand different species worldwide. They have characteristics of both plants and animals; for example, about half of them can photosynthesise and some of these are also predators. When conditions are right, they multiply rapidly causing algal blooms. However, only a very few species are neurotoxic.

What foods can cause ciguatera?

The ciguatera dinoflagellates live on the surfaces of seaweed and corals where they are eaten by small, herbivorous fish. They, in turn, are eaten by larger fish, and the toxin from the dinoflagellate accumulates as this process repeats. So, serious ciguatera poisoning is more likely from eating predator fish high up the food chain than from eating seaweed.

Ciguatera is not caused by eating shellfish, although other types of toxic dinoflagellates can accumulate in shellfish.

Where in the world does ciguatera occur?

Tropical and sub-tropical regions,in Australia as well as in the Pacific and Asia. It can be a major problem for the South Pacific region, particularly in French Polynesia, New Caledonia and Tuvalu.

A paper published in 1987 estimated that worldwide as many as 10-50 thousand people may get ciguatera seafood poisoning each year.

Dr Hoe Chang of NIWA has found one of the dinoflagellate species known to cause ciguatera poisoning on the northeast coast of New Zealand and in the Wellington region, but this species, Ostreopsis siamensis, is generally associated with mild symptoms. A more toxic species, Gambierdiscus toxicus, has been reported in New Zealand, but its presence has yet to be confirmed.

It is important to remember that the illness generally occurs in tropical and subtropical countries, and is not often fatal.

Is it new?

No. Ciguatera seafood poisoning has been known for centuries.

How can I avoid getting ciguatera poisoning?

Unfortunately, it is one of those rare things that is hard to avoid in tropical and subtropical countries, unless you don’t eat seafood, because cooking does not kill ciguatera. It is unlikely that you would get even a mild dose of ciguatera poisoning from eating seafood in New Zealand.

Is seaweed itself toxic?

No.

For media comment

Dr Hoe Chang, NIWA Scientist (expert on toxic dinoflagellates): 04 386 0360
Dr Wendy Nelson, NIWA Principal Scientist (expert on seaweeds): 04 386 0600