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In the last week, twenty people have been poisoned by eating shellfish collected from the Bay of Plenty shoreline according to the Bay of Plenty District Health Board. They are calling it the worst outbreak ever seen in the area. Ten people have required hospital treatment and four people remain in hospital. Symptoms ranged from a tingling sensation around the mouth to difficulty walking.

Paralytic shellfish poisoning (PSP) is contracted by eating contaminated shellfish, such as mussels, pipi, tuatua, cockles, oysters, scallops and kina (sea urchin). Shellfish are filter feeders so can accumulate toxins produced by microscopic algae. The toxins responsible for most shellfish poisonings are water-soluble and heat and acid-stable, so aren’t destroyed by ordinary cooking methods. The main toxin responsible for PSP is called saxitoxin, although other related compounds have been reported. Saxitoxin is one of the most potent natural toxins known. It acts on the voltage-gated sodium channels of nerve cells, preventing normal cellular function leading to paralysis.

Saxitoxin structure (from Wikipedia)

PSP can be fatal in extreme cases. Symptoms usually occur within 12 hours of ingestion, and include nausea, vomiting, diarrhea, or abdominal pain; numbness and tingling around the mouth, face or extremities; difficulty swallowing or breathing; dizziness; double vision; and in severe cases, paralysis and respiratory failure.

The advice is not to collect shellfish from Tairua on the east coast of the Coromandel Peninsula, south to Waihi Beach and along the Bay of Plenty coast to Whakatane Heads in the Eastern Bay of Plenty. This includes Tairua Harbour as well as Tauranga Harbour, Maketu and Waihi estuaries, Matakana and Motiti Islands, and all other inshore islands along this coastline.

What I didn’t realise was that many shellfish can store saxitoxin for several weeks after a harmful algal bloom passes. According to wikipedia, butter clams can store the toxin for up to two years. Think I’ll give them a miss!

Saxidomus gigantea – the butter clam. Photo by Dave Cowles, August 2005.