How toxins build up in shellfish
Shellfish with 2 shells (bivalve shellfish), like mussels or oysters, can filter up to 10 to 12 litres of water an hour. That's more than 200 litres a day for each shellfish. These shellfish filter out phytoplankton (microscopic algae) and other particles as a food source. If these algae are toxic, then the toxins can build up in the shellfish and make them poisonous. These toxins can cause serious illness if you eat the shellfish.
If you get sick after eating shellfish
If someone gets sick after eating shellfish, make sure you:
- phone Healthline for advice on 0800 61 11 16, or get medical help immediately
- advise your nearest public health unit
- keep any leftover shellfish in case it can be tested.
Find contact details for your nearest public health unit
Types of shellfish poisoning
There are 4 main types of poisoning in New Zealand.
Paralytic shellfish poisoning (PSP)
Safe limit: Less than 0.8mg/kg
Symptoms of PSP
Symptoms usually appear within 10 minutes to 3 hours of eating. Symptoms may include:
- numbness and a tingling (prickly feeling) around the mouth, face, and extremities (hands and feet)
- difficulty swallowing or breathing
- dizziness and headache
- nausea and vomiting
- diarrhoea
- paralysis and respiratory failure and in severe cases, death.
Causes of PSP
PSP is caused by a group of chemicals called the saxitoxins and gonyautoxins. These chemicals all differ in their toxicity to humans, and their proportions may vary. It depends on the species of shellfish and the species of algae producing the toxin.
Toxic algae of the species Gymnodinium catenatum, Alexandrium minutum and Alexandrium pacificum commonly cause PSP toxicity in New Zealand shellfish.
Amnesic shellfish poisoning (ASP)
Safe limit: Less than 20mg/kg
Symptoms of ASP
Symptoms usually appear within 24 hours of eating. Symptoms may include:
- vomiting
- nausea
- diarrhoea
- abdominal cramps.
Other symptoms of ASP in severe cases
In more severe cases there can be neurological symptoms. These can take up to 3 days to develop and include:
- headache and dizziness
- disorientation and vision disturbances
- loss of short-term memory
- motor weakness
- seizures
- profuse respiratory secretions
- unstable blood pressure
- cardiac arrhythmia
- coma.
Causes of ASP
ASP is caused by a chemical called domoic acid. Symptoms are mainly gastrointestinal, especially at low toxin levels. However, about a quarter of affected people experience neurological problems including memory loss that may be significant and permanent. Symptoms first appear within 24 hours and neurological difficulties within 48 hours. Toxic algae of the Pseudo-nitzchia genus produce domoic acid.
Diarrhetic shellfish poisoning (DSP)
Safe limit: Less than 0.16mg/kg of okadaic acid equivalents
Symptoms of DSP
Symptoms usually appear within 30 minutes of eating and last for about a day. Symptoms may include:
- diarrhoea
- vomiting
- nausea
- abdominal cramps.
Causes of DSP
DSP is caused by okadaic acid and related compounds. DSP group toxins are mainly produced by the Dinophysis genus.
Neurotoxic shellfish poisoning (NSP)
Safe limit: 0.8 mg/kg brevetoxin-2 equivalents
Symptoms of NSP
Symptoms typically appear within 24 hours of eating. Symptoms may include:
- difficulty swallowing
- double vision
- unsteadiness and tremors (shakes)
- nausea and vomiting
- diarrhoea
- numbness, including tingling of the mouth, lips, hands, and feet
- difficulty distinguishing between hot and cold.
Causes of NSP
In NSP cases, the toxins can attack the nervous system. The toxins are brevetoxins or brevetoxin analogs. NSP is generally produced by the Karenia, Karlodinium, or Gymnodinium genus.
Who to contact
If you have questions about toxic shellfish poisoning, email info@mpi.govt.nz
