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When eaten, poisonous marine animals can make you ill, and may even cause muscular paralysis and death. Use this guide to the poisonous marine animals that can be found in South African seawaters, compiled with the help of leading South African toxicologist Dr Gerbus Müller, before you indulge in sea delicacies this summer.
1. Red tides
Red tides (dinoflagellate blooms) are regular seasonal phenomena that occur on the west coast of South Africa.
These blooms, a type of algae that are food for shellfish, may discolour the sea to various shades of brown, orange, purple, yellow or red.
The majority of blooms are non-toxic, but toxin-producing species, such as Alexandrium catenella and Dinophysis acuminata occur locally. Mussels, oysters and other bivalve shellfish strain these from the water, digest them, and accumulate their poison.
The shellfish do not seem to be harmed by the poison, but are toxic when eaten. Shellfish affected by red tides can be extremely poisonous. It may cause paralysis and has been known to kill humans.
South Africans should take note of three different types of shellfish poisoning:
a. Paralytic shellfish poisoning
Eating mussels that contain the poison saxitoxin, produced by Alexandrium catenella, causes paralytic shellfish poisoning.
Signs and symptoms: Symptoms occur within 30 minutes to two hours after eating the food.
These include a tingling sensation or numbness around the lips (which spreads to the rest of the face and neck), a prickly sensation in fingertips and toes, headache, dizziness, a floating or gliding sensation, vertigo, visual disturbances and a weakness of the legs.
In severe cases, progressive muscular paralysis, with pronounced breathing difficulty, develops. Death due to respiratory failure may occur within 2-24 hours after eating affected food.
Treatment: Symptoms and signs normally clear within 36-48 hours. Luckily, the toxic effect is completely reversible
Treatment is symptomatic and supportive.
a. Diarrhetic shellfish poisoning
This type of poisoning is caused by okadeic acid, which is produced by Dinophysis acuminata.
Signs and symptoms: Symptoms usually develop within four hours after eating a meal.
These include nausea, vomiting, diarrhoea, abdominal pain and rigors.
Treatment: People usually recover within three days of eating the affected food.
Treatment is symptomatic. Pay particular attention to fluid retention by drinking copious amounts of fluids.
a. Neurotoxic shellfish poisoning
This type of poisoning is caused by Gymnodinium species.
Signs and symptoms: Exposure, usually in the surf close to the beach, causes irritation of the eyes, nose and throat with coughing, sneezing and difficulty breathing.
It may lead to asthma attacks in certain people.
Treatment: Treatment is symptomatic and supportive.
How to prevent poisoning: Be aware of red tide warnings and never eat shellfish that originated from the affected area during this time. Red tides usually occur during the late summer and early autumn, but may occur at any time of the year.
An obvious sign to look out for is discolouration of the water.
Check the safety of picking your own mussels (and other bivalve shellfish) by dialling the Red Tide Alert desk in Cape Town on (021) 434 4457.
2. Scombroid poisoning
Scombroid poisoning is caused by the consumption of fish that has undergone autolytic changes which increases its histamine content.
These changes may be tied to the food chain of the fish itself. However, the exact mechanism is unclear.
This is a problem that occurs from time to time, particularly in Cape yellowtail, although tuna and mackerel have also been implicated. Several incidents of scombroid poisoning have occurred in South Africa and all over the world in recent years.
It is not necessarily linked to breaks in the cold chain or improper storage conditions.
Signs and symptoms of poisoning: The onset of the illness occurs rapidly, usually within minutes after the affected food is ingested.
Symptoms include a hot, blotchy flushing of the skin, gastrointestinal symptoms, such as vomiting and diarrhoea, mouth sensations and palpitations. These symptoms seldom last for more than six hours.
Most cases of scombroid poisoning are mild and not life-threatening. The illness can be more serious in patients suffering from heart disease or asthma, or in elderly people.
Treatment: Seek medical attention. Your doctor will prescribe antihistamine treatment.
How to prevent poisoning: Avoid eating Cape yellowtail if you suffer from heart disease or asthma.
Unfortunately, there is no way of telling whether a helping of fish is affected just by looking at it. Although there have been reports that affected fish tastes "peppery", "pungent" or "bitter", this is not an accurate measure to go by.
- (Carine Visagie, Health24, updated December 2008)
