2 February 1993 Fish and Algal Toxins - An Ever-Present Hazard

The occurrence of poisons in shellfish and the presence of toxin-producing algae are not new phenomena. Lincoln University toxicologist Dr Ravi Gooneratne describes some hazardous substances of the aquatic world.

Very little research on fish and algal toxins has been done in New Zealand and it is taking some time to identify the exact organism responsible for the current contamination of shellfish. It would be easier in other countries (eg. Britain, Europe, Scandinavia, USA, Canada), where toxic-producing algae have been studied for many years.

Marine organisms produce many toxic substances, but only a few are involved in human food poisoning.

Many different animals have evolved with the capability of producing toxins to help them survive. They use toxins either to obtain food by killing or paralyzing prey, or as a defence against predators. However, in the case of shellfish (clams, mussels, crabs, lobsters etc.) toxins are not produced by the creatures themselves. It is a food they eat, a type of algae called dinoflagellates, which contains the toxin and the shellfish concentrate the poison when they feed on these.

Although the recent cases of shellfish poisoning have been attributed somewhat glibly to pollution, this is an over-simplification of the problem.  Many other factors have contributed to the recent outbreak.

Algae produce more toxins in warm weather and the presence of nutrients favourable to their growth.

Last year saw a series of events which benefitted the development of toxic algae in coastal waters. The 1992 winter was unusually severe. After a cold, unusually wet October and November, summer temperatures in recent weeks have been normal or above average. This weather pattern combined with the availability of nutrients such as phosphates, nitrates and other pollutants in run off from coastal lands, favoured the rapid reproduction of toxin-producing algae, thus creating a "bloom" or high concentration of algal organisms.

It is also possible that in recent years some toxic dinoflagellates may have been discharged into New Zealand waters with ballast from visiting ships.

Some blooms are red, known as "red tide" or "red water", the colour being due to a pigment called peridinin. Shellfish feeding in "red water" can accumulate the toxin with apparently no toxic effect to themselves, but it is quite toxic to the humans who eat them.

Several toxins are associated with shellfish poisoning. The most important one is a nerve toxin called saxitoxin. This toxin dissolves in water and is resistant to heat. The minimum dose of saxitoxin needed to kill a human is a few milligrams, sufficient to cover a pin head. Based on bodyweight, humans are twice as susceptible to this poison as dogs and four times more susceptible than mice. Mild poisoning can be caused by just 1 milligram of the toxin, the amount found in one to five poisonous mussels or clams.

It is the saxitoxin family of poisons which is suspected of being responsible for the current problem.

The effects of this toxin in humans include a tingling feeling and numbness in the lips, face and neck, nausea, vomiting, diarrhoea, and difficulty in walking due to an effect on the muscles. The symptoms may appear within minutes after eating. Death has very occasionally been reported overseas from paralysis of the muscles used for breathing.

Another toxin produced by dinoflagellate type algae and concentrated in the food chain by over 400 species of fish is ciguatoxin. Since these fish are normally edible, and some are valuable food dishes, it is a common form of food poisoning in many parts of the world, particularly the Pacific area and the Caribbean.

Ciguatoxin, like the saxitoxin in shellfish, is a nerve toxin, so people eating contaminated fish develop symptoms like those in shellfish poisoning plus, sometimes, strange symptoms such as a feeling of loose teeth in the mouth.

Climatic conditions in recent weeks may also have favoured growth of blue-green algae, a fresh water algae, which can also produce toxins.

Blue-green algae is not causing the current problems.

Blue-green algae usually multiply sufficiently in summer months and colour the water blue or green. Mammal, bird, amphibian and fish deaths thought to be due to eating blue-green algae have been reported worldwide for over a century. These have resulted either from the animals drinking water containing the toxin or licking algal material from their skins after swimming in the water.

If a blue-green algal bloom is discovered there is a good chance it is toxic. Although many animal deaths have been reported there are no confirmed reports of humans dying directly from the toxins produced by blue-green algae. There are, however, many reports of human illness associated with the presence of toxic blue-green algae, particularly in North America, Australia, Europe, and the United Kingdom. The symptoms of blue-green algae poisoning include skin rashes, eye irritation, vomiting, diarrhoea, fever and pains in muscles and joints.

There are several other toxins produced by marine organisms including fish. Over 700 species of marine fish are known to be toxic. Poisonous puffer fish, for example, account for over two­ thirds of all food poisoning cases in Japan. In that country puffer fish are called "fugu" and are a delicacy, eaten raw. These fish concentrate a toxin (tetradotoxin) in the eggs and liver. Human poisoning frequently occurs from contamination of the edible parts of the fish by toxic eggs or liver.

Tetradotoxin, like saxitoxin in shellfish, is a nerve toxin and results in paralysis. Death may occur due to respiratory failure. Tetradotoxin is also found in some salamanders and the blue­ ringed octopus.

There are lessons to be learnt from the current poisoning episode. It could be a trigger for the authorities to develop a toxin monitoring programme to safeguard our increasing export fish market.

Toxicology research and teaching needs to expand and receive more support if New Zealand is to maintain its "clean green" image and boost its exporting of aquaculture and fisheries products.

The current problem with shellfish underlines the vulnerability of our primary industries to disease and the quick reaction of overseas markets to any hint of health problems.

Dr Gooneratne holds toxicology qualifications from the University of Saskatchewan, Canada, and was the New Zealand representative at the first Aquatic Toxicology Workshop held at the University of Wisconsin, USA, in 1991. Last year, funded by the British Council, he studied the teaching of toxicology in the United Kingdom as an aid to his development of complementary programmes at Lincoln.

Dr Ravi Gooneratne, Lincoln University, Canterbury, New Zealand.

Ian Collins, Journalist, Lincoln University, Canterbury, New Zealand.