Marine Biotoxins
Blooms of planktonic algae, or 'red tides' have been noted since biblical times. Most of these blooms appear to be harmless and merely colour the water, whilst other species can produce toxins that have a serious effect on the surrounding marine life. Blooms that occur in areas where fish and shellfish are farmed, can cause serious economic losses. This can come about in several ways:
- Some algae are capable of secreting compounds into the water that are actually toxic to the fish. Species such as Heterosigma akashiwo and Cryptosporidium parvum are two such algae that cause large and recurrent fish mortalities. Dense algal blooms, either toxic or non-toxic, can cause oxygen depletion in the water that can cause fish kills. Certain diatoms can cause physical obstructions that clog the gills of fish and shellfish, either by producing a mucous-like substance or by penetrating gill membranes.
- There are around 2,000 species of dinoflagellates in the world, many of which produce potent neurotoxins that are capable of producing poisonings in humans that consume products contaminated with them.
Ciguatera Poisoning
Ciguatera poisoning in humans and domestic animals is caused by potent neurotoxins thought to be produced by benthic dinoflagellates including Gambierdiscus toxicus. These toxins accumulate through the food chain, from small fish grazing on algae on coral reefs into the organs of larger top-order predators such as coral trout, red bass, chinaman fish, mackerels and moray eels.
Over 400 non-lethal cases of ciguatera food poisoning have been recorded in Australia and tropical countries such as French Polynesia report thousands of cases every year.
Symptoms in Humans
- Mild cases: diarrhoea, abdominal pain, nausea and vomiting.
- Extreme cases: numbness and tingling sensation in the extremities. Reversal of hot and cold sensations, low heart rate and blood pressure, rashes. Death through respiratory failure.
Treatment
- See your local doctor as soon as possible.
Prevention
- Avoid eating the species mentioned above. Treat oversize fish with suspicion.
- Avoid eating viscera of any tropical fish species.
Paralytic Shellfish Poisoning (PSP)
Paralytic shellfish poisoning is probably the best known of all the shellfish poisonings, with one of the first cases occurring in 1793 in British Columbia, Canada and over 100 reported deaths and several thousand illnesses being attributed to PSP around the world.
Around 20 species of dinoflagellates have been implicated as producing the alkyloid toxin saxitoxin, that accumulates silently in shellfish. Saxitoxin, actually a group of 18 toxins, is a potent neuromuscular blocking agent that finds its way through shellfish to consumers including man.
A few of the dinoflagellate species that are known to produce this toxin are Gymnodinium catenatum, Alexandrium catenella, A. minutum, A. tamarense and Alexandrium. Most of these dinoflagellates have not been shown to be directly toxic to fish or shellfish.
Symptoms in Humans
- Mild cases: Tingling or numbness around lips, spreading to face and neck. Headache, nausea, vomiting and diarrhoea.
- Extreme cases: Muscular paralysis, respiratory difficulty and death may occur due to respiratory paralysis two to twenty four hours after ingestion.
Treatment
- See your local doctor as soon as possible.
Prevention
- Avoid eating shellfish in areas where 'red tides 'are known to occur. Cooking, and discarding the cooking fluids afterwards, diminishes the amount of poison ingested. If there is any possibility of poisoning, the shellfish should be tested by the public health laboratories.
- Affected shellfish cannot be identified visually. If in doubt, don't eat it.
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| Paralytic Shellfish Poisons |
Diarrhetic Shellfish Poisoning (DSP)
Diarrhetic shellfish poisoning was first recognised in 1976 in Japan. Between the years of 1976 and 1982 there were more than 1,300 diagnosed cases in Japan, with the armoured dinoflagellates Dinophysis fortii and D. acuminata being the algae responsible.
Ingestion of shellfish contaminated with the diarrhetic shellfish toxins (akodoic acid, dinophysistoxins and pectenotoxins) causes severe gastrointestinal disturbances in humans. Although no mortalities have been reported, the symptoms can last for two to three days.
Symptoms in Humans
- Mild cases: after 30 minutes to a few hours, diarrhoea, nausea, vomiting and abdominal pain.
- Extreme cases: chronic exposure can result in tumours of the stomach.
Treatment
- See your local doctor as soon as possible.
Prevention
- As for Paralytic Shellfish Poisoning
Amnesic Shellfish Poisoning (ASP)
Amnesic shellfish poisoning was first recognised in 1987 in Prince Edward Island, Canada where it was the cause of three deaths and 105 cases of acute human poisoning. Although the problem does not appear to have reached Australia to date, other strains of the diatom responsible have been identified in Australian waters.
The amnesic shellfish toxin, domoic acid, is produced by the diatoms Nitzschia pungens f. multiseries and Nitzschia pseudodelicatissima which accumulates in shellfish and affects consumers of such shellfish.
High performance liquid chromatography or mouse bioassay techniques can be used to detect the toxin and shellfish containing more than 20 parts per million domoic acid are considered unfit for human consumption.
Symptoms in Humans
- Mild cases: after three to five hours, nausea, vomiting, diarrhoea and abdominal cramps.
- Extreme cases: decreased sensitivity to deep pain, dizziness, hallucinations, confusion, short-term memory loss and seizures.
Treatment
- See your local doctor as soon as possible.
Prevention
- As for Paralytic Shellfish Poisoning
Gill Damage and Irritation
Certain species of algae can seriously harm fish and shellfish by producing mucus which can clog the gills and cause suffocation, or mechanically obstruct and damage the gills. Several species of diatoms within the genus Thalassiosira form gelatinous masses that have been noted as clogging the gills of farmed oysters in Japan, and gelatinous Thalassiosira blooms have been seen in Australia in New South Wales and the Gulf of Carpentaria. One prymnesiophyte, Phaeocystis pouchetii, as well as producing mucus that can foul fishing nets, also produces acrylic acid that is highly irritant to fish gills.
The diatom Chaetocerus convolutus, although being non-toxic to fish, can cause fish deaths when its setae break off and penetrate the gill membranes. Small fish are affected first and death is caused by capillary haemorrhage, suffocation from over-production of mucus and possibly from secondary infections. The setae can also induce multiple granulomas in more chronic cases.
Algae that kill fish
Blooms of unicellular plankton algae have been associated with fish kills since biblical times. These blooms can become so dense as to colour the water a reddish hue ('red tides').
Most of these blooms appear to be completely harmless (e.g. Trichodesmium, Noctiluca and Scrippsiella), unless the cells become so concentrated that they cause fish deaths by the depletion of oxygen in the water. Algal blooms produce oxygen during the day but consume it at night, so affected fish often die in the early hours of the morning.
The decay process of a dying bloom, as well as depleting oxygen by bacterial respiration, can produce ammonia, sulphides and other toxins harmful to fish.
Other algae can actually produce compounds that are lethal to fish and shellfish. These include species such as Gyrodinium aureolum, Gymnodinium spp., Chrysochromulina polylepis, Cryptosporidium parvum and Heterosigma akashiwo. Heterosigma akashiwo is a chloromonad that has caused kills of thousands of caged fish in Japan, Canada, Scotland and Ireland.
The exact mechanism with which H. akashiwo kills fish is not known, but copious amounts of mucus occur on the gills, probably produced by the alga. In some cases hyperplasia of the gill epithelial surfaces was noted, however in most cases, no obvious histological problems were observed.
Cryptosporidium parvum, a small golden-brown flagellate, secretes its toxin into the surrounding water and is the case of large recurrent fish mortalities in Europe and the Middle East. These toxins affect cell permeability, leading to osmotic imbalances. The toxicity of C. parvum is promoted by phosphorus deficiency.
Taints and Flavours
Some algae and diatoms have the ability to impart off-flavours or bitter taints to shellfish, rendering them unpalatable.
In 1987 in Port Phillip Bay, Melbourne, a bloom of the diatom Rhizosolenia chunii occurred and 3 species of shellfish within the bay (mussels, flat oysters and scallops) developed a powerful bitter taint. The taint was so persistent and unpleasant that the mussels from the bay were unmarketable for 7 months, causing a loss of approximately $1 million.
A similar effect was reported in 1987 in Alaska where 'bitter crab disease' was described in Alaskan tanner crabs. This was attributed to actual infection with a dinoflagellate of the genus Hematodinium.
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