3 Public health concerns
3.1 Surveys in different parts of the world have found that between about 45% and 90% of blooms of blue-green algae produce toxins. These toxins are largely retained within the blue-green algal cells during their development and growth phases and are released, in the main, on cell death.
3.2 Blue-green algae of several genera can produce a range of toxins including neuro- and hepatotoxins and lipopolysaccharides. An algal bloom may contain more than one species, each producing the same or different toxins, either singly or in combination. In addition, the toxicity of one species might change over time to a pattern that might vary for different places on a particular water body. Further information on algal toxins is given in Annex B.
3.3 Evidence of toxicity comes from reports of the effects of exposure of people and of animals to algal blooms and from laboratory investigations of algal toxins.
3.4 In 1989, a group of soldiers took part in canoe training, including rolling and swimming exercises, at Rudyard Lake in Staffordshire. Two became severely ill with atypical pneumonia; others reported abdominal pains, vomiting, diarrhoea, blistering of the mouth and sore throats. Further incidents of effects on human health have occurred after recreational contact with blue-green algal scums and blooms in UK inland waters in recent years. The effects were probably associated with exposure to blue-green algae and ingestion of the toxin-containing blue-green algal scum.
3.5 Gastroenteritis, neurological effects and acute hepatocellular damage have been reported from other countries. Illnesses and deaths of haemodialysis patients, probably resulting from blue-green algal toxins in inadequately-treated water, occurred in Brazil in 1996. Further exposures of haemodialsis patients to blue-green algal toxins, followed by illness, occurred in Brazil in 2001.
3.6 Ingestion of hepatotoxic and neurotoxic scums of blue-green algae are reported to have caused the deaths of cattle, sheep, dogs and birds. There is also evidence that blue-green algal toxins have been major contributors to fish kills and deaths of other aquatic animals.
3.7 Another potential source of intoxication for both animals and humans is bioaccumulation of algal toxins in the food chain. The principal concern here would be accumulation of algal toxins in shellfish including freshwater and brackish-water mussels and in fish. However, no cases of intoxication from this source have been reported to date in Scotland.
3.8 Episodes of blue-green algal contamination of drinking water supplies occur periodically. In September 1997, a massive blue-green algal bloom affected the main water supply loch on Westray, Orkney Isles, and resulted in a ban on the use of water for drinking, cooking and washing. Large quantities of water treatment chemicals were needed to reduce blue-green algal concentrations to a level where even a reduced throughput could be maintained and aluminium levels in the final water eventually rose to a level considered unfit for consumption. The water had also become unacceptable due to taste and odour. No blue-green algal toxins were detected. The water authority arranged for potable water to be transported as bottled water and in tankers to serve the human population. Fortunately, the very large cattle herd on the island at the time was able to continue to drink the loch water without ill effect. In July 2005, consumers of water from the Loch of Boardhouse supply in Orkney complained of an earthy taste and musty odour. A visual check of the loch identified green growth around the loch consistent with an algal or cyanobacterial bloom. Analysis confirmed mixed blue-green algal species, predominantly Anabaena, resulting in high levels of 2-methylisobornereol and geosmin. Blue-green algal toxin (microcystin) concentrations were below 1 microgram per litre. A temporary powdered activated carbon dosing plant was installed which improved the taste and odour of the final treated water. The blue-green algae had virtually disappeared by mid-August. Approximately 4,000 consumers were inconvenienced by disruption to their water supply and were supplied with bottled water. A similar problem with blue-green algae and geosmin tainting affected the taste and odour of water from the Glenfarg reservoir in 2006, also resulting in the use of carbon dosing.
3.9 There are occasional reports of animal deaths attributed by their owners to contact with blue-green algal scums. However, objective evidence is not always available to confirm an association with toxin exposure. In summer of 2003 there was good evidence to suggest that the deaths of two dogs in Fife were associated with ingestion of blue-green algal sludge at Town Loch in Dunfermline. Restrictions were imposed and were supported by ongoing monitoring and shoreline deposits were safely removed and disposed of. Another dog death on Shetland reported to SEPA in 2006 was investigated and cyanotoxin analysis suggested that toxin exposure was a strong candidate cause of death. Further incidents have been reported involving dogs and calves where the associations were circumstantial. As often occurs with such incidents, there were gaps in the recognition of a possible link and in the investigation, such that it was difficult to establish a definitive cause.
3.10 Surveillance by HPS using the Scottish Environmental Incident Surveillance System ( SEISS), from 2002 onwards, identifies between 30 to 40 incidents being reported, by SEPA, Local Authorities and NHS Boards annually. Algal blooms are inherently complex (Paragraph 3.2) and assessment of the associated risks to public health is not straightforward. Such assessments should therefore take account of specialist advice ( Annex C). Where advice is not immediately available, action of the kind described below may still be appropriate.