cyanobacterial toxins are well recognized as a cause of livestock poisoning , which has been extensively reported in the americas , europe , asia and australasia . livestock are inevitably vulnerable to poisoning as they are restricted in access to water by topography and by fences , and hence may have no choice but to drink water infested by toxic cyanobacteria . most stock deaths have resulted from the formation of cyanobacterial water - blooms in ponds and lakes on farms , but several major poisoning events were through water blooms on rivers and drinking water reservoirs [ 2 , 3 ] . the symptoms of poisoning by the main toxic cyanobacteria in drinking water reservoirs overlap with a range of other gastrointestinal illnesses , largely caused by infectious disease organisms . as a consequence during an outbreak of enteric disease the pathogens are investigated first , as the most probable cause , and only after exhaustive exploration are toxins of any type evaluated . agricultural chemicals and industrial pollutants such as heavy metals are likely to be next suspected , with cyanobacterial toxins ignored until well after the event . epidemiological data for human poisoning by cyanobacterial toxins only exists for a small number of events . the most well characterized case was the poisoning of renal dialysis patients in a clinic in caruaru , brazil , in 1996 . in this instance the patients treated in a dialysis clinic during one week suffered severe illness following perfusion , with hepatic failure and , in more than 50 cases , death . investigation of the water treatment unit at the clinic found contamination of the filters by two types of cyanobacterial toxin , microcystins and cylindrospermopsins [ 5 , 6 ] . because of the severity of the poisoning a thorough investigation was carried out , which showed up major defects in the operation of the water treatment unit at the clinic . exposure to toxins through renal dialysis is a particularly potent route of poisoning , equivalent to an intravenous injection in the case of water soluble toxins . the volume of water used in perfusion is large , about 120l , so that the total amount of toxin to which a dialysis patient is exposed is much greater than possible through drinking water . exposure to cyanobacterial toxins through consumption of contaminated drinking water has however also resulted in poisoning . the earliest demonstration of this was in 1983 , when the population of a rural town in australia was supplied with drinking water from a reservoir carrying a dense water bloom of a toxic species of cyanobacterium , microcystis aeruginosa . the controlling authority dosed the reservoir with copper sulphate to destroy the cyanobacteria , which caused the cells to lyse and release toxin into the water . epidemiological data for liver injury in the affected population , a control population and comparison of the time periods before the bloom , during the bloom and lysis , and afterwards , showed clearly that liver damage had occurred only in the exposed population and only at the time of the water bloom . in another less well characterized event , about 140 children and 10 adults were hospitalized , after the water supply authorities treated a cyanobacterial bloom in a small drinking water supply reservoir with copper sulphate , to resolve taste and odour problems . within a week severe later work on this strain of cyanobacterium lead to the identification of an alkaloid cytotoxin , with considerable liver toxicity . cyanobacteria are a normal component of the worldwide biota , with a wide tolerance of climatic conditions and environment . as a very ancient life - form they occupy every conceivable ecological niche , and their abundance is limited by nutrient and light availability . in aquatic systems cyanobacteria are always present , through the population density varies from very small numbers to more than 10 organisms / ml . there is a strong relationship between phosphorus concentration in the water and cyanobacterial numbers and also a similar though less linked relationship between dissolved nitrate / ammonia and cyanobacteria . thus in general toxic cyanobacterial species will be presentin all water bodies , with numbers dependent on the available nutrients and light . as human population density rises the inflow of nutrients into water bodies increases through agricultural fertilizer use , urban run - off and sewage discharge . cyanobacteria can utilize nutrients competitively with eukaryotic phytoplankton , and will proliferate more successfully at lower nutrient concentrations than the green algae . as a result many rivers , lakes and reservoirs worldwide develop high cyanobacterial cell concentrations , especially in the summer months , which appear as greenish suspensions in the water . some species float to the surface under warm , still conditions , forming scums with extreme cell concentrations above 110 cells / ml . dried scums often appear blue - green or red through liberation of phycocyanin pigment , leading to the common name of these organisms - blue - green algae . the scum - forming cyanobacterial species are largely toxic , and the majority of domestic animal poisonings have occurred from the animals drinking scum . cell populations carry over from year to year , and once a reservoir or lake has an established water bloom of cyanobacteria in summer , it is very difficult to reverse this phenomenon . cyanobacteria proliferate in warmer weather , and often form extensive blooms in late summer . with an increase in global temperatures , cyanobacterial populations are likely to increase also . with the growth in human populations , demand for drinking water has resulted in water is being drawn from water bodies carrying substantial cyanobacterial populations , thus presenting a risk to human populations . annual or even permanent blooms of toxic cyanobacteria are becoming increasingly common in drinking water reservoirs . to give an illustration the three main reservoirs supplying brisbane in australia all carry substantial populations of the toxic cylindrospermopsis raciborskii . this cyanobacterium forms dense layers 510 m below the surface , so that the first indication of the proliferation of the organism may be the blocking of filters in the drinking water treatment plant . other examples are the main drinking water supply reservoirs for the cities of sao paulo in brasil and lodz in poland , which contain heavy blooms of the toxic microcystis aeruginosa in summer . they are produced by several genera of cyanobacteria growing in freshwater which have the capacity to form dense waterblooms and floating scums at the edge of lakes and rivers . the neurotoxins are alkaloid compounds , fast acting and have caused many deaths of dogs and livestock . this toxin has been found in three common genera of cyanobacteria , anabaena , aphanizomenon and planktothrix , all filamentous planktonic organisms capable of high cell concentrations and potential scum formation . there has been no clear evidence of human poisoning from these organisms , though a coroner in wisconsin in 2003 resolved that the death of a male teenager who was diving and playing in a pond containing neurotoxic anabaena had died as a consequence of ingestion of these cyanobacteria   to verify this cause of death , evidence of toxin in the gastrointestinal tract or tissues was required , however there has been no published report of the presence of toxin . by contrast , dogs poisoned by anatoxin - a have shown the toxin in stomach contents . the compound is stable in the environment , as exhibited by the dogs having died after eating decaying lumps of cyanobacteria on the lakeside . anatoxin - a has been identified in the water of lakes in north america and in europe , which are largely used for recreation . there is the possibility of consumption of moderate quantities of water during swimming and especially water skiing , and hence a risk exists for anatoxin - a poisoning of recreational water users . many authorities in developed countries have warning procedures for cyanobacterial blooms at popular recreational areas , to reduce risk to water users . there has been little attention paid to the assessment of risk to drinking water consumers from anatoxin - a , largely because of the rapid excretion of the toxin from the body , no evidence of residual effects and low free - water concentrations in lakes . anatoxin - a(s ) is much less common in cyanobacterial waterblooms , though it was first identified following cattle deaths in the usa . the alkaloid closely resembles an organophosphorus insecticide ( figure 1 ) , and acts as an anticholinesterase . the characteristic feature of this poisoning is excessive salivation , which is the reason for the designation ( s ) . the compound is highly unstable and unlikely to persist in water supplies , and as a result is also unlikely to present any risk . the saxitoxin - type neurotoxins are well known as the cause of paralytic shellfish poisonings , which have resulted in many hundreds of human deaths worldwide . as a result , legislation controls the allowable concentration of saxitoxins in shellfish harvested for human consumption ( 80g/100 g fresh shellfish tissue ) and there is a substantial monitoring program in many countries . saxitoxins are however not limited to marine waters , and also occur in freshwater cyanobacteria . anabaena , aphanizomenon and lyngbya genera of cyanobacteria have species that produce saxitoxins [ 2123 ] . the massive waterbloom of anabaena circinalis on 1,000 km of the darling river in australia in 1991 killed a large number of sheep and cattle , and also resulted in detectable neurotoxicity in town water supplies . saxitoxins are heat - stable molecules , which are not easily removed in conventional water treatments unless ph and chlorine residuals are carefully controlled , but can be effectively removed by ozone or activated carbon . the toxicity of saxitoxin is considerable , as the alkaloid blocks sodium conduction in axons preventing nerve impulse transmission , leading to paralysis . acute poisoning in humans is unlikely to occur from contaminated water supplies , as the human body can tolerate about 100g of saxitoxin without ill effect , which translated to drinking water is 50g / l assuming 2l water drunk per day . no cumulative effects have been demonstrated , though there is limited evidence of resistance to toxicity in exposed human populations . new zealand is considering a maximum acceptable value in drinking water of 3g / l of saxitoxin equivalents in their new drinking water guidelines , which should be ratified shortly . this value has also been proposed in australia . the data on which this value was based were the intraperitoneal toxicity of saxitoxin to mice , and incorporated a safety factor of 1,000 . there are no data for the concentrations of saxitoxin - type neurotoxins in drinking water , and there were no reports of neurotoxic symptoms in the town population when neurotoxicity was detected in the drinking water supply . these toxins have received the greatest attention , as they are the source of the most likely risk to consumers of drinking water . the predominant genera of cyanobacteria forming the peptide toxins called microcystins are microcystis , planktothrix and anabaena . species from these genera are common in europe , the americas , africa and asia , and poisoning of domestic animals has been widely reported . only two epidemiological investigations have so far shown human injury from microcystin in drinking water , one in australia   and one in china ( unpublished ) . as a consequence of the frequency of cyanobacterial blooms containing hepatotoxins in drinking water reservoirs , the who carefully examined the need for the major toxins , the microcystins , to be included in the drinking water guidelines . an expert group was established to examine the whole issue of cyanobacterial toxins in drinking water , which resulted in a comprehensive assessment of the risks involved . the outcome was a recommendation that the microcystins should be included among the chemicals for which guideline values be determined . these peptide toxins are cyclic , and contain a majority of d - amino acids ( figure 2 ) . the positions shown as [ x ] and [ y ] are l - amino acids , and are variable between species and strains of cyanobacteria . the most abundant variant has l - leucine ( l ) and l - arginine ( r ) respectively at [ x ] and [ y ] ( microcystin - lr ) . the amino acid at the left of the molecule is unique , is connected into the ring through an amino group at the -carbon atom , and has the trivial name of adda . microcystins are resistant to digestion in the gastrointestinal tract of eukaryotes , as peptide bonds linking to the d - amino acids are not susceptible to normal hydrolytic enzymes . the toxins are concentrated into the liver by an active transport system , similar to the bile acid transporter . microcystins specifically inhibit protein phosphatases 1 and 2a , which have a vital role in cell control and in intracellular structure . acute poisoning is through destruction of the liver architecture , leading to blood loss into the liver and hemorrhagic shock . later death is through liver failure with massive destruction of hepatocytes , seen in large animal deaths and human fatalities [ 5 , 33 ] . chronic exposure to these toxins in drinking water led to ongoing active liver injury in mice . there is experimental evidence for tumour promotion by microcystins , and limited data for carcinogenesis in rodents . in rural areas in southern china some villages showed hyper - endemic rates of hepatocellular carcinoma , which have been shown to be linked to hepatitis , aflatoxin in the food , and drinking surface water . microcystins in the ponds and ditches used as water sources were suspected of contributing to the cancer rates [ 36 , 37 ] . hence they present a risk to consumers in less developed regions and countries who are collecting water from surface sources to drink . many lakes , ponds , ditches and streams in rural and outer urban areas suffer from eutrophication through excessive nutrient leaching from housing , sewage , and intensive agricultural use , leading to cyanobacterial proliferation . in tropical and temperate regions of the world the genus microcystis is the most abundant cyanobacterium forming toxic blooms , with toxin concentrations sufficient to poison domestic animals . if these contaminated water sources are used for human consumption , there is a risk of human poisoning . conventional western drinking water treatment may not be effective under bloom conditions in removing microcystins from drinking water and hence there is a risk to consumers . advanced water treatment using ozone and activated carbon will reliably remove microcystins . who have carried out an assessment of the safe level of microcystins in drinking water , based on data from a subchronic toxicity trial in mice , with supporting data from growing pigs . the calculation used the no observed adverse effect level for male mice during a 13 week oral toxicity trial , of 40g of microcystin - lr / kg / day . this was used to calculate a tolerable daily intake ( tdi ) for safe human consumption , by the incorporation of uncertainty or safety factors . while these are subjective , a factor of 10 for interspecies uncertainty between rodents and humans , a further 10 for variability in sensitivity between people , and an uncertainty of 10 for inadequate data , possible tumour promotion and lack of lifetime exposure are generally accepted . from this value the guideline value ( also called the reference dose and the maximum acceptable concentration ) was calculated from the standard bodyweight of 60 kg , an assumption of the proportion of the dose from drinking water of 0.8 ( some may come from food and particularly blue - green algal diet supplements ) and a standard water consumption of 2l / day . this value was determined from the toxicity of microcystin - lr , so the who chemical safety committee set the guideline value for microcystin  lr . since there are some 60 variants of the molecule , and some highly toxic blooms do not contain any of the lr variant , it is necessary to interpret this as toxicity equivalent to microcystin - lr . where this guideline value has been adopted as the basis for national legislation , the need for monitoring of all the toxin variants has been recognized and the equivalent total toxicity calculated . individual countries have also adopted a higher standard bodyweight , and a different proportion of the consumption from drinking water . all of the guideline values adopted so far lie between 1 and 2g / l of microcystin equivalents , which for practical purposes are the same . a larger potential adjustment to this value may result from re - classification of microcystin as a carcinogen , rather than a non - carcinogenic poison . while there is experimental evidence for tumour promotion by microcystin in liver , skin and colon , the only data indicating carcinogenesis have been obtained by continued very high intraperitoneal doses of toxin in mice which cause extensive liver damage . in china there is ongoing investigation into the relationship between surface water consumption and cancer of liver and colon . this issue is discussed in detail elsewhere , concluding that there is insufficient evidence at present to determine that microcystin is a probable carcinogen but the possibility requires continual evaluation . these alkaloid cytotoxins were relatively recently discovered , following the widespread human poisoning at palm island , australia due to contamination of the water supply . cyanobacteria from the supply reservoir were collected , cultured and evaluated for toxicity , showing potent toxicity to liver , kidney , adrenals , lymphoid cells and other tissues in mice . subsequent investigation of the oral toxicity of the cyanobacterium responsible , cylindrospermopsis raciborskii , further demonstrated the tissue damage caused by the toxin [ 42 , 43 ] . the toxic alkaloid was isolated and identified as a potent inhibitor of protein synthesis [ 11 , 44 ] . there are on - going investigations into the mechanism of cylindrospermopsin toxicity , which may involve activated metabolites of the alkaloid [ 44 , 45 ] . the alkaloid has several reactive groups , including a hydroxymethyl uracil , which may be vulnerable to biological oxidation ( figure 3 ) . cylindrospermopsin has been found in water bodies that have blooms of the cyanobacterial species aphanizomenon ovalisporum , and umezakia natans as well as those with cylindrospermopsis[48 , 49 ] , and recently in germany , in the absence of any of those species . it is apparent from this data that cylindrospermopsin is likely to occur widely in freshwater sources , and that only a beginning has been made in identifying species producing this toxin . monitoring of water supplies for cylindrospermopsin has found concentrations in natural water bodies , reservoirs and in drinking water that are above 10g / l , which is a cause for concern [ 51 , 52 ] . on the basis of the experimental toxicity of cylindrospermopsin , the reported human poisoning associated with cylindrospermopsis and the toxin concentrations measured in water bodies , it was apparent that risk assessment for this toxin in drinking water was required . a subchronic oral exposure trial of cylindrospermopsin in male mice provided a no observed adverse effect level of 30g of cylindrospermopsin / kg / day . on the assumption of standard uncertainty factors and the total intake arising from drinking water , a guideline value of 1g / l resulted . examination of the molecular structure of cylindrospermpsin indicated that it may be able to interact with dna or rna in cells , through the uracil group , assisted by the planar shape of the molecule . if this proves to be the case , then evaluation of the possible carcinogenicity of the molecule is required . preliminary data indicated that cylindrospermopsin may form dna adducts , and there is evidence for clastogenicity and micronucleus formation in a cultured human white cell line incubated with cylindrospermopsin . a preliminary trial of carcinogenicity in mice indicated the presence of excess tumors , providing support for a more definitive carcinogenicity trial . it is premature at present to attempt to classify cylindrospermopsin as a possible human carcinogen , because of the very limited current data . further experimental and epidemiological research on this toxin is required to clarify these issues , and cylindrospermopsin is now on the  candidate contaminant list of the us environmental protection agency . there are two areas in which more data is necessary to make a clear case for national action on minimizing health risks from cyanobacterial toxins . the first is the need for widespread monitoring for the presence of toxic cyanobacterial species and toxins in drinking water sources , to identify the abundance of locations of potential risk . this is in progress in europe , and an initial survey has been carried out through the american water works association in the usa . from data arising from these surveys , the extent of the problem has become apparent , and the location of a proportion of the populations at most risk . the second and more difficult aspect is the need for epidemiological studies on at - risk populations to quantify the adverse health effects . exposure biomarkers will have to be developed , in addition to quantitating the toxin concentrations in tap water . the commonly used clinical measures of liver and kidney function , and clinical records for hepatoenteritis , provide relevant health information . the earlier data on population injury from microcystins indicates the clinical parameters of particular interest . cyanobacteria are a normal component of the worldwide biota , with a wide tolerance of climatic conditions and environment . as a very ancient life - form they occupy every conceivable ecological niche , and their abundance is limited by nutrient and light availability . in aquatic systems cyanobacteria are always present , through the population density varies from very small numbers to more than 10 organisms / ml . there is a strong relationship between phosphorus concentration in the water and cyanobacterial numbers and also a similar though less linked relationship between dissolved nitrate / ammonia and cyanobacteria . thus in general toxic cyanobacterial species will be presentin all water bodies , with numbers dependent on the available nutrients and light . as human population density rises the inflow of nutrients into water bodies increases through agricultural fertilizer use , urban run - off and sewage discharge . cyanobacteria can utilize nutrients competitively with eukaryotic phytoplankton , and will proliferate more successfully at lower nutrient concentrations than the green algae . as a result many rivers , lakes and reservoirs worldwide develop high cyanobacterial cell concentrations , especially in the summer months , which appear as greenish suspensions in the water . some species float to the surface under warm , still conditions , forming scums with extreme cell concentrations above 110 cells / ml . dried scums often appear blue - green or red through liberation of phycocyanin pigment , leading to the common name of these organisms - blue - green algae . the scum - forming cyanobacterial species are largely toxic , and the majority of domestic animal poisonings have occurred from the animals drinking scum . cell populations carry over from year to year , and once a reservoir or lake has an established water bloom of cyanobacteria in summer , it is very difficult to reverse this phenomenon . cyanobacteria proliferate in warmer weather , and often form extensive blooms in late summer . with an increase in global temperatures , with the growth in human populations , demand for drinking water has resulted in water is being drawn from water bodies carrying substantial cyanobacterial populations , thus presenting a risk to human populations . annual or even permanent blooms of toxic cyanobacteria are becoming increasingly common in drinking water reservoirs . to give an illustration the three main reservoirs supplying brisbane in australia all carry substantial populations of the toxic cylindrospermopsis raciborskii . this cyanobacterium forms dense layers 510 m below the surface , so that the first indication of the proliferation of the organism may be the blocking of filters in the drinking water treatment plant . other examples are the main drinking water supply reservoirs for the cities of sao paulo in brasil and lodz in poland , which contain heavy blooms of the toxic microcystis aeruginosa in summer . they are produced by several genera of cyanobacteria growing in freshwater which have the capacity to form dense waterblooms and floating scums at the edge of lakes and rivers . the neurotoxins are alkaloid compounds , fast acting and have caused many deaths of dogs and livestock . this toxin has been found in three common genera of cyanobacteria , anabaena , aphanizomenon and planktothrix , all filamentous planktonic organisms capable of high cell concentrations and potential scum formation . there has been no clear evidence of human poisoning from these organisms , though a coroner in wisconsin in 2003 resolved that the death of a male teenager who was diving and playing in a pond containing neurotoxic anabaena had died as a consequence of ingestion of these cyanobacteria   to verify this cause of death , evidence of toxin in the gastrointestinal tract or tissues was required , however there has been no published report of the presence of toxin . by contrast , dogs poisoned by anatoxin - a have shown the toxin in stomach contents . the compound is stable in the environment , as exhibited by the dogs having died after eating decaying lumps of cyanobacteria on the lakeside . anatoxin - a has been identified in the water of lakes in north america and in europe , which are largely used for recreation . there is the possibility of consumption of moderate quantities of water during swimming and especially water skiing , and hence a risk exists for anatoxin - a poisoning of recreational water users . many authorities in developed countries have warning procedures for cyanobacterial blooms at popular recreational areas , to reduce risk to water users . there has been little attention paid to the assessment of risk to drinking water consumers from anatoxin - a , largely because of the rapid excretion of the toxin from the body , no evidence of residual effects and low free - water concentrations in lakes . anatoxin - a(s ) is much less common in cyanobacterial waterblooms , though it was first identified following cattle deaths in the usa . the alkaloid closely resembles an organophosphorus insecticide ( figure 1 ) , and acts as an anticholinesterase . the characteristic feature of this poisoning is excessive salivation , which is the reason for the designation ( s ) . the compound is highly unstable and unlikely to persist in water supplies , and as a result is also unlikely to present any risk . the saxitoxin - type neurotoxins are well known as the cause of paralytic shellfish poisonings , which have resulted in many hundreds of human deaths worldwide . as a result , legislation controls the allowable concentration of saxitoxins in shellfish harvested for human consumption ( 80g/100 g fresh shellfish tissue ) and there is a substantial monitoring program in many countries . saxitoxins are however not limited to marine waters , and also occur in freshwater cyanobacteria . anabaena , aphanizomenon and lyngbya genera of cyanobacteria have species that produce saxitoxins [ 2123 ] . the massive waterbloom of anabaena circinalis on 1,000 km of the darling river in australia in 1991 killed a large number of sheep and cattle , and also resulted in detectable neurotoxicity in town water supplies . saxitoxins are heat - stable molecules , which are not easily removed in conventional water treatments unless ph and chlorine residuals are carefully controlled , but can be effectively removed by ozone or activated carbon . the toxicity of saxitoxin is considerable , as the alkaloid blocks sodium conduction in axons preventing nerve impulse transmission , leading to paralysis . acute poisoning in humans is unlikely to occur from contaminated water supplies , as the human body can tolerate about 100g of saxitoxin without ill effect , which translated to drinking water is 50g / l assuming 2l water drunk per day . no cumulative effects have been demonstrated , though there is limited evidence of resistance to toxicity in exposed human populations . new zealand is considering a maximum acceptable value in drinking water of 3g / l of saxitoxin equivalents in their new drinking water guidelines , which should be ratified shortly . this value has also been proposed in australia . the data on which this value was based were the intraperitoneal toxicity of saxitoxin to mice , and incorporated a safety factor of 1,000 . there are no data for the concentrations of saxitoxin - type neurotoxins in drinking water , and there were no reports of neurotoxic symptoms in the town population when neurotoxicity was detected in the drinking water supply . these toxins have received the greatest attention , as they are the source of the most likely risk to consumers of drinking water . the predominant genera of cyanobacteria forming the peptide toxins called microcystins are microcystis , planktothrix and anabaena . species from these genera are common in europe , the americas , africa and asia , and poisoning of domestic animals has been widely reported . only two epidemiological investigations have so far shown human injury from microcystin in drinking water , one in australia   and one in china ( unpublished ) . as a consequence of the frequency of cyanobacterial blooms containing hepatotoxins in drinking water reservoirs , the who carefully examined the need for the major toxins , the microcystins , to be included in the drinking water guidelines . an expert group was established to examine the whole issue of cyanobacterial toxins in drinking water , which resulted in a comprehensive assessment of the risks involved . the outcome was a recommendation that the microcystins should be included among the chemicals for which guideline values be determined . these peptide toxins are cyclic , and contain a majority of d - amino acids ( figure 2 ) . the positions shown as [ x ] and [ y ] are l - amino acids , and are variable between species and strains of cyanobacteria . the most abundant variant has l - leucine ( l ) and l - arginine ( r ) respectively at [ x ] and [ y ] ( microcystin - lr ) . the amino acid at the left of the molecule is unique , is connected into the ring through an amino group at the -carbon atom , and has the trivial name of adda . microcystins are resistant to digestion in the gastrointestinal tract of eukaryotes , as peptide bonds linking to the d - amino acids are not susceptible to normal hydrolytic enzymes . the toxins are concentrated into the liver by an active transport system , similar to the bile acid transporter . microcystins specifically inhibit protein phosphatases 1 and 2a , which have a vital role in cell control and in intracellular structure . acute poisoning is through destruction of the liver architecture , leading to blood loss into the liver and hemorrhagic shock . later death is through liver failure with massive destruction of hepatocytes , seen in large animal deaths and human fatalities [ 5 , 33 ] . chronic exposure to these toxins in drinking water led to ongoing active liver injury in mice . there is experimental evidence for tumour promotion by microcystins , and limited data for carcinogenesis in rodents . in rural areas in southern china some villages showed hyper - endemic rates of hepatocellular carcinoma , which have been shown to be linked to hepatitis , aflatoxin in the food , and drinking surface water . microcystins in the ponds and ditches used as water sources were suspected of contributing to the cancer rates [ 36 , 37 ] . hence they present a risk to consumers in less developed regions and countries who are collecting water from surface sources to drink . many lakes , ponds , ditches and streams in rural and outer urban areas suffer from eutrophication through excessive nutrient leaching from housing , sewage , and intensive agricultural use , leading to cyanobacterial proliferation . in tropical and temperate regions of the world the genus microcystis is the most abundant cyanobacterium forming toxic blooms , with toxin concentrations sufficient to poison domestic animals . if these contaminated water sources are used for human consumption , there is a risk of human poisoning . conventional western drinking water treatment may not be effective under bloom conditions in removing microcystins from drinking water and hence there is a risk to consumers . advanced water treatment using ozone and activated carbon will reliably remove microcystins . who have carried out an assessment of the safe level of microcystins in drinking water , based on data from a subchronic toxicity trial in mice , with supporting data from growing pigs . the calculation used the no observed adverse effect level for male mice during a 13 week oral toxicity trial , of 40g of microcystin - lr / kg / day . this was used to calculate a tolerable daily intake ( tdi ) for safe human consumption , by the incorporation of uncertainty or safety factors . while these are subjective , a factor of 10 for interspecies uncertainty between rodents and humans , a further 10 for variability in sensitivity between people , and an uncertainty of 10 for inadequate data , possible tumour promotion and lack of lifetime exposure are generally accepted . from this value the guideline value ( also called the reference dose and the maximum acceptable concentration ) was calculated from the standard bodyweight of 60 kg , an assumption of the proportion of the dose from drinking water of 0.8 ( some may come from food and particularly blue - green algal diet supplements ) and a standard water consumption of 2l / day . this value was determined from the toxicity of microcystin - lr , so the who chemical safety committee set the guideline value for microcystin  lr . since there are some 60 variants of the molecule , and some highly toxic blooms do not contain any of the lr variant , it is necessary to interpret this as toxicity equivalent to microcystin - lr . where this guideline value has been adopted as the basis for national legislation , the need for monitoring of all the toxin variants has been recognized and the equivalent total toxicity calculated . individual countries have also adopted a higher standard bodyweight , and a different proportion of the consumption from drinking water . all of the guideline values adopted so far lie between 1 and 2g / l of microcystin equivalents , which for practical purposes are the same . a larger potential adjustment to this value may result from re - classification of microcystin as a carcinogen , rather than a non - carcinogenic poison . while there is experimental evidence for tumour promotion by microcystin in liver , skin and colon , the only data indicating carcinogenesis have been obtained by continued very high intraperitoneal doses of toxin in mice which cause extensive liver damage . in china there is ongoing investigation into the relationship between surface water consumption and cancer of liver and colon . this issue is discussed in detail elsewhere , concluding that there is insufficient evidence at present to determine that microcystin is a probable carcinogen but the possibility requires continual evaluation . these alkaloid cytotoxins were relatively recently discovered , following the widespread human poisoning at palm island , australia due to contamination of the water supply . cyanobacteria from the supply reservoir were collected , cultured and evaluated for toxicity , showing potent toxicity to liver , kidney , adrenals , lymphoid cells and other tissues in mice . subsequent investigation of the oral toxicity of the cyanobacterium responsible , cylindrospermopsis raciborskii , further demonstrated the tissue damage caused by the toxin [ 42 , 43 ] . the toxic alkaloid was isolated and identified as a potent inhibitor of protein synthesis [ 11 , 44 ] . there are on - going investigations into the mechanism of cylindrospermopsin toxicity , which may involve activated metabolites of the alkaloid [ 44 , 45 ] . the alkaloid has several reactive groups , including a hydroxymethyl uracil , which may be vulnerable to biological oxidation ( figure 3 ) . cylindrospermopsin has been found in water bodies that have blooms of the cyanobacterial species aphanizomenon ovalisporum , and umezakia natans as well as those with cylindrospermopsis[48 , 49 ] , and recently in germany , in the absence of any of those species . it is apparent from this data that cylindrospermopsin is likely to occur widely in freshwater sources , and that only a beginning has been made in identifying species producing this toxin . monitoring of water supplies for cylindrospermopsin has found concentrations in natural water bodies , reservoirs and in drinking water that are above 10g / l , which is a cause for concern [ 51 , 52 ] . on the basis of the experimental toxicity of cylindrospermopsin , the reported human poisoning associated with cylindrospermopsis and the toxin concentrations measured in water bodies , it was apparent that risk assessment for this toxin in drinking water was required . a subchronic oral exposure trial of cylindrospermopsin in male mice provided a no observed adverse effect level of 30g of cylindrospermopsin / kg / day . on the assumption of standard uncertainty factors and the total intake arising from drinking water , a guideline value of 1g / l resulted . examination of the molecular structure of cylindrospermpsin indicated that it may be able to interact with dna or rna in cells , through the uracil group , assisted by the planar shape of the molecule . if this proves to be the case , then evaluation of the possible carcinogenicity of the molecule is required . preliminary data indicated that cylindrospermopsin may form dna adducts , and there is evidence for clastogenicity and micronucleus formation in a cultured human white cell line incubated with cylindrospermopsin . a preliminary trial of carcinogenicity in mice indicated the presence of excess tumors , providing support for a more definitive carcinogenicity trial . it is premature at present to attempt to classify cylindrospermopsin as a possible human carcinogen , because of the very limited current data . further experimental and epidemiological research on this toxin is required to clarify these issues , and cylindrospermopsin is now on the  candidate contaminant list of the us environmental protection agency . there are two areas in which more data is necessary to make a clear case for national action on minimizing health risks from cyanobacterial toxins . the first is the need for widespread monitoring for the presence of toxic cyanobacterial species and toxins in drinking water sources , to identify the abundance of locations of potential risk . this is in progress in europe , and an initial survey has been carried out through the american water works association in the usa . from data arising from these surveys , the extent of the problem has become apparent , and the location of a proportion of the populations at most risk . the second and more difficult aspect is the need for epidemiological studies on at - risk populations to quantify the adverse health effects . exposure biomarkers will have to be developed , in addition to quantitating the toxin concentrations in tap water . the commonly used clinical measures of liver and kidney function , and clinical records for hepatoenteritis , provide relevant health information . the earlier data on population injury from microcystins indicates the clinical parameters of particular interest . the cyanobacterial toxins provide a risk to human health when the population of toxic cyanobacteria in drinking water sources rises to bloom proportions . the present assessments of guideline values for these toxins as chemical , non - carcinogenic contaminants indicate that a safe concentration in drinking water is in the region of 1g / l , a concentration that has been exceeded in numerous water storages . carcinogenicity of these toxins is not yet established , though both microcystins and cylindrospermopsin have caused excess tumors in rodent experiments . with the increased eutrophication of water supplies and global warming , cyanobacterial populations and
cyanobacterial toxins have caused human poisoning in the americas , europe and australia . there is accumulating evidence that they are present in treated drinking water supplies when cyanobacterial blooms occur in source waters . with increased population pressure and depleted groundwater reserves 
 , surface water is becoming more used as a raw water source , both from rivers and lakes / reservoirs . 
 additional nutrients in water which arise from sewage discharge , agricultural run - off or storm water result in overabundance of cyanobacteria , described as a  water bloom. the majority of cyanobacterial water - blooms are of toxic species , producing a diversity of toxins . 
 the most important toxins presenting a risk to the human population are the neurotoxic alkaloids ( anatoxins and paralytic shellfish poisons ) , the cyclic peptide hepatotoxins ( microcystins ) and the cytotoxic alkaloids ( cylindrospermopsins ) . at the present time the only cyanobacteral toxin family that have been internationally assessed for health risk by the who are the microcystins , which cause acute liver injury and are active tumour promoters . based on sub - chronic studies in rodents and pigs , a provisional guideline level for drinking water of 1g / 
 l of microcystin - lr has been determined . 
 this has been adopted in legislation in countries in europe , south america and australasia . 
 this may be revised in the light of future teratogenicity , reproductive toxicity and carcinogenicity studies . 
 the other cyanobacterial toxin which has been proposed for detailed health risk assessment is cylindrospermopsin , a cytotoxic compound which has marked genotoxicity , probable mutagenicity , and is a potential carcinogen . 
 this toxin has caused human poisoning from drinking water , and occurs in water supplies in the usa , europe , asia , australia and south america . 
 an initial health risk assessment is presented with a proposed drinking water guideline level of 1g / l . 
 there is a need for both increased monitoring data for toxins in drinking water and epidemiological studies on adverse health effects in exposed populations to clarify the extent of the health risk .