Saxitoxins
(endorsed 2011)
Guideline
Due to the lack of adequate data, no guideline value is set for concentrations of saxitoxins. However given the known toxicity, the relevant health authority or drinking water regulator should be notified immediately if blooms of Anabaena circinalis (Dolichospermum circinalis) [1] or other producers of saxitoxins are detected in sources of drinking water.
General description
There are three types of cyanobacterial neurotoxins: anatoxin a, anatoxin a-s and the saxitoxins. The saxitoxins include saxitoxin, neosaxitoxin, C-toxins and gonyautoxins (Chorus and Bartram 1999 Chapter 3). The anatoxins seem unique to cyanobacteria, while saxitoxins are also produced by various dinoflagellates under the name of paralytic shellfish poisons (PSPs). A number of cyanobacterial genera can produce neurotoxins, including Anabaena (Dolichospermum), Oscillatoria, Cylindrospermopsis, Cylindrospermum, Lyngbya and Aphanizomenon, but to date in Australia, neurotoxin production has only been detected from Anabaena circinalis (Dolichospermum circinalis), and the Australian isolates appear to produce only saxitoxins (Velzeboer et al. 1998). As with most toxic cyanobacteria, A. circinalis (D. circinalis) tends to proliferate in calm, stable waters, particularly in summer when thermal stratification reduces mixing.
The toxicity of individual populations of A. circinalis (D. circinalis) is variable, and one extensive survey of the toxicity across the Murray-Darling Basin indicated that 54% of field samples tested were neurotoxic (Baker and Humpage, 1994). A natural population may consist of a mixture of toxic and non-toxic strains and this is believed to explain why population toxicity may vary over time and between samples (Chorus and Bartram 1999 Chapter 3).
The saxitoxins are a group of carbamoyl and decarbamoyl alkaloids that are either non-sulfated (saxitoxins), singly-sulfated (gonyautoxins), or doubly-sulfated (C-toxins). The various types of toxins vary in potency, with saxitoxin having the highest toxicity. The prevalent toxins in Australian blooms of A. circinalis are the C-toxins. These can convert in the environment or by acidification or boiling to more potent toxins (Negri et al. 1997, Ravn et al. 1995). The half-lives for breakdown of a range of different saxitoxins in natural water have been shown to vary from 9 to 28 days, and gonyautoxins may persist in the environment for more than three months (Jones and Negri, 1997).
Australian significance
Blooms of A. circinalis (D. circinalis) have been recorded in many rivers, lakes, reservoirs and dams throughout Australia, and A. circinalis (D. circinalis) is the most common organism in riverine blooms in the Murray-Darling Basin (Baker and Humpage 1994). In temperate parts of Australia blooms typically occur from late spring to early autumn. The first reported neurotoxic bloom of A. circinalis (D. circinalis) in Australia occurred in 1972 (May and McBarron 1973). The most publicised blooms occurred in the Murray-Darling System in 1991, 2009 and 2010 (NSWBGATF 1992, NSW Office of Water 2009, MDBA 2010). The first bloom extended over 1,000 kilometres of the Darling-Barwon River system in New South Wales (NSWBGATF 1992). A state of emergency was declared, with a focus on providing safe drinking water to towns, communities and landholders. Stock deaths were associated with the occurrence of the bloom but there was little evidence of human health impacts. The blooms in 2009 and 2010 affected several hundred kilometres of the River Murray on the border between NSW and Victoria and included Anabaena, Microcystis and Cylindrospermopsin. Alerts were issued about risks to recreational use, primary contact by domestic users, livestock and domestic animals. A bloom of A. circinalis (D. circinalis) in a dam in New South Wales was shown to have caused sheep deaths (Negri et al. 1995).
Relatively low numbers of A. circinalis (D. circinalis) (below 2,000 cells/mL) can produce offensive tastes and odours in drinking water due to the production of odorous compounds such as geosmin.
Treatment of drinking water
The first line of defence against cyanobacteria is catchment management to minimise nutrient inputs to source waters. Source water management techniques to control cyanobacterial growth include maintaining flow in regulated rivers; water mixing techniques to eliminate stratification and reduce nutrient release from sediments in reservoirs; and the use of algicides in dedicated water supply storages Caution is necessary in using algicides if a bloom has developed because these agents will disrupt cells and liberate saxitoxins which are largely intracellular and can otherwise be removed by cell removal as noted below. Once intracellular toxins are released they are much more difficult to manage. Saxitoxins will eventually be released into the water phase when a developed bloom declines and algal cells lyse, reinforcing the need to prevent blooms as far as possible. Algicide use should be in accordance with local environment and chemical registration regulations. Where multiple intakes are available, withdrawing water selectively from different depths can minimise the intake of high accumulations of cyanobacterial cells at the surface.
Water treatment processes can be highly effective in removing both cyanobacterial cells and saxitoxins. As with other cyanotoxins, a high proportion of saxitoxins remain intracellular unless cells are lysed or damaged, and can therefore be removed by coagulation and filtration in a conventional treatment plant (Chorus and Bartram 1999 Chapter 9). It should be noted that using oxidants such as chlorine or ozone to treat water containing cyanobacterial cells, while killing the cells, will also result in the release of free toxin; therefore pre-chlorination or pre-ozonation are not recommended without a subsequent step to remove dissolved toxins.
Saxitoxins are adsorbed from solution by both granular activated carbon and powdered activated carbon. Because powdered activated carbon may be a more practical option for intermittent or emergency use, it is important to seek advice and carefully select the most appropriate type for toxin removal, as carbons vary significantly in performance for different compounds. Ozone and normal doses of chlorine may not be entirely effective in destroying saxitoxins. Destruction of saxitoxins by chlorine is dependent on both pH and the particular toxin, and toxin destruction only occurs at relatively high pH (Drikas et al. 2002). Boiling is not effective for destruction of saxitoxins.
If treatment is instituted in response to the presence of toxin-producing cyanobacteria, the effectiveness of the process needs to be confirmed by testing for toxin in the product water.
Method of identification and detection
The established method for measuring toxicity due to the presence of saxitoxins/PSPs is the mouse bioassay (Hollingworth and Wekell 1990) which provides a result in terms of equivalence to μg saxitoxin activity (STX-eq). This is the standard method used in association with the shellfish industry and recognised by Foods Standards Australia and New Zealand. Where appropriate standards are available, the analytical technique of high performance liquid chromatography with post-column derivatisation can be used to quantify a range of saxitoxins in both water and cell material (Rositano et al. 1998, Chorus and Bartram 1999 Chapter 13). This information can then be used to derive an estimate of total toxins in terms of saxitoxin equivalents (STX-eq) using a conversion based on specific mouse toxicities given by Oshima (1995) (see Rositano et al. 1998).
A number of immunoassay procedures (ELISA), developed for application to contaminated shellfish, are available for detection of saxitoxins. These assays are highly sensitive to the individual toxins against which antibodies have been generated, however they all show poor cross-reactivity to other saxitoxins. In particular, if antibodies have been generated against STX, there is virtually no response to the C toxins (Cembella and Lamoureux 1993), which are the predominant toxins in some cyanobacteria such as neurotoxic A. circinalis (D. circinalis), and thus these assays may be very poor in determining these compounds
Cyanobacteria are detected by light microscopy, identified using morphological characteristics, and counted per standard volume of water (Hotzel and Croome 1999). Practical keys for the identification are provided in Baker and Fabbro (2002).
Health considerations
There is no evidence of human health effects caused directly by consuming water containing saxitoxin-producing cyanobacteria or PSP-producing dinoflagellates. There are, however, numerous reports of human toxicity associated with consumption of shellfish containing relatively high concentrations of PSPs (Kao 1993). Paralytic shellfish poisoning is an acute disorder that can lead to paraesthesia of the mouth and throat progressing to the neck and extremities, dizziness, weakness, ataxia and muscular paralysis with associated symptoms including nausea, vomiting, thirst and tachycardia. Symptoms can occur within 5 minutes and in fatal cases, death occurs within 2-12 hours. In non-fatal cases, intoxication generally resolves within 1-6 days. The toxin is rapidly cleared by urinary excretion. There are no known chronic effects but long-term animal studies are lacking.
In addition, it has been shown that saxitoxins can accumulate in the Australian freshwater mussel Alathyria condola by filter feeding on A. circinalis (D. circinalis) (Negri and Jones, 1995), and the consumption of contaminated shellfish from water affected by A. circinalis (D. circinalis) blooms therefore represents a potential alternative route of human exposure.
Derivation of HEALTH ALERT
There are insufficient toxicity data to establish a guideline value. An analysis of data from reported events of paralytic shellfish poisoning found that most cases of illness were associated with consumption of in excess of 200 μg STX-eq per person, with a low effect level of 124 μg STX-eq. A health alert value of 3 μg STX-eq/L of drinking water can be calculated for acute exposure associated with occurrence of intermittent blooms of cyanobacteria based on the approach described in Fitzgerald et al. (1999).
where:
124 μg STX-eq is the Low Observed Adverse Effect Level (LOAEL) from published human poisonings (Fitzgerald et al. 1999).
0.5 is the proportion of total daily intake attributed to the consumption of water.
2 L/day is the average amount of water consumed by an adult.
10 is the safety factor derived from use of a LOAEL rather than a NOAEL.
Based on Australian monitoring data, this would require cell densities exceeding 20,000 cells/mL (biovolume of 5 ; based on a mean cell volume of 250 ). Water associated with cell densities of this magnitude would normally be malodorous and unpalatable, with the threshold for off-tastes in water being 1,000-2,000 cells/mL.
Notification procedures
It is recommended that a notification procedure be developed by water and health authorities. A tiered framework should be considered. Initial notification to health authorities could be provided when numbers of A. circinalis (D. circinalis) reach 30% of the density equivalent to 3 μg/L STX-eq/L (6,000 cells/mL; biovolume 1.5 ), while an alert could be provided when cell numbers are equivalent to 3 μg/L STX-eq/L (20,000 cells/mL; biovolume 5 ). For saxitoxin producing species other than A. circinalis (D. circinalis), notifications and alerts should be based on biovolumes.
In all cases, cell numbers should only be used as preliminary signals and as triggers for toxin testing to enable assessment of potential health risks.
Footnote
A change of nomenclature has been proposed for Anabaena to Dolichospermum (Wacklin P, Hoffmann L and Komarek J (2009). Nomenclature validation of the genetically revised cyanobacterial genus Dolichospermum (Ralfs ex Bornet et Flahault) comb nova. Fottea 9: 59-64). Both names are cited due to common usage of Anabaena and recognising that references cited use the name Anabaena.
NOTE: Important general information is contained in PART II, Chapter 5
References
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Baker PD, Fabbro LD (2002). A Guide to the Identification of Common Blue-Green Algae (Cyanoprokaryotes) in Australian Freshwaters. CRCFE Identification Guide No. 25, Cooperative Research Centre for Freshwater Ecology, Albury.
Chorus I, Bartram J (eds) (1999). Toxic Cyanobacteria in Water: A guide to their public health consequences, monitoring and management. E&FN Spon, London.
Drikas M, Newcombe, G, Nicholson B (2002). Water treatment options for cyanobacteria and their toxins. In: Blue-Green Algae: Their significance and management within water supplies. CRC for Water Quality & Treatment Occasional Paper 4, pp 75-92.
Fitzgerald DJ, Cunliffe DA, Burch MD (1999). Development of health alerts for cyanobacteria and related toxins in drinking water in South Australia. Environmental Toxicology, 14(1):203-207.
Hollingworth T, Wekell MM (1990). Paralytic Shellfish Poison In: Official Methods of Analysis of the Association of Official Analytical Chemists, 15th edition. Association of the Official Analytical Chemists, Washington DC, pp 881-882.
Hotzel G, Croome R (1999). A Phytoplankton Methods Manual for Australian Freshwaters. LWRRDC Occasional Paper 22/99, Land and Water Resources Research and Development Corporation, Canberra.
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Kao CY (1993). Paralytic Shellfish Poisoning. In: I Falconer, Ed, Algal Toxins in Seafood and Drinking Water. Academic Press, London, pp 75-86,
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Negri AP, Jones GJ, Blackburn SI, Oshima Y, Onodera H (1997). Effect of culture and bloom development and of sample storage on paralytic shellfish poisons in the cyanobacterium Anabaena circinalis. Journal of Phycology, 33:26-35.
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Rositano J, Nicholson B, Heresztyn T, Velzeboer R (1998). Characterisation and Determination of PSP Toxins in Neurotoxic Cyanobacteria and Methods for Their Removal from Water. UWRAA Research Report No. 148. Urban Water Research Association of Australia, Melbourne.
Velzeboer RMA, Baker PD, Rositano J (1998). Characterisation of Saxitoxins Produced by the Cyanobacterial Genus Anabaena in Australia. UWRAA Research Report No. 135. Urban Water Research Association of Australia, Melbourne.
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