Contents
1.1 Document purpose and scope
1.2 Target audience
1.3 Document structure and overview
References
Safe drinking-water is of paramount importance for human health. Throughout history, access to drinking-water has been a prerequisite for the development of civilisations – and the loss of access often a key factor for their decline. Recognising the vital role of drinking-water for public health, the World Health Organization (WHO) dedicates a significant share of its efforts to promote the safety of water for today and for the future (Onda et al., 2012). With a human population reaching 10 billion by the mid of this century, the pressure on global drinking-water resources will not cease, and ongoing efforts in research, management and governance are needed to recognise, understand and mitigate health risks associated with water use. This includes further uses of water involving human exposure, particularly for recreation, and, depending on specific local or regional circumstances, also for irrigating crops, cooling water or dust suppression, for example.
Health hazards recognised in water today comprise infectious microorganisms (e.g., bacteria, viruses and protozoa causing gastrointestinal diseases), geogenic substances (e.g., arsenic, fluoride, uranium), industrial and agricultural chemicals (e.g., perfluorinated chemicals [PFCs], pesticides) and toxins produced by cyanobacteria – the subject of the first edition of “Toxic Cyanobacteria in Water” (Chorus & Bartram, 1999) and of the present volume.
Among the hazards considered in the Guidelines for Drinking Water Quality (GDWQ; WHO, 2017), infectious microorganisms are the most significant causes of mortality on a global scale, causing a substantial burden of disease via diarrhoeal illnesses such as cholera, cryptosporidiosis or retroviral enteritis (James et al., 2018; Roth et al., 2018; Prüss-Ustün et al., 2019). In contrast, the contribution of toxic chemicals in water to morbidity and mortality is rarely acute, and aside from a few geogenic chemicals, the impacts on health are less visible and less clearly attributable to chemicals. This applies particularly for carcinogenic compounds, the impacts of which accumulate over time. Thus, considering global causes of mortality and morbidity, data to estimate the disease burden through exposure to chemicals in water are typically lacking.
This is also true for cyanobacterial toxins: only a relatively low number of recorded cases of acute human intoxication are clearly attributable to these toxins (Wood, 2016). Nonetheless, like with other toxins potentially found in drinking-water, exposure to low, subacute concentrations is possible because drinking-water is an indispensable part of the human diet, and hence, exposure is difficult to avoid – abstinence or replacement from alternative sources for longer periods is not a feasible option in most settings.
Compared to other agents that may occur in water and that are covered in the GDWQ, the occurrence and behaviour of cyanotoxins is fundamentally different and consequently requires different management approaches. On the one hand, the producing cyanobacteria need to be addressed as microorganisms that can proliferate in surface waters – but which are not in themselves infectious – requiring measures to reduce their occurrence that shift management from microbiology to ecology. On the other hand, their toxins are chemicals and need to be addressed as such, including the derivation of values for maximally tolerable concentrations and the development of technical methods to reduce their concentration through drinking-water treatment.
Other unique characteristics include:
Cyanotoxins are among the most toxic naturally occurring compounds: lethal doses are in the same range as some toxins from mushrooms (amanitin, phaloidin) or plants (aconitine, strychnine, atropine).
Cyanotoxins occur worldwide in many lakes, reservoirs and rivers used as sources of drinking-water or for recreational activity.
Contact with toxic cyanobacteria is difficult to avoid without implementing severe restrictions: most people who enjoy swimming in natural waters most likely have been in contact with toxic cyanobacteria.
The occurrence of toxic cyanobacterial blooms is often not perceived as a danger by the public in the same way as a spill of an industrial toxin or chemical with the same hazard potential would be, because it may be regarded as “natural” and hence innocuous.
Cyanotoxins are produced naturally within surface waters and are not, like most chemicals for which guideline values have been set or proposed, directly introduced by human activity. For many of the anthropogenic contaminants, legislation regulating their use and release into the environment has successfully reduced concentrations in ground or surface waters an approach that is not practicable for cyanobacterial toxins.
The control of toxigenic cyanobacteria is complex and typically requires efforts on scales beyond the water supply and waterbody with its immediate environment, potentially including the management of entire catchments and requiring longer-term investments (e.g., in sewage management) as well as political decisions with wider impact (e.g., on fertiliser use).
Thus, cyanobacteria and their toxins pose specific challenges, and guidance with respect to their management warrants a dedicated WHO publication.
Cyanobacteria have been present in natural ecosystems since the Precambrian Era, some 2 billion years ago (Wilmotte, 1994), and the production of cyanotoxins is probably an equally ancient characteristic (Rantala et al., 2004). The first scientific report on toxic cyanobacteria dates from the late 19th century (Francis, 1878), but earlier historical records have been interpreted as similar poisoning events (Codd et al., 2015). Studies on cyanobacterial toxins in lake sediments found microcystins (Zastepa et al., 2017) and cylindrospermopsin (Waters, 2016) in layers deposited well before the 20th century. In comparison with more recent sediments, in most cases, the assumed historic concentrations were, however, much lower than those found in today’s eutrophic lakes.
In large parts of the world, waterbody eutrophication started accelerating in the middle of the 20th century, in the wake of urbanisation and industrialisation. Since that time, massive cyanobacterial blooms have occurred in many lakes and reservoirs in which this phenomenon was not known before. Therefore, it is not the biosynthesis of toxins itself that created a new health hazard, but the more recent significant proliferation of toxic cyanobacteria in waterbodies as a result of human activities. This health hazard most probably will gain growing importance as cyanobacterial blooms are expected to increase at the scale at which eutrophication is expected to increasingly occur in many regions of the world (Huisman et al., 2018).
Whether or not global warming is likely to increase cyanobacterial proliferation depends on specific conditions in a particular waterbody. In order to support the inclusion of climate change scenarios in risk assessment and management (e.g., water safety planning), this book includes information on how these conditions may influence cyanobacterial growth and bloom formation.
Cyanobacteria can produce a huge diversity of secondary metabolites, the biosynthetic pathways of which are known for a number of individual compounds or compound classes, respectively. Only a small share of the known metabolites shows toxic effects, but these cyanotoxins have caused numerous cases of poisoning of farm or wild animals, which demonstrate their toxic potential (Wood, 2016; Svirčev et al., 2019) and which suggests that animal illnesses and deaths are sentinel events for human health risks (Hilborn & Beasley, 2015). A large body of evidence from experimental studies with laboratory animals has elucidated their mode of action: some cyanotoxins are highly neurotoxic and others can damage the liver, kidney or other organs when ingested.
Epidemiological studies have looked for chronic effects in human populations exposed to toxic cyanobacteria, and indeed, a number of studies since the mid-19th century associate symptoms with cyanotoxin exposure. The key caveat of several of these anterior studies is the lack of data on the dose to which the population might have been exposed and a lack of analytical tools for detecting other hazards at that time, such as molecular techniques for the detection of pathogenic viruses. However, although our current knowledge may question some of the epidemiological evidence frequently quoted to highlight the cyanotoxin hazard, the evidence from animal experiments is clear and sufficient to derive guideline values for a range of cyanotoxins.
In this respect, cyanotoxins are in line with most other substances for which World Health Organization (WHO) has set guideline values: this is not typically done because a substance has been widely shown to cause human illness or result in fatalities through water consumption, but rather because a substance has significant toxic properties and water is recognised as a relevant pathway for exposure. Given the widespread occurrence of cyanobacteria – as compared to the occurrence of many purely anthropogenic contaminants in water – cyanotoxins are likely to occur more widely and more often in concentrations of potential concern than many of the other chemicals considered in the Guidelines for Drinking Water Quality (WHO, 2017).
1.1 Document purpose and scope
The second edition of “Toxic Cyanobacteria in Water” presents the state of knowledge regarding the impact of cyanobacterial toxins on health through the use of water and provides guidance on assessing and managing the risks of cyanobacteria and their toxins in order to protect drinking-water sources and recreational waterbodies. It further provides an overview of exposure through other important sources, including food, use of dietary supplements and through dialysis.
This edition is an update of the first edition of this publication, which was published 20 years ago (Chorus & Bartram, 1999). In addition to updating the state of knowledge specifically related to cyanobacteria and their toxins, this updated edition accounts for developments in and best practices for water supply management, namely, water safety planning, as well as the broader state of knowledge on climate change, eutrophication and others.
Water safety planning (see Box 1.1) is a comprehensive preventive risk assessment and risk management approach, and is a critical component of WHO’s Framework for Safe Drinking-Water, to most effectively ensure drinking-water safety. Most importantly, the Water Safety Plan (WSP) approach systematically addresses all steps in a water supply from catchment to consumer (Bartram et al., 2009).
While the concept of WSP development is tailored to drinking-water supplies, many of its elements can be applied to the assessment and management of other potential exposure routes. For food safety – fish and shellfish in the context of this volume – the related concept of HACCP (Hazard Analysis Critical Control Points; from which WSPs were developed) applies and can readily be linked to WSP elements. The WSP approach is currently being developed for application to other areas of water management, that is, as Sanitation Safety Plans and Recreational Water Safety Plans. Among the hazards relevant to water, cyanobacteria are often likely to expose people through multiple pathways, and adopting a WSP approach will provide the most effective approach to protecting their health.
Box 1.1 Developing a Water Safety Plan (WSP)
Drinking-water safety often relies heavily on the verification of compliance to water quality standards. However, by the time laboratory results show noncompliance, the population served will already have consumed the water and become exposed – and in the case of pathogens, many people may thus become ill. Therefore, “end-of-pipe” monitoring alone is insufficient to guide management decisions. The WSP approach shifts the emphasis of drinking-water quality management to a holistic risk-based approach that covers all pro...
