Cyanobacterial Toxins of Drinking Water Supplies: Cylindrospermopsins and Microcystins - Chapter 13 (end) pdf

259 13 Emerging Issues The whole research field of toxic cyanobacteria and associated hazard to human health is relatively new, with the first international meeting on the topic at Wright State University, Dayton, Ohio, in 1980 (Carmichael 1981). Since that time there have been very considerable developments in understanding of the organisms, toxins, health impacts, and processes for the removal of unwanted substances in the treat- ment of drinking water. A landmark development in public safety was the determi- nation in 1998 by the World Health Organization (WHO) of a provisional Guideline Value for microcystin-LR in drinking water. This has been followed by individual national and regional governments adopting a regulatory standard for microcystins in drinking water, which has been closely related to the WHO guideline of 1.0 µ g/L. The regulations have varied from the WHO guideline only by specifying toxicity equivalent to microcystin-LR, thus including all microcystins, and through different population body weights and assumed proportion of the toxin in drinking water. In addition, New Zealand and Brazil have adopted Maximum Acceptable Values or Guideline Values for cylindrospermopsin of 3.0 and 15 µ g/L, respectively, in drinking water. The increasing acceptance that cyanobacterial toxins in drinking water may be a hazard to human health has raised a series of currently unanswered questions, which fall roughly into three areas: ecological, health, and water treatment. In the ecological area, the dominant question is how to respond to the changing world, in which population pressure and global warming are together intensifying the likeli- hood of the appearance of toxic cyanobacteria in water sources. In the health field, the largest unresolved issue is the relationship between cyanobacterial toxins and human cancer. In water treatment, the main question is the most cost-effective way of providing safe water for the population, including safety from cyanobacterial toxins. 13.1 ECOLOGICAL ISSUES The continuing growth of the overall world population, together with the need for additional agricultural production, is placing increased demands on water supply. Intensification of agriculture has resulted in greater use of water for irrigation as well as greater nutrient loads within water catchments. In more arid areas, including those with Mediterranean climates, groundwater has often provided the bulk of the supply for drinking and for irrigation. In parts of the U.S. and Australia, groundwater depletion and deterioration in quality are forcing a reexamination of the use of surface water, which was earlier rejected because of eutrophication or contamination. TF1713_C013.fm Page 259 Tuesday, October 26, 2004 2:25 PM Copyright 2005 by CRC Press 260 Cyanobacterial Toxins of Drinking Water Supplies In circumstances where demand for irrigation water has much depleted water flow in rivers, the flow reduction is frequently accompanied by eutrophication in the summer months. This may be exacerbated by discharge of treated or untreated sewage into the rivers. Use of such supplies as sources of drinking water increasingly leads to problems with toxic cyanobacteria in the drinking water. Use of eutrophic water for drinking water supply raises the need for fast and clear identification of toxic organisms. The recent development of genetic approaches offers the most promising advance, since the presence or absence of toxin-coding genes provides the key information for potential hazard. With the use of genetic identification of toxic potential, there is no need for species identification or toxin measurement in the first instance. If a potential hazard requires quantitation, which is secondary to the initial identification, then cell numbers or biovolume and toxin concentration are required. The genetic methods lend themselves to development of automated techniques capable of handling multiple samples rapidly. They can also be developed as simple color test kits, capable of use in basic laboratories or even at the water’s edge. Global warming has implications for the world distribution of cyanobacterial species. In particular, the organisms now mainly restricted to subtropical and tropical regions will be enabled to grow in cooler latitudes. This is already evident from the increased range of Cylindrospermopsis raciborskii , which was first characterized as a tropical species in Indonesia and has since been found in Australia, Thailand, Brazil, the U.S., Canada, France, Germany, and Hungary. The other widely distrib- uted species producing cylindrospermopsin is Aphanizomenon ovalisporum , reported in the Mediterranean region and Australia in the subtropics. This too can be expected to increase its range with global warming. C. raciborskii is capable of producing a divergent range of toxins, including cylindrospermopsins, saxitoxins, and so far unidentified neurotoxins and hepatotox- ins. As a species that can form dense concentrations in the lower epilimnion, out of sight of surface observation, it poses a problem for reservoir managers, who may not detect its presence until the cyanobacterial filaments block the filters at a treat- ment plant. As the filaments leak toxin into the water, potentially high concentrations of dissolved cylindrospermopsin may reach consumers. The need for effective reservoir and catchment management is becoming more important as eutrophication increases. In both areas of management the problems are intractable, with reservoir management often the only option. The most common technique for controlling cyanobacterial blooms (in countries where it is legal to do so) is to add herbicide to the water. Copper sulfate is the most widely used herbicide, added in some reservoirs every 14 days in summer. This has highly adverse impacts on the local ecology and can lead to the appearance of copper-resistant organisms. It also releases toxin when the cyanobacterial bloom lyses. The other technique in wide use is compressed-air destratification, in which diffusers on the reservoir bottom send streams of air upward, mixing the deeper and surface layers. This has had variable success, leaving the issue of a cost-effective method of reservoir manage- ment open for further investigation. The need for catchment management on a long-term basis to reduce nutrient inputs to water storage sites is becoming more important and also more difficult. TF1713_C013.fm Page 260 Tuesday, October 26, 2004 2:25 PM Copyright 2005 by CRC Press Emerging Issues 261 Intensity of land use in catchments inevitably rises, with increases in population leading to pressure for residential development, agricultural intensification, and access to recreational facilities. Control of catchments is made more difficult by private land ownership, increasing land value, and — in less developed countries — occupation by squatters. In many instances effective catchment management is a political impossibility. Some of the most polluting activities can be controlled by regulations, such as discharge of high-nutrient waste by agroindustry and wastewater treatment plants into drinking water storage catchments, but lower-intensity dis- charges — such as unsewered residences, high-density but small-scale livestock farming, and the use of fertilizer — are practically uncontrollable. Studies on optimal management strategies for partially polluted drinking water catchments are required to provide a factual basis for political decision making. 13.2 HEALTH ISSUES Although there is much basic information on the acute toxicity of microcystins and cylindrospermopsins, there are very significant gaps in our knowledge of chronic effects. In particular little is known at present on possible actions of cylindrosperm- opsin on teratogenesis, reproductive health, and carcinogenesis in humans or exper- imental mammals. The increasing threat posed by cylindrospermopsin in drinking water as a result of global warming makes the urgency of research more acute. The present preliminary data raise the possibility of cylindrospermopsin being a potent human carcinogen. This requires investigation by carcinogenicity trials using formal, established protocols. These trials must be supported by studies of the molecular mechanisms of cylindrospermopsin carcinogenicity as well as genotoxicity studies. Where possible, epidemiological studies of cancer rates in human populations known to be exposed to cyanobacterial toxins in drinking water should be undertaken. Because of the urgent need for the WHO to set a provisional Guideline Value for cylindrospermopsin in drinking water, the available subchronic toxicity data for experimental animals should be used in the absence of carcinogenicity data. There are several relevant studies, in particular one following the protocols of the Organi- zation for Economic Cooperation and Development (OECD) for such trials, which can provide a sufficient basis for a provisional determination. The carcinogenicity of microcystins is still an unresolved issue. The evidence for tumor promotion in experimental animals is very strong, but the relevance of this to human cancer has not yet been clearly shown. The most informative data come from southern China, where the rate of human hepatocellular carcinoma has been related to hepatitis, aflatoxin in the diet, and the drinking of surface water. The component of surface water that is proposed as the active agent in the cancer rate is microcystin. Because microcystin is taken up and excreted through the gut, there is also the possibility of increased rates of gastrointestinal tumors in the human population caused by microcystin. This awaits further exploration. In the mouse colon, tumor precursors were shown to grow faster with microcystin in the drinking water. Until an exposure marker for cyanobacterial toxin intake in the human population is demonstrated, it is very difficult to carry out effective human epidemiology. This TF1713_C013.fm Page 261 Tuesday, October 26, 2004 2:25 PM Copyright 2005 by CRC Press 262 Cyanobacterial Toxins of Drinking Water Supplies requires the comparison of known rates of intake of the postulated carcinogen with cancer rates, so that a dose–response relationship can be demonstrated. Without accurate measures of exposure, only indicative information can be acquired. An exposure marker that directly relates to toxin intake would remove the need to measure the actual intake of individuals, as it would give a cumulative intake measurement. Research in this area on both cylindrospermopsin and microcystin is needed. Since the recognition that endocrine-disrupting compounds in drinking water may harm human health, the number of chemical compounds that may require to be monitored in drinking water has become very large. Together with the present chemical substances for which Guideline Values or Maximum Contaminant Levels have been set, water utilities may in the future be expected to measure very large numbers of substances that are at concentrations below detection limits in most instances. To alleviate the necessity of carrying out uninformative analyses for compounds that are not likely to be present at detectable levels in any particular water supply, and to provide a proactive approach to safe drinking water, the WHO is developing a quite different methodology for drinking water safety. It is entitled Water Safety Plan , which would be required to be set up individually for each drinking water supply utility (WHO 2003). The basic approach is that of risk assessment at each stage of the drinking water process by evaluating the potential risks of harmful contaminants from the catchment and source water and their reduction, the risks and risk reduction within the treatment system, and risks of contamination and risk reduction in the distribution system. This would be done on an individual plant basis, so that the critical risks are assessed and minimized, and monitoring is based on the risk assessment. It is intended to include a process of continual revision and improve- ment, with legislative backing for the whole process and formal approval of the plan. The New Zealand government is implementing such an approach in 2005 on the basis of a structure set up and publicized in 2001 with the title How to Prepare and Develop Public Health Risk Management Plans for Drinking Water Supplies (New Zealand Ministry of Health 2001). The supporting information includes risk management and alert levels for cyanobacterial contamination of drinking water reservoirs, which are ongoing management problems for New Zealand drinking water utilities. The Australian National Health and Medical Research Council is developing a comparable approach to safety in drinking water based on planning of multiple barrier risk reduction and assessment of critical control points (Australian Depart- ment of Health 2003). When the WHO Water Safety Plan final document is available it is likely that a range of other countries will evaluate this new approach to drinking water safety. 13.3 WATER TREATMENT There are several emerging issues related to water treatment that arise through legislative and technical development. Now that countries are adopting as regulations the WHO provisional Guideline Value for microcystin in drinking water, the need TF1713_C013.fm Page 262 Tuesday, October 26, 2004 2:25 PM Copyright 2005 by CRC Press Emerging Issues 263 for standardized test protocols for these toxins becomes acute. The increased interest in applying Hazard Analysis at Critical Control Points will reduce water industry requirements for analysis in circumstances where toxins are unlikely, but will not remove the need for accurate and straightforward analytical techniques that can be applied when needed. Cross-laboratory validation of methods is essential and has commenced. Similarly, when a WHO Guideline Value for cylindrospermopsins is determined, standardized analytical methods will be required, which can be undertaken with normal laboratory facilities and at moderate cost. An enzyme-linked immunosorbent assay may be the most feasible. Treatment technologies are in a process of continual development, with the newer approaches of titanium oxide/UV oxidation and membrane filtration offering the possibility of lower operating costs than the present highly effective ozone/activated carbon treatment. Conventional coagulation/sedimentation offers some protection from cyanobacterial toxins, but only with focused operational provisions in place. As toxic blooms become more common and occur to a greater extent in drinking water storage sites, upgrading of water treatment practices will be increasingly necessary. Because a substantial proportion of the population in the developing world does not have access to treated water, consumption of cyanobacterial toxins is more likely among them. Often surface water is the only supply, with seasonal eutrophication. Very low-cost methods of purifying water from pathogens and toxins are needed. Boiling is common practice to reduce enteric disease transmission, but it does not remove microcystins or cylindrospermopsins. It is possible that low-cost slow sand filters can be developed, based on 44-gal drums half filled with sand, for remote and rural families. These would reduce hazards from protozoa, bacteria, and metab- olizable organic contaminants as well as to cyanobacterial toxins. REFERENCES Australian Department of Health (2003). Public consultation draft framework for Management of Drinking Water Quality: A Preventive Strategy from Catchment to Consumer. www.health.gov.au/nhmrc/publications/synopses/eh19syn.htm. Carmichael, W. W. (1981). The Water Environment . New York, Plenum Press. New Zealand Ministry of Health (2001). How to Prepare and Develop “Public Health Risk Management Plans for Drinking Water Supplies .” www.moh.govt.nz/water. WHO (2003). Guidelines for Drinking Water Quality. Third Edition. Draft Chapter 4, “Water Safety Plans.” www.who.int/docstore/water_sanitation_health/GDWQ/Updat- ing/draftguide1/draftchap4.htm. TF1713_C013.fm Page 263 Tuesday, October 26, 2004 2:25 PM Copyright 2005 by CRC Press . considerable developments in understanding of the organisms, toxins, health impacts, and processes for the removal of unwanted substances in the treat- ment of drinking water. A landmark development in public. to microcystin-LR, thus including all microcystins, and through different population body weights and assumed proportion of the toxin in drinking water. In addition, New Zealand and Brazil have. cylindrospermopsin of 3.0 and 15 µ g/L, respectively, in drinking water. The increasing acceptance that cyanobacterial toxins in drinking water may be a hazard to human health has raised a series of currently