Occupational and Environmental Medicine 1 994;53:649-651 649

Occupational and Environmental Medicine 1 994;53:649-651 649 Occupational and Occup Environ Med: first published as 10.1136/oem.53.10.649 on 1 October 1996. Downloaded from Environmental Medicine Editorial Drinking water quality: new challenges for an old problem The provision of safe drinking water has been a high pri- population does not necessarily protect subgroups that ority in public health ever since John Snow removed the are more susceptible to water borne organisms. Patients handle from the Broad Street pump more than a century with HIV infection and AIDS, for example, are at risk of ago to quell an epidemic of cholera in London. Today, severe and intractable diarrhoea from Cryptosporidium, a three main issues surrounding the provision of safe drink- parasite which has recently been identified as an impor- ing water are emerging. These are: to better define the tant contaminant in drinking water.3 As these patients burden of gastroenteritis attributable to drinking water represent only a very small fraction of the community, and the organisms which cause it; to identify those water local measures, such as the use of specialised water filters treatment systems which are cost effective and safe; and in their homes, may be an appropriate preventive mea- to determine the optimal methods for monitoring drink- sure.4 In contrast, patients taking acid suppressing med- ing water to ensure its continuing safety. ication to prevent peptic ulceration are common;- and have been found to require a lower infective dose of some water borne organisms to develop clinical disease.67 True burden of disease Once the true burden of endemic gastrointestinal dis- Defining the amount of gastroenteritis that is attributable ease from drinking water has been identified, the next to drinking water is difficult, especially when cases are major step is to define the specific organisms responsible. widely distributed in time and space. Large outbreaks of Knowledge of this is important because it will help to gastroenteritis that occur over a short period and in a identify the water treatment processes which will be most confined area are usually relatively easy to identify. In the effective. On the basis of the most recent data, newly Milwaukee outbreak, for example, an estimated 400 000 emerging organisms-including parasites such as Crypto- cases of diarrhoea occurred over a 14 day period in a sin- sporidium and viruses such as Norwalk-are likely to be gle city and were easily visible above the background inci- making a greater contribution to water borne endemic dence.1 In contrast, smaller outbreaks involving fewer gastroenteritis than are bacteria.89 people, over a longer period, and over a wide geographi- Concern about the threat from Cryptosporidium in cal area, are more difficult to detect from the background drinking water, in the wake of the Milwaukee outbreak, http://oem.bmj.com/ incidence without the use of complex surveillance sys- led to a major workshop in the United States in 1994 to tems. consider methods to reduce the public health impact of What is almost impossible to answer, without a spe- this organism. cific aetiological study, is whether water borne gastroenteritis is endemic and hence contributes significantly to the background incidence of disease. Recent research has Water treatment processes suggested that much water borne gastroenteritis is Water treatment systems in current use have been pri- on September 24, 2021 by guest. Protected copyright. endemic and causes a greater burden of disease than that marily developed to control pathogenic bacteria and occurring during the highly publicised outbreaks. A improve the aesthetic quality of water. Chlorine is highly Canadian intervention study, for example, found that effective against pathogenic bacteria, but usually has little almost one third of the gastroenteritis occurring outside a effect on many parasites found in drinking water." The known epidemic was attributable to drinking unfiltered addition of flocculation and deep sand filtration to disin- water. In this study, households were randomised to fection systems are used primarily to reduce the turbidity receive either their normal tap water or bottled water of drinking water, which can bring about a substantial from which all organisms had been removed. Those who reduction in the number of cryptosporidial cysts, but received tap water had 30% more gastroenteritis than result in little effect on viruses. Even in water treated those who drank bottled water. As participants knew with all these methods, and in which all current water which type of water they were receiving, this may have quality guidelines are being met, large outbreaks can still influenced their reporting of gastrointestinal symptoms, occur. Unusual weather conditions may contribute to and therefore this type of study will need to be repeated this, as in the Milwaukee outbreak, which occurred after with participants blinded. heavy rains caused increased turbidity and proliferation This study was conducted in an area that received of cryptosporidial cysts in the water.'2 drinking water which had been treated with a combina- Water treatment is expensive. In some cases, major tion of flocculation, chlorination, and sand filtration, and changes to treatment systems are clearly justified, as it which generally met all water quality standards. In areas was in Milwaukee where chlorine resistant pathogens where the water quality is substantially worse, a greater were the cause of the problem. In contrast, in some pro- proportion of the background incidence of gastroenteritis tected water catchment systems, where the quality of the is likely to be due to drinking water. source water is very high after chlorination alone, there Providing drinking water that does not significantly may be little to be gained by upgrading the water treat- contribute to endemic gastroenteritis among the entire ment process. The source and site of microbiological 650 Editorial contamination are also important determinants of the assay for Cryptosporidium, for example, requires large vol- type of water treatment system required. For example, if umes of water when low levels of detection are required, Occup Environ Med: first published as 10.1136/oem.53.10.649 on 1 October 1996. Downloaded from Cryptosporidium enters the system downstream of a physi- and can cost several hundred dollars to perform. More cal barrier such as sand filtration, further upgrading of importantly, however, the relevance to health of such a the system at the level of the source water will have little result is unknown. Cryptosporidium is often found at low effect. Before decisions can be made about the most concentrations in water samples from cities where no appropriate treatment for a particular water system, it is recognised outbreak has occurred." This may indicate important to know how many cases of disease are attrib- that the cysts are not viable or are below the infective utable to drinking the water in that system, and what dose. An arbitrary detection level of 10-30 oocysts per effect the introduction of various water treatment litre has been set on the basis that recognised outbreaks processes is likely to have on reducing this number, so have usually been associated with higher concentrations that the cost per episode of human disease prevented can than this.'7 If a single sample contains organisms at this be calculated. level, then the water authority should consider additional In recent years, concern has been expressed about the sampling, expedited processing of samples, and attention safety of some methods used to disinfect and otherwise to other characteristics of the performance, of the treat- treat drinking water. Chlorine, for example, is known to ment plant. react with organic matter within drinking water distribu- An accurate, cheap, and effective means of testing tion systems to form low levels of trihalomethanes. There water to ensure that it is safe from all pathogens does not is some epidemiological evidence that trihalomethanes currently exist. It is hoped that new technology that is may be carcinogenic in humans.'3 This evidence is not easier to perform, such as the polymerase chain reaction, strong, however, as many of the cancer studies were eco- will provide the basis for such assays, although the rele- logical in design with poor indices of exposure relating to vance of a positive test in terms of human health needs to water intake and hence have resulted in considerable be clarified before such tests are routinely advocated. uncertainty in the ingested dose of trihalomethanes. In summary, the effective prevention of gastroenteritis Recently, the ability of epidemiological methods to detect from drinking water requires a large amount of data health effects from low level environmental exposures, which are currently not available. Firstly, we need to often the case in chemical contamination of drinking know what proportion of endemic gastroenteritis from water, has been seriously questioned. 14 Nevertheless, each specific organism is attributable to drinking water. potential adverse effects from chemical additives used in Armed with this knowledge we then need to know what the provision of drinking water need to be considered in water treatment systems will remove these organisms and any cost benefit analyses of different water treatment sys- assure that these methods are both safe and cost effec- tems. tive. To ensure that water quality is maintained we need In some

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