Water-borne pathogens are agents that cause disease. They are a growing international hazard, not to mention a global economic burden. Waterborne pathogens can also kill; the vast majority of preventable deaths occur in children below five years of age, particularly in developing countries.
No country is immune. Even in OECD countries, the number of outbreaks reported in the last decade demonstrates that transmission of pathogens by drinking water remains a significant problem and that, despite substantial advances in recent years, access to safe drinking water is still a major public health challenge.
In 1993, a major outbreak of gastro-intestinal illness caused by a parasite commonly harboured by cattle, Cryptosporidium, was reported in Milwaukee, the largest city in the US state of Wisconsin. Some 400,000 residents were infected, and it caused more than 60 deaths. Cost estimates for this outbreak alone exceeded $54 million. The outbreak occurred in water that met guidelines for traditional indicators of microbial contamination. It revealed the vulnerability of US water systems. For OECD countries, the Milwaukee outbreak underscored the severe consequences of waterborne diseases. More recent outbreaks have involved E. coli O157:H7. In spring 2000 in Walkerton, Ontario (Canada), one such outbreak resulted in over 2,300 cases of infection and six deaths.
The number of outbreaks reported in the last decade demonstrates that transmission of pathogens by drinking water remains a significant problem and that, despite substantial advances in recent years, access to safe drinking water is still a major public health challenge.
There are probably a mix of reasons for the outbreaks: the discharge of greater quantities of wastewater, the ageing of water treatment infrastructure, inadequate treatment, and the increasing occurrence, or perhaps the increasing recognition and detection, of organisms resistant to conventional disinfection.
Contamination of water distribution systems can be caused via cross-connections, back-siphonage, corrosion, or construction and repairs of the distribution system. Waterborne epidemics can also be caused by contaminated groundwater.
This was probably the case in Sweden when from 1980 to 1999, 116 outbreaks of waterborne diseases were reported, affecting about 58,000 people. An organism detected was Campylobacter, though about 70% of the outbreaks were due to unknown agents causing acute gastrointestinal illness. And between 1991 and 2000, 41 outbreaks were reported in the UK, with more than 3,768 reported cases of illness. Most of these outbreaks were due to Campylobacter and Cryptosporidium, an emerging pathogen that many water supply systems struggle to cope with.
These cases emphasise the urgency of reviewing the effectiveness and reliability of methods, management approaches, and technologies for guaranteeing the microbiological safety of drinking water.
Assessment of the microbial quality of drinking water is based largely on culture techniques. These do not detect specific waterborne pathogens but rely on the monitoring of indicator bacteria like coliforms and enterococci, which reveal the potential presence of microbial pathogens of faecal origin.
The use of bacteria as indicators has proved successful in preventing the spread of waterborne cholera and typhoid, and protecting against bacterial pathogens such as salmonella and shigella. But it is not as reliable for detecting contamination by viruses and protozoa.
Moreover, traditional means of assessing microbial water quality were originally developed for natural waters and are less suited for monitoring water after disinfection. They were most commonly used for testing drinking water at the tap; this end-product testing comes too late.
Various aspects of the supply chain have to be monitored, requiring different techniques, parameters and approaches, and, above all, an integrated management method that takes the local context and needs into account.
The World Health Organisation and OECD have produced a guidance document as a basis for risk management decisions at every point in the system. It gives guidance on selecting and using various parameters and technologies to meet specific information needs and to support safe practice throughout the water system: catchment protection and assessment, assessment of source-water quality and of treatment efficiency, and monitoring of drinking water quality at the point of leaving the treatment facility and throughout the distribution system. It is in effect a total system approach for improved drinking water quality.
The aim is to control each treatment step so as to prevent contaminants from reaching the consumer. Consideration is also given to tolerable risk, water-quality targets, public health status, and education. Thus, risk management can no longer be confined to a single organisation or agency; national, regional and local governments, water authorities, water supply agencies, and public health authorities all play a role. This creates significant challenges for co-ordination as well as production of useful and compatible data since each of these stakeholders has specific responsibilities and information needs.
But systems are not enough, detection techniques also have to be improved. This is where science comes in. Emerging molecular methods are likely to make a significant contribution by increasing the chances of detecting a pathogen from an implicated source of drinking water, particularly in the case of viruses with no readily available or rapid method of culture. These include the likes of rotaviruses, astroviruses, caliciviruses, and the hepatitis A virus.
Traditional methods for detecting viruses are based on tissue-culture techniques that can take several weeks. Thanks to rapid advances in biotechnological research of the last few years, a wide range of new genetic (nucleic-acid- based) and immunological tools are now available and some molecular techniques appear particularly promising. They can offer faster, more sensitive and specific ways of detecting micro-organisms. For example, genotyping, or molecular characterisation, is a powerful new tool for identifying the source of microbial contaminants and is already in routine use for detecting Cryptosporidium in some OECD countries. On the horizon are methods based on micro-arrays and biosensors.
Advances in semiconductors and computers are expected to allow the next generation of microbial sensors to be small and simple devices which are quick to respond. The future thus holds the promise of new indicators for detecting both existing and emerging pathogens.
At present, water industry data are comprehensive and complex but underutilised, especially by the public health community. The epidemiological data, in contrast, are relatively heterogeneous, reactive, and limited in their risk identification potential.
A compatible data system that all countries and water-system authorities could draw on, particularly health and technical data, would boost water risk management.
EPISYS, an experimental system being used in north-eastern England in a collaboration between the health sector and the water industry (North of Tyne Communicable Disease Control Unit and Northumbrian Water Limited), might serve as a model. The project uses technical data and healthcare outcome data in a bid to obtain real-time, geographically correlated health data. Its person- or symptom-based system is timely and sensitive, and amenable to rapid, community-level, response.
So far, the system has been found to detect traditional episodes (salmonella in school children), track non-notifiable illnesses (viral community outbreaks), and detect episodes for which there is otherwise no data (viral illness, data from multiple sources).
As ever, resources are needed to increase the usefulness and broad applicability of the new technologies in the pipeline. Many challenges remain in the pursuit of safe drinking water for all. Water shortage today affects places as diverse as Italy and Malaysia. By 2025 water shortage may affect as many as 3 billion people. As a consequence, a large part of mankind today is increasingly forced to use surface water, water drawn from polluted rivers, irrigation canals, ponds and lakes and to re-use wastewater. Sustainable strategies will require significant effort from both public and private sectors and capital expenditure. It will not only be essential to improve the integration of the world’s regulatory approaches, but the science of monitoring and data collection as well. Only then will demand for safe drinking water be met.
CRC/IWA (2002), Drinking Water and Infectious Diseases: Establishing the Links.
IWA/OECD/WHO (2003), Assessing Microbial Safety of Drinking Water: Improving Approaches and Methods.
OECD (1997), Biotechnology for Water Use and Conservation, OECD, Paris.
OECD (1999), The Price of Water: Trends in OECD Countries, OECD, Paris.
Ronchi, E. (1999), “Molecular Technologies for Safe Drinking Water”, OECD Observer No 215, OECD, Paris
©OECD Observer No 236, March 2003