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Why does WHO recommend testing for coliphages to ensure water quality?

Jun 10, 2021

The WHO emphasizes some microbiological aspects to ensure water quality

The fourth edition of the World Health Organization’s (WHO) Guidelines for drinking-water quality (GDWQ), published in 2018, builds on over 50 years of guidance by WHO on drinking-water quality, which has formed an authoritative basis for the setting of national regulations and standards for water safety in support of public health.

Ensuring the microbial safety of drinking-water supplies is based on applying multiple barriers from the source to the consumer to prevent contamination of drinking water or reduce it to levels that are not harmful to health. Safety is enhanced by implementing multiple barriers, such as protection of water resources, proper selection and operation of a series of treatment steps, and management of distribution systems (piped or otherwise) to maintain and protect the quality of treated water.

The main strategy is to prevent or reduce the entry of pathogens into water sources and reduce reliance on treatment processes to remove pathogenic organisms.

Generally speaking, the most significant microbiological risks are associated with the ingestion of water contaminated with human or animal feces (including poultry feces). Feces can be a source of pathogens such as bacteria, viruses, protozoa and helminths, protozoa and helminths.

When setting health protection goals related to microbial safety, the most critical concerns are pathogenic organisms from feces. Often, the microbiological quality of water can have abrupt variations over a wide range of concentrations. As a result, there can be sudden increases in pathogen levels, which can significantly increase the risk of illness and trigger waterborne disease outbreaks. In addition, many people may be exposed to the infection before contamination is detected.

For these reasons, ensuring the microbial safety of drinking water cannot rely on end-product testing alone, even if it is performed frequently. To systematically ensure the safety of drinking water and protect public health, should give special attention to the water safety framework and implement comprehensive water safety plans. 

Generally, verification of the microbiological quality of drinking water includes testing for Escherichia coli as an indicator of fecal contamination. However, in practice, analysis of thermotolerant coliform bacteria may be an acceptable alternative to Escherichia coli analysis as an indicator of fecal contamination in many circumstances. While E. coli is valid, it has limitations. Enteric viruses and protozoa are more resistant to disinfection; consequently, the absence of E. coli will not necessarily indicate an absence from these organisms. Under certain circumstances, the inclusion of more resistant indicators, such as bacteriophages and/or bacterial spores, should be considered. In addition, viruses and protozoa that are more resistant to conventional environmental conditions or treatment technologies, including filtration and disinfection, may be present in treated drinking water in the absence of E. coli. Retrospective studies of waterborne disease outbreaks have shown that complete reliance on assumptions surrounding the absence or presence of E. coli may not guarantee safety. Therefore, it may be desirable to include more resistant microorganisms, such as bacteriophages or bacterial spores, as indicators of persistent microbiological hazards in certain circumstances.


Their incorporation in monitoring programs, including control and surveillance programs, should be assessed, and surveillance programs should be evaluated concerning local circumstances and scientific criteria. For example, such events may include the use of a water source that is known to be contaminated with viruses and intestinal parasites or when such contamination is suspected due to the impact of human and livestock waste.

Failure to ensure drinking water safety can expose the community to the risk of outbreaks of intestinal diseases and other infections. Outbreaks of waterborne diseases are especially to be avoided because of their ability to simultaneously infect large numbers of people and, potentially, a high proportion of the community.

Although water can be a significant source of infectious organisms, many of the diseases transmitted through water can also be transmitted by other routes, such as person-to-person contact, food ingestion, droplets, and aerosols. In certain circumstances, in the absence of waterborne outbreaks, these routes may be more important.

The recent European Parliament Directive on Drinking Water already implies testing for coliphages in water as a viral indicator of fecal contamination. Thus, the regulation is already in force, and the Member States will have to implement it within one and a half years.

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