77 MicrobialMicrobial aspectsaspects A conceptual framework for Introduction implementing the Guidelines (Chapter 1) (Chapter 2) hehe greatestgreatest riskrisk toto FRAMEWORK FOR SAFE DRINKING-WATER SUPPORTING TTpublicpublic healthhealth fromfrom INFORMATION microbesmicrobes in in water water is is asso-asso- Health-based targets Public health context Microbial aspects ciatedciated with with consumptionconsumption (Chapter 3) and health outcome (Chapters 7 and 11) ofof drinking-waterdrinking-water thatthat isis Water safety plans Chemical aspects (Chapter 4) (Chapters 8 and 12) contaminatedcontaminated withwith hu-hu- System Management and Radiological Monitoring manman andand animalanimal excreta,excreta, assessment communication aspects althoughalthough otherother sourcessources (Chapter 9) Acceptability and routes of exposure Surveillance and routes of exposure aspects (Chapter 5) maymay also also be be signifi significant. cant. (Chapter 10) ThisWaterborne chapter focusesout- onbreaks organisms have beenfor which asso- Application of the Guidelines in specific circumstances thereciated is withevidence, inadequate from (Chapter 6) outbreak studies or from treatment of water sup- Climate change, Emergencies, prospectiveplies and unsatisfactorystudies in Rainwater harvesting, Desalination systems, Travellers, Planes and non-outbreakmanagement of situations, drinking- ships, etc. ofwater diseases distribution. being caused For byexample, ingestion in distributionof drinking-water, systems, inhalation such outbreaks of water have droplets been linked or dermal to cross-connections, contact with drinking-watercontamination andduring their storage, prevention low andwater control. pressure For and the purposeintermittent of the supply. Guidelines, Water - theseborne routes outbreaks are considered are preventable waterborne. if an integrated risk management framework based on aChapter multiple-barrier 11 (Microbial approach fact fromsheets) catchment provides to additional consumer detailed is applied. information Implementing on individualan integrated waterborne risk management pathogens, framework as well as onto keepindicator the water microorganisms. safe from contamination in distribution systems includes the protection of water sources, the proper selection 7.1and Microbialoperation of hazards drinking-water associated treatment with processes, drinking-water and the correct management Infectiousof risks within diseases the caused distribution by pathogenic systems (forbacteria, further viruses information, and parasites see the (e.g. supporting proto- zoadocument and helminths) Water safety are the in distributionmost common systems and ;widespread Annex 1). health risk associated with drinking-water.This chapter The focuses public healthon organisms burden is for determined which there by theis evidence,severity and from incidence outbreak ofstudies the illnesses or from associated prospective with pathogens,studies in theirnon-outbreak infectivity andsituations, the population of diseases exposed. being Incaused vulnerable by ingestion subpopulations, of drinking-water, disease outcome inhalation may of be water more droplets severe. or dermal contact 117117 with drinking-water and their prevention and control. For the purpose of the Guide- lines, these routes are considered waterborne. Chapter 11 (Microbial fact sheets) provides additional detailed information on individual waterborne pathogens, as well as on indicator microorganisms. 7.1 Microbial hazards associated with drinking-water Infectious diseases caused by pathogenic bacteria, viruses and parasites (e.g. proto- zoa and helminths) are the most common and widespread health risk associated with drinking-water. The public health burden is determined by the severity and incidence of the illnesses associated with pathogens, their infectivity and the population exposed. In vulnerable subpopulations, disease outcome may be more severe. 117 117a GUIDELINES FOR DRINKING-WATER QUALITY 7. MICROBIAL ASPECTS Breakdown in water supply safety (source, treatment and distribution) may lead to large-scale contamination and potentially to detectable disease outbreaks. In some Infectious diseases caused by pathogenic cases, low-level, potentially repeated con- bacteria, viruses, protozoa and helminths tamination may lead to significant spor- are the most common and widespread adic disease, but public health surveillance health risk associated with drinking‑water. is unlikely to identify contaminated drink- ing-water as the source. Waterborne pathogens have several properties that distinguish them from other drinking-water contaminants: • Pathogens can cause acute and also chronic health effects. • Some pathogens can grow in the environment. • Pathogens are discrete. • Pathogens are often aggregated or adherent to suspended solids in water, and pathogen concentrations vary in time, so that the likelihood of acquiring an in- fective dose cannot be predicted from their average concentration in water. • Exposure to a pathogen resulting in disease depends upon the dose, invasiveness and virulence of the pathogen, as well as the immune status of the individual. • If infection is established, pathogens multiply in their host. • Certain waterborne pathogens are also able to multiply in food, beverages or warm water systems, perpetuating or even increasing the likelihood of infection. • Unlike many chemical agents, pathogens do not exhibit a cumulative effect. Quantitative microbial risk assessment (QMRA), a mathematical framework for evaluating infectious risks from human pathogens, can assist in understanding and managing waterborne microbial hazards, especially those associated with sporadic disease. 7.1.1 Waterborne infections The pathogens that may be transmitted through contaminated drinking-water are diverse in characteristics, behaviour and resistance. Table 7.1 provides general infor- mation on pathogens that are of relevance for drinking-water supply management. Waterborne transmission of the pathogens listed has been confirmed by epidemio- logical studies and case histories. Part of the demonstration of pathogenicity involves reproducing the disease in suitable hosts. Experimental studies in which healthy adult volunteers are exposed to known numbers of pathogens provide information, but these data are applicable to only a part of the exposed population; extrapolation to more vulnerable subpopulations is an issue that remains to be studied in more detail. Table 7.2 provides information on organisms that have been suggested as possible causes of waterborne disease but where evidence is inconclusive or lacking. The spectrum of pathogens may change as a result of host, pathogen and environmental changes such as fluctuations in human and animal populations, reuse of wastewater, changes in lifestyles and medical interventions, population movement and travel, selective pressures for new pathogens and mutants or recombinations of existing pathogens. The immunity of individuals also varies considerably, whether acquired 118 GUIDELINES FOR DRINKING-WATER QUALITY GUIDELINES FORDRINKING-WATER 118 Table 7.1 Pathogens transmitted through drinking-watera Type species/ Health Persistence in Resistance to Relative Important Pathogen genus/groupb significancec water suppliesd chlorinee infectivityf animal source Bacteria Burkholderia B. pseudomallei High May multiply Low Low No Campylobacter C. coli High Moderate Low Moderate Yes C. jejuni Escherichia coli – High Moderate Low Low Yes Diarrhoeagenicg E. coli – E. coli O157 High Moderate Low High Yes Enterohaemorrhagic Francisella F. tularensis High Long Moderate High Yes 7. MICROBIALASPECTS Legionella L. pneumophila High May multiply Low Moderate No Mycobacteria (non‑ Mycobacterium avium Low May multiply High Low No tuberculous) complex 119 Salmonella typhi High Moderate Low Low No Other salmonellae S. enterica High May multiply Low Low Yes S. bongori Shigella S. dysenteriae High Short Low High No Vibrio V. cholerae O1 and High Short to longh Low Low No O139 Viruses Adenoviridae Adenoviruses Moderate Long Moderate High No Astroviridae Astroviruses Moderate Long Moderate High No Caliciviridae Noroviruses, High Long Moderate High Potentially Sapoviruses Hepeviridae Hepatitis E virus High Long Moderate High Potentially Picornaviridae Enteroviruses, High Long Moderate High No Parechoviruses, Hepatitis A virus Reoviridae Rotaviruses High Long Moderate High No 7. MICROBIALASPECTS 7. MICROBIALASPECTS 119 Table 7.1 (continued) Type species/ Health Persistence in Resistance to Relative Important Pathogen genus/groupb significancec water suppliesd chlorinee infectivityf animal source Protozoa Acanthamoeba A. culbertsoni High May multiply High High No Cryptosporidium C. hominis/parvum High Long High High Yes Cyclospora C. cayetanensis High Long High High No Entamoeba E. histolytica High Moderate High High No Giardia G. intestinalis High Moderate High High Yes Naegleria N. fowleri High May multiply Low Moderate No Helminths Dracunculus D. medinensis High Moderate Moderate High No 7. MICROBIALASPECTS a This table contains pathogens for which there is some evidence of health significance related to their occurrence in drinking‑water supplies. More information on these and other pathogens is presented in chapter 11. 119a b The type species listed (e.g. L. pneumophila) are those most commonly linked to waterborne transmission but other species may also cause disease. c Health significance relates to the incidence and severity of disease, including association