Page 2: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Enteric Protozoa: Giardia and Cryptosporidium
Where treatment is required for enteric protozoa, the guideline for Giardia and Cryptosporidium is a health-based treatment goal of a minimum 3 log removal and/or inactivation of cysts and oocysts. Depending on the source water quality, a greater log removal and/or inactivation may be required. Treatment technologies and watershed or wellhead protection measures known to reduce the risk of waterborne illness should be implemented and maintained if source water is subject to faecal contamination or if Giardia or Cryptosporidium has been responsible for past waterborne outbreaks.
Protozoa are a diverse group of microorganisms. Most are free-living organisms that can reside in fresh water and pose no risk to human health. Some enteric protozoa, such as Giardia and Cryptosporidium, are pathogenic and have been associated with drinking water related outbreaks. They may be found in water following direct or indirect contamination by the faeces of humans or other animals. Person-to-person transmission is a common route of transmission of both Giardia and Cryptosporidium.
Health Canada recently completed its review of the health risks associated with enteric protozoa in drinking water. This Guideline Technical Document reviews and assesses identified health risks associated with enteric protozoa in drinking water. It evaluates new studies and approaches and takes into consideration the methodological limitations for the detection of protozoa in drinking water. From this review, the guideline for protozoa in drinking water is a health-based treatment goal of a minimum 3 log reduction of enteric protozoa.
The health effects associated with exposure to Giardia and Cryptosporidium, like those of other pathogens, depend upon features of the host, pathogen and environment. The host's immune status, the (oo)cyst's infectivity and the degree of exposure are all key determinants of infection and illness. Infection with Giardia or Cryptosporidium can result in both acute and chronic health effects.
Theoretically, a single cyst of Giardia would be sufficient to cause infection. However, studies have shown that the dose required for infection is usually more than a single cyst and is dependent on the virulence of the particular strain. Typically, Giardia is non-invasive and results in asymptomatic infections. Symptomatic giardiasis can result in nausea, diarrhoea (usually sudden and explosive), anorexia, an uneasiness in the upper intestine, malaise and occasionally low-grade fever or chills. The acute phase of the infection commonly resolves spontaneously, and organisms generally disappear from the faeces. Some patients (e.g., children) suffer recurring bouts of the disease, which may persist for months or years.
As is the case for Giardia and other pathogens, a single organism of Cryptosporidium can potentially cause infection, although studies have shown that more than one organism is generally required. Individuals infected with Cryptosporidium are more likely to develop symptomatic illness than those infected with Giardia. Symptoms include watery diarrhoea, cramping, nausea, vomiting (particularly in children), low-grade fever, anorexia and dehydration. The duration of infection depends on the condition of the immune system. Immunocompetent individuals usually carry the infection for a maximum of 30 days. In immunocompromised individuals, infection can be life-threatening and can persist throughout the immunosuppression period.
Giardia cysts and Cryptosporidium oocysts can survive in the environment for extended periods of time, depending on the characteristics of the water. They have been shown to withstand a variety of environmental stresses, including freezing and exposure to seawater. (Oo)cysts are commonly found in Canadian source waters. The sudden and rapid influx of these microorganisms into source waters, for which available treatment may not be sufficient or adequate, is likely responsible for the increased risk of infection associated with transmission through drinking water.
Giardia and Cryptosporidium are common causes of waterborne disease outbreaks; Giardia is the most commonly reported intestinal protozoan in Canada, North America and worldwide.
The multi-barrier approach is the best approach to reduce enteric protozoa and other waterborne pathogens in drinking water. Source water assessments should be part of routine vulnerability assessments and/or sanitary surveys. They should include routine and targeted monitoring for Giardia and Cryptosporidium. Monitoring of source water for protozoa can be targeted by using information about sources of faecal contamination from a sanitary survey, together with historical data on rainfall, snowmelt, river flow and turbidity, to help to identify the conditions that are likely to lead to peak events. A method that allows for the simultaneous detection of these protozoans is available and has been validatedfor surface water. Where monitoring for Giardia and Cryptosporidium is not feasible (e.g., small supplies), (oo)cyst concentrations can be estimated. Estimates should be based on a source water assessment along with other water quality parameters that can provide information on the risk and/or level of faecal contamination in the source water.
Once the source water quality has been characterized, pathogen removal targets and effective treatment barriers can be established in order to achieve safe levels in the finished drinking water. In general, all water supplies should be disinfected, and an adequate concentration of disinfectant residual should be maintained throughout the distribution system at all times. The combination of physical removal (e.g., filtration) and disinfection barriers (e.g., UV light) is the most effective way to reduce protozoa in drinking water, because of their resistance to commonly used disinfectants such as chlorine. Treatment systems that rely solely on chlorine as the treatment barrier will require large CT values to effectively inactivate Giardia. In the case of Cryptosporidium, extremely large CT values will be required, which would prohibit the use of chlorine for the inactivation of Cryptosporidium oocysts.
Although the absence of Escherichia coli and total coliforms does not necessarily indicate the absence of enteric protozoa, they remain the best available indicators for verifying microbiological drinking water quality. The application and control of a multi-barrier, source-to-tap approach, in combination with monitoring of a variety of indicators (e.g., turbidity, chlorine residual, E. coli ), can be used to verify that the water has been adequately treated and is therefore of an acceptable microbiological quality.
Quantitative microbial risk assessment (QMRA) can be used as part of a multi-barrier approach to help provide a better understanding of risk related to a water system. QMRA uses source water quality data, treatment barrier information and pathogen-specific characteristics to estimate the burden of disease associated with exposure to pathogenic microorganisms in a drinking water source. Through this assessment, variations in source water quality and treatment performance can be evaluated for their contribution to the overall risk. Such analysis can be used to assess the adequacy of existing control measures or the requirement for additional treatment barriers or optimization and help establish limits for critical control points.
Specific enteric protozoa whose characteristics make them a good representative of all similar pathogenic protozoa are considered in QMRA to select a reference protozoan. It is assumed that controlling the reference protozoan would ensure control of all other similar protozoa of concern. Cryptosporidium parvum and Giardia lamblia have been selected as the reference protozoa for this risk assessment because of their high prevalence rates, potential to cause widespread disease, resistance to chlorine disinfection and the availability of a dose-response model for each organism.
Note: Specific guidance related to the implementation of the drinking water guideline should be obtained from the appropriate drinking water authority in the affected jurisdiction.
Exposure to Giardia and Cryptosporidium should be limited by implementing a source-to-tap approach to protect the quality of drinking water. This approach includes assessing the entire drinking water system, from the source water through the treatment and distribution systems to the consumer, in order to identify risks and appropriate measures to mitigate those risks.
Source water assessments should be part of routine vulnerability assessments and/or sanitary surveys. They should include routine monitoring for Giardia and Cryptosporidium in order to establish a baseline, followed by long-term targeted monitoring. Monitoring of source water for protozoa can be targeted by using information about sources of faecal contamination from a sanitary survey, together with historical data on rainfall, snowmelt, river flow and turbidity, to help to identify the conditions that are likely to lead to peak events. Assessments should also include identification of potential sources of human and animal faecal contamination in the watershed/aquifer and potential pathways and/or events (low to high risk) by which protozoa can make their way into the source water and affect water quality. Sources of human faecal matter, such as sewage treatment plant effluents, sewage lagoon discharges and improperly maintained septic systems, have the potential to be significant sources of Giardia and Cryptosporidium. Faecal matter from agricultural animals, wildlife and other animals are also considered an important source of Giardia and Cryptosporidium species capable of causing illness in humans.
It is important to conduct a comprehensive assessment of groundwater sources to classify them as either groundwater under the direct influence of surface water or groundwater considered to be less vulnerable to faecal contamination (i.e., those not under the direct influence of surface water). These assessments should include, at a minimum, a hydrogeological assessment, an evaluation of well integrity, and a sanitary survey of activities and physical features in the area. Groundwater considered to be less vulnerable to faecal contamination, if properly classified, should not have protozoa present. However, even these groundwater sources will have a degree of vulnerability and should be periodically reassessed.
Assessments of water quality need to consider the "worst-case" scenario for that source water. For example, there may be a short period of poor source water quality following a storm. This short-term degradation in water quality may in fact embody most of the risk in a drinking water system. Collecting and analysing source water samples for Giardia and Cryptosporidium can provide important information for determining the level of treatment and mitigation (risk management) measures that should be in place to reduce the concentration of (oo)cysts to an acceptable level. Where source water sampling and analysis for Giardia and Cryptosporidium are not feasible (e.g., small supplies), (oo)cyst concentrations can be estimated. Estimates should take into account information obtained from the source water assessment along with other water quality parameters that can provide information on the risk and/or level of faecal contamination in the source water. Because these estimates will have a high level of uncertainty, additional factors of safety during engineering and design or upgrade of the treatment plant or a greater log reduction than calculated using a QMRA approach should be applied in order to ensure production of drinking water of an acceptable microbiological quality.
The information obtained from source water assessments is a key component of carrying out site-specific risk assessments. This information should be used along with treatment and distribution system information to help assess risks from source to tap. This document suggests the use of QMRA as a tool that can help provide a better understanding of the water system by evaluating the impacts of variations in source water quality and treatment process performance on the overall risk, including the potential impact of hazardous events, such as storms, contamination events or the failure of a treatment barrier. The resulting analysis can be used to assess the adequacy of existing control measures, to determine the need for additional treatment barriers or for optimization and to help establish limits for critical control points.
Where treatment is required, a minimum 3 log removal and/or inactivation of Giardia and Cryptosporidium (oo)cysts is required. In many surface water sources, a greater log reduction may be necessary.
Reductions can be achieved through physical removal processes, such as filtration, and/or by inactivation processes, such as ultraviolet light disinfection. Generally, minimum treatment of supplies derived from surface water sources or groundwater under the direct influence of surface waters should include adequate filtration (or equivalent technologies) and disinfection. The appropriate type and level of treatment should take into account the potential fluctuations in water quality, including short-term water quality degradation, and variability in treatment performance. Pilot testing or other optimization processes may be useful for determining treatment variability. In systems with a distribution system, a disinfectant residual should be maintained at all times.As part of the multi-barrier approach, a variety of indicators (e.g., turbidity, chlorine residual, E. coli ) should be routinely monitored in order to verify that the water has been adequately treated and therefore meets the health-based treatment goal. These indicators can also be used for assessing the distribution system and to verify that the microbiological quality of the water is being maintained through the distribution system to the consumer's tap.
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