Giardia

(endorsed 2011)

Guideline

No guideline value is set for Giardia due to the lack of a routine method to identify human infectious strains in drinking water. If such a guideline were established, it would be well below 1 organism per litre and would involve testing of impractically large volumes of water.

A multiple barrier approach operating from catchment to tap is recommended to minimise the risk of Giardia contamination. Protection of catchments from human and animal wastes is a priority. Operation of barriers should be monitored to ensure effectiveness and that microbial health-based targets are being met.

Although routine monitoring for Giardia is not recommended, investigative testing may be required in response to events that could increase the risk of contamination (e.g., heavy rainfall, increased turbidity, treatment failure). If Giardia is detected in drinking water, the relevant health authority or drinking water regulator should be notified immediately. All necessary measures to assess and minimise public health risks should be implemented as soon as possible.

General description

Although known as a human parasite for 200 years, Giardia has been regarded seriously as an agent of disease only since the 1960s. It has been identified as an important waterborne pathogen, and linked to many outbreaks of illness associated with drinking water, particularly in North America. Although the importance of this organism has been established, there are large gaps in knowledge about it, and there are no tests for identifying the presence of human infectious species in water.

Giardia has a relatively simple life cycle involving two stages: a flagellate that multiplies in the intestine, and an infective thick-walled cyst that is shed intermittently but in large numbers in faeces. Concentrations of cysts as high as 88,000 per litre in raw sewage and 240 per litre in surface water have been reported (Wallis et al. 1996). Giardia is typically present in larger numbers in Australian sewage than Cryptsoporidium. Cysts are robust and can survive for weeks to months in fresh water.

There are a number of species of Giardia, but human infections (giardiasis) are usually assigned to one, G. intestinalis (= G. lamblia and G. duodenalis). G. intestinalis infections have been reported from domestic and wild animals, but the host range of human infectious species is uncertain. Although substantial advances have been made in the sampling and counting of cysts, there are currently no established methods to identify human infectious organisms in water. Waterborne outbreaks of giardiasis have generally been linked to consumption of untreated or unfiltered surface water and contamination with human waste.

Consumption of contaminated drinking water is only one of several mechanisms by which transmission (faecal-oral) can occur. Recreational waters, including swimming pools, are also emerging as an important source of giardiasis. However, excluding outbreaks, by far the most likely route of transmission is by direct contact with a human carrier. Transmission of Giardia can also occur by contact with infected animals and occasionally through contaminated food.

Australian significance

Outbreaks of giardiasis in Australia often involve close communal groups. In day-care centres, for instance, as many as 20% of children may carry Giardia without symptoms (Grimmond et al. 1988). Infection is endemic and is significant among children and adults in the wider community, and sources of this infection are difficult to identify. Giardiasis is notifiable in some states and territories.

The most publicised incident of drinking-water contamination in Australia occurred from July to September 1998 in Sydney. High numbers of Cryptosporidium (see Fact Sheet) and Giardia were reported for treated water, and boil-water notices were issued for three million residents. No increase in illness was detected in association with the contamination despite increased epidemiological surveillance. An epidemiological study in Queensland showed no correlation between infection and source of drinking water, point-of-use treatment (boiling or filtration) or recreational contact with water (Boreham and Phillips 1986). Another study identified contact with septic tank waste or contaminated soil as a possible mechanism of infection (Boreham et al. 1981). An outbreak of illness associated with drinking water was reported in Victoria when mixed infections due to Cryptosporidium and Giardia followed contamination of a private water supply by overflow from a septic tank (Lester 1992).

Preventing contamination of drinking water

Approaches applied to Cryptosporidium will usually be as or more effective against Giardia.

A multiple barrier approach operating from catchment to tap should be implemented to minimise the risk of contamination by Giardia. Protection of water catchments from contamination by human and animal wastes is a priority. Water from unprotected catchments is likely to be subject to contamination by Giardia and treatment, including effective filtration or enhanced disinfection, will be required to remove these organisms and ensure a safe supply. The lower the quality of source water, the greater the reliance on water treatment processes.

Water catchments should be surveyed for potential sources of contamination, and source water should be subject to investigative and event-based testing for Giardia, to:

• assess risk factors for contamination;

• provide a basis for catchment management to reduce these risks; and

• determine the level of water treatment required.

It has been reported that increases in turbidity associated with rainfall events may signal increased numbers of Giardia (Atherholt et al. 1998), although Australian data indicate that there is no uniform relationship that is applicable across different catchments (CRC 2007).

Groundwater from confined aquifers or from depth should be free from contamination by Giardia. However, bores need to be well maintained and protected from intrusion of surface and subsurface contamination. Integrity should be monitored using traditional indicators of faecal contamination.

Giardia cysts are more resistant than enteric bacteria to chlorine, but not as resistant as Cryptosporidium. The time required for a 90% (1 log 10) kill at 1 mg/L free chlorine is of the order of 25-35 minutes. Other chemical disinfectants such as ozone are more effective. The United States Environmental Protection Agency (USEPA) has published C.t tables specifying chlorine concentrations (C) and contact times (t) required to inactivate Giardia cysts over a range of conditions (see USEPA 1999). In addition, C.t tables have been provided for chloramines, ozone and chlorine dioxide. More recently it has been shown that ultraviolet (UV) light disinfection is effective against Giardia and this technology is being increasingly adopted for drinking-water treatment. The USEPA has developed detailed guidance on the application of UV for inactivation of Giardia, including the relationship between dose and log reduction as well as aspects of plant design, process validation and operational issues (USEPA 2006). Ensuring that UV light doses or chemical disinfectant concentrations and contact times are at all times greater than specified values provides a practical means of ensuring that Giardia cysts are inactivated and are not a threat to public health.

The USEPA National Primary Drinking Water Standards prescribe comprehensive treatment (including filtration and disinfection) of most surface waters to protect drinking water from contamination by Giardia. Operational procedures in water treatment plants should be carefully examined where Giardia cysts are suspected or known to be present in the raw water, to ensure that optimum removal is achieved and maintained. Filtration plants should be operated by trained and skilled personnel.

The integrity of distribution systems should be maintained. Storages for treated water should be roofed, backflow prevention measures should be applied and faults and burst mains should be repaired in a way that will prevent contamination.

Method of identification and detection

USEPA has published a standard method for Giardia applying the same principles as those used for isolating Cryptosporidium oocysts from water (USEPA 2005). Testing for Giardia should include exacting quality control procedures and determination of recovery efficiencies.

In the past, techniques such as excystation and vital dye staining have been used to assess cyst viability or infectivity; however it is now recognised that these methods are unreliable and may overestimate human infection risks. There are currently no established methods to identify human infectious organisms in water.

Health considerations

Infection by Giardia may reduce absorption of nutrients and cause diarrhoea. In most cases, illness is self limiting, but in some cases chronic infection with intermittent diarrhoea can occur. Specific treatments are available.

Derivation of guideline

No guideline value is proposed for Giardia, and routine monitoring of distribution systems, including outlets from water treatment plants, is not recommended because of the lack of a reliable and efficient method to identify human infectious organisms. In addition, current risk assessment models suggest that impractically large volumes of water would need to be tested to provide a meaningful indication of health risk.

Investigative testing of drinking water may be required if Giardia contamination is suspected. This could occur in association with a major rainfall event, which could lead to a marked decrease in water quality and a marked increase in the numbers of Giardia in source water, suboptimal operation of treatment processes, a breakdown in treatment plant operations, or a fault within the distribution system. Monitoring may also be required in response to suspected waterborne giardiasis.

When an incident of concern leads to the testing of distribution systems for Giardia, the relevant health authority or drinking water regulator should be notified immediately. If Giardia is detected in finished water, the relevant health authority or drinking water regulator should again be notified immediately.

Comprehensive protocols should be developed by water and health agencies to deal with detections of Giardia in drinking water and should describe approaches for interpreting the health and operational significance. In responding to incidents or detection, the health authority or drinking water regulator may choose to do so in consultation with the water authority and/or an expert panel. Credible public communication is essential. Responses could include further sampling to confirm the presence and source of the organisms; testing for the presence of viable organisms; increased disinfection; the issuing of advice, including boil-water notices, to the public; and enhanced surveillance to detect possible increases in community giardiasis.

References

Atherholt TB, LeChevallier MW, Norton WD, Rosen JS (1998). Effect of rainfall on Giardia and Crypto. Journal of the American Waterworks Association, 90(9):66-80.

Boreham PFL, Phillips RE (1986). Giardiasis in Mount Isa, north west Queensland. Medical Journal of Australia, 144:524-528.

Boreham PFL, Dondey J, Walker R (1981). Giardiasis among children in the city of Logan, South East Queensland. Australian Paediatric Journal, 17:209-212.

CRC (2007). Source Water Quality Assessment and the Management of Pathogens in Surface Catchments and Aquifers. Cooperative Research Centre for Water Quality and Treatment Research Report 29.

Grimmond TR, Radford AJ, Brownridge T, Farshid A, Harris C, Turton P, Wordsworth K (1988). Giardia carriage in aboriginal and non-aboriginal children attending urban day-care centres in South Australia. Australian Paediatric Journal, 24:304-305.

Lester R (1992). A mixed outbreak of cryptosporidiosis and giardiasis. Update. Quarterly Bulletin of Infectious Diseases, Health Department Victoria, 1:14-15.

USEPA (United States Environmental Protection Agency) (1999). Disinfection Profiling and Benchmarking Guidance Manual. EPA 815-R-99-013, USEPA, Cincinatti, USA.

USEPA (United States Environmental Protection Agency) (2005) Method 1623: Cryptosporidium and Giardia in Water by Filtration/IMS/FA.

USEPA (United States Environmental Protection Agency) (2006). Ultraviolet Disinfection Guidance Manual. EPA 815-D-03-007, USEPA, Cincinatti, USA.

Wallis PM, Erlandsen SL, Isaac-Renton JL, Olson ME, Robertson WJ, van Keulen H (1996). Prevalence of Giardia cysts and Cryptosporidium oocysts and characterization of Giardia spp isolated from drinking water in Canada. Applied and Environmental Microbiology, 62:2789-2.