Cryptosporidium

(endorsed 2011)

Guideline

No guideline value is set for Cryptosporidium 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 from catchment to tap is recommended to minimise the risk of Cryptosporidium 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 Cryptosporidium 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 Cryptosporidium 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

In recent years, Cryptosporidium has come to be regarded as one of the most important waterborne human pathogens in developed countries. Over 30 outbreaks associated with drinking water have been reported in North America and Britain, with the largest infecting an estimated 403,000 people (Mackenzie et al. 1994). Recent research has led to improved methods for testing water for the presence of human infectious species, although such tests remain technically demanding and relatively expensive.

Cryptosporidium is an obligate parasite with a complex life cycle that involves intracellular development in the gut wall, with sexual and asexual reproduction. Thick-walled oocysts, shed in faeces are responsible for transmission. Concentrations of oocysts as high as 14,000 per litre in raw sewage and 5,800 per litre in surface water have been reported (Madore et al. 1987). Oocysts are robust and can survive for weeks to months in fresh water under cold conditions (King and Monis 2007).

There are a number of species of Cryptosporidium, with C. hominis and C. parvum identified as the main causes of disease (cryptosporidiosis) in humans. C. hominis appears to be confined to human hosts, while the C. parvum strains that infect humans also occur in cattle and sheep. C. parvum infections are particularly common in young animals, and it has been reported that infected calves can excrete up to 10 billion oocysts in one day. Waterborne outbreaks of cryptosporidiosis have been attributed to inadequate or faulty treatment and contamination by human or livestock (particularly cattle) waste. C. hominis and C. parvum can be distinguished from one another and from other Cryptosporidium species by a number of genotyping methods. Infectivity tests using cell culture techniques have also been developed.

Consumption of contaminated drinking water is only one of several mechanisms by which transmission (faecal-oral) can occur. Recreational waters, including swimming pools, are an important source of cryptosporidiosis and direct contact with a human carrier is also a common route of transmission. Transmission of Cryptosporidium can also occur by contact with infected farm animals, and occasionally through contaminated food.

Australian significance

The most publicised incident of drinking-water contamination in Australia occurred in July-September 1998 in Sydney. High numbers of Cryptosporidium and Giardia (see Fact Sheet) were reported in 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. The incident highlighted the lack of a method (at that time) to determine whether detected organisms were infective for humans.

Cryptosporidiosis is a notifiable condition in all Australian states and territories. A case-control study of sporadic cases in Adelaide and Melbourne from 1998 to 2001 indicated that person-to-person contact and public swimming pools were the most common risk factors for infection (Robertson et al. 2002). Outbreaks associated with contaminated swimming pools have occurred in several Australian states. In South Australia, a relatively large number of illnesses were recorded in 1990-91 but no source was identified (Weinstein et al. 1993). The only known outbreak of illness associated with drinking water occurred in Victoria, when a mixture of 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

A multiple barrier approach operating from catchment to tap should be implemented to minimise the risk of contamination by Cryptosporidium. 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 Cryptosporidium and treatment, including effective filtration, 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 Cryptosporidium, 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 Cryptosporidium (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 Cryptosporidium. 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.

Cryptosporidium oocysts are extremely resistant to chlorine and will not be killed by concentrations that can be practically used in drinking water. Other chemical disinfectants such as ozone are more effective (Bouchier 1998). More recently it has been shown that ultraviolet (UV) light disinfection is effective against Cryptosporidium, and this technology is being increasingly adopted for drinking-water treatment. The United States Environmental Protection Agency (USEPA) has developed detailed guidance on the application of UV for inactivation of Cryptosporidium, 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 are at all times greater than specified values provides a practical means of ensuring that Cryptosporidium oocysts are inactivated and are not a threat to public health.

Due to their small size (4-6 µm), Cryptosporidium oocysts can challenge removal by granular media-based filtration processes; however, well designed and operated systems can provide a high level of removal. Membrane filtration processes that provide a direct physical barrier can represent a viable alternative for the effective removal of Cryptosporidium. Where Cryptosporidium oocysts are suspected or known to be present in the raw water, the design and operation of water treatment plants should be carefully examined to ensure that required performance is achieved and maintained. For granular media-based systems, there should be particular attention to ensuring that coagulation/flocculation is optimal, turbidity is monitored from all filters, backwash water is handled appropriately, and increases in turbidity are minimised during filter start-up and operation, to avoid sudden flow surges (see Badenoch 1995, Bouchier 1998). A turbidity limit of 0.2 NTU or less for effluent from individual filters has been shown to provide optimal removal of Cryptosporidium and other classes of enteric pathogens (Xagoraraki et al. 2004).

The performance of filtration plants should be monitored continuously and treated water of a constant quality should be produced, irrespective of the quality of the raw water.

Filtration plants should be operated by trained and skilled personnel. Failure of water treatment processes, including failure to meet specified targets for turbidity (or particle counts), represents a potential risk of oocyst contamination of the drinking-water supply.

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

There are a number of different methods for isolating Cryptosporidium oocysts from water. Most quantitative methods involve concentration of relatively large volumes of water and fluorescent staining of the concentrated material. Many methods are adapted from USEPA Method 1623 (USEPA 2005), which uses immunomagnetic beads to separate oocysts from contaminating debris. The use of any method should include exacting quality control procedures and determination of recovery efficiencies. The Cryptosporidium Proficiency Testing program operated by the National Association of Testing Authorities, Australia sets out performance requirements for laboratories undertaking Cryptosporidium testing, but does not mandate a specific test methodology.

Different species and strains of Cryptosporidium may be distinguished using a range of genotyping techniques, which have been developed by different laboratories. An international trial has been undertaken to compare the utility of six techniques to characterise C. hominis and C. parvum isolates (Chalmers et al. 2005).

In the past, techniques such as excystation and vital dye staining have been used to assess oocyst viability or infectivity; however it is now recognised that these methods are unreliable and may overestimate human infection risks. Cell culture techniques provide more accurate measures of human infectivity although correlation can be variable (Rochelle et al. 2002; Schets et al. 2005). Cell culture may be coupled with a number of molecular techniques using the polymerase chain reaction to amplify genetic markers. This provides more rapid and sensitive tests for infectivity (Di Giovanni et al. 1999, Keegan et al. 2003).

Although there have been considerable advances in methodology, identification of human infectious Cryptosporidium oocysts in water remains a technically demanding and relatively expensive process.

Health considerations

Infection of normally healthy people by Cryptosporidium can result in self-limiting diarrhoea that usually resolves within a week but can last for a month or more. Illness varies according to age and immune status. Chronic infections may occur, and can be life threatening, in some severe immunodeficiency conditions (advanced stages of AIDS, severe combined immune deficiency, specific T-cell deficiency) (Bouchier 1998, Chief Medical Officer 1999). People with severe immunodeficiency conditions may also be more susceptible to infection by species other than C. hominis and C. parvum.

Derivation of guideline

No guideline value is proposed for Cryptosporidium 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 C. parvum. 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 (Haas et al. 1996). Human dose response trials have indicated that infectivity for different isolates of Cryptosporidium varies widely (Chappell et al. 2006).

Investigative testing of drinking water may be required if Cryptosporidium 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 Cryptosporidium in source water, sub-optimal 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 cryptosporidiosis.

When an incident of concern leads to the testing of distribution systems for Cryptosporidium, the relevant health authority or drinking water regulator should be notified immediately. If Cryptosporidium 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 Cryptosporidium 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 infectious organisms and the specific presence of C. hominis or C. parvum; issuing advice, including boil-water notices, to the public; and enhanced surveillance to detect possible increases in community cryptosporidiosis.

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.

Badenoch J (1995). Cryptosporidium in water supplies. Second Report of the Group of Experts. Department of the Environment, Department of Health, HMSO.

Bouchier I (1998). Cryptosporidium in water supplies. Third Report of the Group of Experts. Department of the Environment Transport and the Regions, Department of Health, HMSO.

Chalmers RM, Ferguson C, Cacciò S, Gasser RB, Abs EL, Osta YG, Heijnen L, Xiao L, Elwin K, Hadfield S, Sinclair M, Stevens M (2005). Direct comparison of selected methods for genetic categorisation of Cryptosporidium parvum and Cryptosporidium hominis species. International Journal for Parasitology 35(4):397-410.

Chappell CL, Okhuysen PC, Langer-Curry R, Widmer G, Akiyoshi DE, Tanriverdi S, Tzipori S (2006). Cryptosporidium hominis: Experimental challenge in healthy adults. American Journal of Tropical Medicine and Hygiene, 75(5):851-857.

Chief Medical Officer’s Update 23 August (1999). Cryptosporidium in water: clarification of the advice to the immunocompromised. Department of Health, London.

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.

Di Giovanni GD, Hashemi FH, Shaw NJ, Abrams FA, LeChevallier MW, Abbaszadegan M (1999). Detection of infectious Cryptosporidium parvum oocysts in surface and filter backwash water samples by immunomagnetic separation and integrated cell culture-PCR. Applied and Environmental Microbiology, 65(8):3427-32.

Haas CN, Crockett C, Rose JB, Gerba CP, Fazil A (1996). Risk assessment of Cryptosporidium parvum oocysts in drinking water. Journal of the American Waterworks Association, 88(9):131-136.

Keegan AR, Fanok S, Monis PT, Saint CP (2003). Cell culture-Taqman PCR assay for evaluation of Cryptosporidium parvum disinfection. Applied and Environmental Microbiology, 69(5):2025-11.

King BJ, Monis PT (2007). Critical processes affecting Cryptosporidium oocyst survival in the environment. Parasitology, 134(3):309-23.

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

MacKenzie WR, Hoxie NJ, Proctor ME, Gradus MS, Blair KA, Paterson DE, Kazmicrczak JJ, Addiss DG, Fox KR, Rose JB, Davis JP (1994). A massive outbreak in Milwaukee of Cryptosporidium infection transmitted through the public water supply. New England Journal of Medicine, 331:161-167.

Madore MS, Rose JB, Gerba CP, Arrowood MJ, Sterling CR (1987). Occurrence of Cryptosporidium oocysts in sewage effluents and selected surface waters. Journal of Parasitology, 73: 702-5.

Robertson B, Sinclair MI, Forbes AB, Veitch M, Kirk M, Cunliffe D, Willis J, Fairley CK (2002). Case-control studies of sporadic cryptosporidiosis in Melbourne and Adelaide, Australia. Epidemiology and Infection, 128:419-431.

Rochelle PA, Marshall MM, Mead JR, Johnson AM, Korich DG, Rosen JS and De Leon R (2002) Comparison of in vitro cell culture and a mouse assay for measuring infectivity of Cryptosporidium parvum. Applied and Environmental Microbiology, 68:3809-3817.

Schets FM, Engels GB, During M and Roda Husman AM (2005) Detection of infectious Cryptosporidium oocysts by cell culture immunofluorescence assay: applicability to environmental samples. Applied and Environmental Microbiology, 71:6793-6798.

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.

Weinstein P, Macaitis M, Walker C, Cameron S (1993). Cryptosporidial diarrhoea in South Australia: an exploratory case-control study of risk factors for transmission. Medical Journal of Australia, 158:117-119.

Xagoraraki, I., Harrington GW, Assavasilavasukul P. and Standridge JH. (2004). Removal of emerging waterborne pathogens and pathogen indicators by pilot-scale conventional treatment. Journal of the American Water Works Association, 96(5):102-113.