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Risikovurdering af Giardia og Cryptosporidium i vand

Summary and conclusions

This project includes a risk assessment for human infection by the protozoa Giardia and Cryptosporidium by exposure to water in Denmark. Several of such risk assessments have been conducted in other countries, but these assessments are difficult to apply to the Danish situation. This is mainly due to the facts that in Denmark more than 99% of the drinking water is produced from groundwater, that the number of bathers vary from different countries, and that the rules for swimming pools are different.

The purpose of the project has been to perform a risk assessment for infection by Giardia and Cryptosporidium through intake of drinking water and exposure to bathing water, surface water, wastewater and water in swimming pools. The assessments can eventually be used by the Danish Environmental Agency for defining quality values or maximum acceptable concentration of the protozoa in the mentioned water types.

The biology and epidemiology shows that Giardia cysts and Cryptosporidium oocysts are the infective stadium of the protozoa's life cycle, and that they are 5-10 m and 4-6 m, respectively. Both protozoa have low infectious doses; 10 Giardia cysts or 30 Cryptosporidium oocysts can infect humans. The infections are characterized by e.g. diarrhea, nausea, vomiting and dehydration. For Cryptosporidium the infection period is typically 1-2 weeks, and for Giardia 1-3 weeks. There is no treatment for Cryptosporidium infection, whereas the Giardia infection can be treated. Both infections are often self-limited, but can in rare cases become chronic.

In this risk assessment it was not distinguished between the individual Giardia and Cryptosporidium species or genotypes, because only very seldom the genotypes diagnosed in human giardiasis or cryptosporidiosis are reported. Furthermore, it is not clear yet, which genotypes infect humans. Usually the detection methods for water and other environmental samples will not include identification of the species, since most Giardia and Cryptosporidium species cannot be differentiated morphologically. Additionally, most of the published studies do not distinguish between live and dead organisms.

Many different detection methods are used for Giardia and Cryptosporidium and the uncertainty of the concentrations given in literature is very high. Therefore, results from different laboratories can be difficult to compare. Despite this, it is possible to compare different investigations, because the difference in concentration between different locations is higher than the uncertainty of the measurements within each location. The difference between the results is due to e.g. recovery, which is influenced by the distinguishing between living and dead organisms, the water quality, and the age of the (oo)cysts (a common term for both cysts and oocysts). Each step in the concentration and detection processes may loose (oo)cysts, and therefore the recovery generally is low and varying.

Giardiasis and cryptosporidiosis are not mandatory to register in Denmark and the actual number of cases per year is not known. In year 2000 approximately 1600 cases of giardiasis were diagnosed and every year approximately 200 cases of cryptosporidiosis are diagnosed. The actual yearly number is estimated to 60,000 cases of giardiasis and 10,000 cases of cryptosporidiosis, although these estimates are quite uncertain.

This report includes a risk assessment of four scenarios: infection by drinking water, by contact with floods by waste water in cities, by recreational use of surface water and by using swimming pools. Every scenario is discussed in two sub-analysis.

Different strategies for the quantitative risk assessments are used in each scenario, because the data were very different in the different scenarios. Thus the risk assessment of floods by wastewater in cities is based on collected measurements of the concentration of Giardia and Cryptosporidium in untreated wastewater. This is also the case for the risk assessment of recreational use of fresh surface waters, but for marine surface waters the risk assessment is based on rough estimations of dilution ratios. The quantitative risk assessment of swimming pools is based on an incident-based modeling of occurrence, since there are no available measurements of the Giardia and Cryptosporidium concentrations in swimming pools. These different strategies should of course be considered when the different scenarios are compared. Overall it is evaluated that it is acceptable to compare the results from the different scenarios.

Infection by drinking water was not assessed quantitatively since there are not sufficient data available for this. A qualitative assessment indicated that the public water supply offers a high level of protection, but showed also some cases where contamination of the water supply could occur, e.g. by breakages in the distribution system. There is probably a higher risk in the high number of small (less than 9 consumers) water supplies without hygienic protection barriers. Such contaminations however, will be very local and will only affect few persons.

Table 0-1. Summery of calculated typical values for infection by the different scenarios.

1 Typical is the 50%-fractile, i.e. the median value
2 ?: No quantitative risk assessment.

The results of the quantitative risk assessments are summarized in table 0-1. The calculated values for risk per exposure can be used for an estimate for the number of infected in Denmark per year – and the typical (median) values are given in table 0-1. There is of course a certain uncertainty on estimates of number of infected per year – but it was beyond the project to estimate a confidence interval.

Floods of wastewater in cities are no major risk, since less than one person is estimated to be infected per year. Of the investigated scenarios recreational use of especially marine surface water is the route of infection, which results in most cases of giardiasis, whereas swimming pools are the most important route of infection for cryptosporidiosis. Hot water swimming pools are the route of infection, which result in the highest number of infections among the three quantitative scenarios.

In terms of risk per exposure Giardia is a larger problem in recreational use than in swimming pools, whereas Cryptosporidium is a larger problem in swimming pools than in recreational use. This is mainly due to that generally fewer Giardia cysts than Cryptosporidium oocysts are needed to cause infection and that the environmental concentration generally is higher for Giardia cysts than Cryptosporidium oocysts. Furthermore, Giardia is much more sensitive to chlorination than Cryptosporidium.

The risk of infection caused by floods is small because the intake of water per exposure is very small (around 1 mL), and because the annual number of exposure is only 15,000. The number of exposures to surface water (5 millions) in connection to recreational use is much higher, and the water intake is higher (around 40 mL per exposure). Therefore the estimated number of infected in connection with recreational use of surface water is higher, despite lower concentrations of Giardia cysts and Cryptosporidium in these environments than during flood events.

In connection to recreational use of surface water the highest risk for infection is by Giardia in marine waters The calculated risk for infection during recreational use of surface waters is more uncertain than for the other scenarios, because only few Danish data on (oo)cyst concentration are available for fresh water, and the concentration in marine surface waters is based on rough estimations of dilution of freshwater discharged into the marine water.

In connection to swimming pools the highest risk is for Cryptosporidium in hot water pools, where typically 141 are infected per year. The highest uncertainty in the estimations regarding the risk of infection in swimming pools is the estimation of prevalence of persons in the population excreting (oo)cysts.

Surprisingly, the investigated scenarios (excluding drinking water) for water related infection only account for 2% of the annual cases of Cryptosporidium and less for Giardia. In evaluating the values it is important to consider that they are average values. This means that a single outbreak e.g. in a swimming pool can result in several hundreds of infected although this will happen rarely. Furthermore the 2% can be 2 to 10 times higher because of uncertainties in the calculations and in the estimation of number of infected per year in Denmark. This uncertainty is estimated, and not a result of an uncertainty calculation.

There are no Danish or European recommendations regarding acceptable risk, but the US Environmental Protection Agency recommends that the risk should be less than 1 infected by Giardia or Cryptosporidium per 10,000 per year (i.e. the risk is less than 10^-4 per year). These recommended values are touched or exceeded by a typical number of exposures for the scenarios risk of giardiasis through recreational use of fresh waters and risk of cryptosporidiosis through hot water swimming pools.

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Version 1.0 Februar 2006, © Miljøstyrelsen.