A3.5 Selection of reference pathogens

The calculation of LRVs cannot reasonably be undertaken for all enteric pathogens in drinking water. Reference pathogens (for protozoa, bacteria and viruses) are selected as a conservative model for the group of microorganisms that they represent. If the drinking water is protected from the reference pathogen, then it is assumed that the drinking water is protected from all pathogens in that group. Due to variations in characteristics, behaviours and susceptibilities one bacterium, one virus and one protozoan are selected considering:

• Disease prevalence and outbreaks via drinking water: This gives an insight into the organisms that are capable of causing disease, and also those that are likely to be excreted into the environment.

• Persistence: Which pathogens are expected to be of concern due to the scientific evidence for their persistence in drinking water?

• Infectivity: Which pathogens are most infectious? For which pathogens are scientific data available to support the characterisation of a dose-response relationship?

• Disease burden: Which pathogens are of most concern in terms of their impact on community disease burden?

Estimated prevalence rates for six candidate reference pathogens are summarised in Table A3.1 (Gibney et al. 2014).

Table A3.1 Case numbers and mortality for selected pathogens causing acute gastroenteritis in Australia in 2010 (Gibney et al. 2014)

Pathogen	Cases of disease per 1000 population	Deaths per 1 million population
Campylobacter	34.7	2.3
Salmonella	3.2	4.0
Rotavirus⁽¹⁾	26.5	0.9
Norovirus	97.6	0.8
Giardia	27.5	0
Cryptosporidium	8.8	0

Note:

1. for the entire population assuming an unvaccinated population.

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There is limited synthesised information on the causative agents of drinking water-related outbreaks in Australia. Data are available for the period 2001–2007 (Dale et al. 2010) based on collation of information on outbreaks of gastroenteritis since 2001 for all transmission routes by OzFoodNet, an Australian national network for the surveillance of foodborne diseases. The data demonstrated that a very small number of outbreaks from those reported (10 out of 6515 cases, <0.2%) could be attributed to drinking water. Of 6515 gastroenteritis outbreaks, 54 (0.83%) outbreaks were classified as either waterborne or suspected waterborne. Drinking water was the suspected source for 19% (10) of these outbreaks and 78% (42) were attributed to recreational water. Of the drinking water outbreaks implicated pathogens were found on all but one occasion. These included Salmonella (five outbreaks), Campylobacter jejuni (three outbreaks) and Giardia (one outbreak). Of the 42 outbreaks attributed to recreational water, all except one were reported to be due to Cryptosporidium contamination of swimming pools.

Protozoa

Cryptosporidium has been selected as the reference protozoan due to its resistance to chlorine disinfection, high infectivity and the relatively large amount of data available for characterisation. This is consistent with the Australian Guidelines for Water Recycling (Phase 1) (NRMMC, EPHC and AHMC 2006).

Bacteria

Campylobacter has been selected as the reference bacterial pathogen. The selection of Campylobacter is a more conservative option than other candidate bacteria since it is significantly more infectious. This is consistent with the Australian Guidelines for Water Recycling (Phase 1) (NRMMC, EPHC and AHMC 2006) and the WHO Guidelines (2014).

Virus

The reference virus is a combination of culture-based enumeration data for adenovirus and dose-response and health impact data for norovirus. This is because it is desirable to rely on enumeration data from culture methods, yet the viruses of most health significance (currently norovirus) cannot be readily and routinely detected using culture-based methods. This approach follows the advice of some of the earliest proponents of risk assessment (Regli et al. 1991).