A3.4 Defining the health outcome target
It is not possible to achieve zero health risk from the consumption of treated drinking water. Rather, a tolerable risk needs to be set as a health outcome target that can be considered to represent a target of safety across a population.
The metric selected to define the health outcome target is the DALY. The DALY allows aggregation of health impacts to provide an overall measure of burden of disease (Leder et al. 2012). Box A3.1 provides an overview of the DALYs. DALYs also allow the comparison of the effects from different health complaints.
The microbial health outcome target for enteric pathogens that applies to Australian drinking water supplies is 1 x DALY pppy, also referred to as 1 µDALY.
Overview of Disability Adjusted Life Years (DALYs)
One Disability Adjusted Life Year (DALY) can be thought of as one year of healthy life lost. The sum of these DALYs across the population is referred to as the burden of disease. This can be thought of as a measurement of the gap between current health status and an ideal health situation where the entire population lives to an advanced age, free of disease and disability.
DALYs for a disease or health condition are calculated as the sum of the Years Lived with Disability (YLD) for people living with the health condition or its consequences and Years of Life Lost (YLL) due to premature death in the population: DALY = YLD + YLL.
The YLD corresponds to the number of years lived with disability multiplied by the average duration of the disease and a weight factor that reflects the severity of the disease. The scale of severity ranges from 0 (perfect health) to 1 (dead). The basic formula for YLD is the following: YLD = DW x L x I; where: DW = disability weight, L = average duration of the case until remission or death (years) and I = number of incident cases.
The YLL corresponds to the number of deaths multiplied by the standard life expectancy at the age at which death occurs. The formula for YLL is the following: YLL = N x L; where: N = number of deaths and L = standard life expectancy at age of death in years.
Example calculation of DALYs for Norovirus infections
Severities of health impacts (disabilities) are weighted within the range of 0 for good health to 1 for death (Salomon et al. 2015). Severities for outcomes of microbial infection include:
0.074 for mild diarrhoea
0.188 for moderate diarrhoea
0.247 for severe diarrhoea
Using an Australian example, infection with Norovirus causes:
mild diarrhoea lasting 2.1 days in 92.2% of cases
moderate diarrhoea lasting 2.4 days in 7.2% of cases
severe diarrhoea lasting 7.2 days in 0.6% of cases
death (severity rating of 1) in 0.0008% of elderly cases with an average YLL of 7.1 years
The DALY per 1,000 Norovirus cases is then:
= (0.074 x 2.1/365 x 922) + (0.188 x 2.4/365 x 72) + (0.247 x 7.2/365 x 6) + (1 x 7.1 x 0.008).
= 0.40 + 0.089 + 0.029 + 0.057
= 0.57
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