Environmental Engineering Reference
In-Depth Information
infected is (1
P inf ). For an annual probability, n = 365. So, the annual probability
is given as
365
P ann ¼ 1 ð 1 P inf Þ
ð 6 : 4 Þ
Now, infections are necessary to cause disease, but not all infections will result
in symptoms of illness. In general, economists and epidemiologists express the
disease burden in terms of Disability Adjusted Life Years or DALYs. We interpret
DALYs to be years lived with a disability plus lives lost due to a hazard, such as an
E. Coli outbreak. DALYs take into account both illness outcomes as well as the
duration and severity of the illness. Thus, the calculation of DALYs per infection is
given as
DALY ¼ X
n
P ill j inf
ð
Þ XPoutcome i j ill
ð
Þ X Durationi i XSeverityi i
ð 6 : 5 Þ
i ¼ 1
where:
n
is the total number of outcomes;
P ill j inf
ð
Þ
is the probability of illness given the infection
P outcome i j ill
ð
Þ
is the probability of outcome i given illness
Duration i
is the duration in years of outcome i
Severity i
is the severity weighting for outcome i
The advantage of using DALYs is that it can take account of not only illnesses
like enteric illnesses but also more serious disease outcomes such as the Guillain-
Barre syndrome associated with Campylobacter. Disease burdens vary widely
depending on locality. For example, the disease burden per 1,000 cases of rotovirus
diarrhea is 480 DALYs in low-income areas, whereas in high-income areas, it is
only 14 DALYs per 1,000 cases. Other examples of disease burden estimates for
drinking water contaminants are reproduced in Table 6.3 .
The disease burden based on DALYs would be calculated using the number of
infections per year, i.e. maximizing Eq. 6.3 above for the population multiplied by
the DALY contribution per infection.
It should be obvious that the calculations involved are not so simple. But for
freshwater beaches in southern California, QMRA has been used to determine the
safety or otherwise of a particular beach.
6.3.4.2 Implementing QMRA
Figure 6.5 shows the main steps in the implementation of a QMRA procedure, from
the
first step of estimating the mean probability density of a given pathogen, to
estimating the costs of infections based on DALYs per person per year. It is also
necessary to distinguish between baseline hazards as well the extra
burden
of
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