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estimates of burden have been extrapolated from data on the prevalence
of infection, where is it assumed that only a fraction of infections, those
which are intense, are associated with ascariasis. 16,39
An essential prerequisite to estimating the global burden of ascariasis is
a framework for assessing its burden relative to all other health condi-
tions. The first comprehensive approach to estimating the global health
burden, including ascariasis, was provided by the Global Burden of
Disease (GBD) study. 40 e 41 For a summary measure of population health
this study uses the Disability-Adjusted Life Years (DALYs), which
incorporates both years of life lost from premature death (YLLs) and years
of life lived with disability (YLDs) into a composite estimate. YLLs are
computed by multiplying the number of deaths at each age x by a stan-
dard life expectancy at age x. YLDs are computed as the prevalence of
different disease and injury sequelae multiplied by a disability weight for
each sequela. Disability weights are a quantification of the severity of
health loss associated with each sequel and range from 0 to 1, where 0 is
commensurate with perfect health and 1 is commensurate with death.
The original GBD study
referred to here as the 1990 study
e
produced results for 1990 which were initially published in the World
Development Report 1993: Investing in Health 42 and separately in
1994 and 1997. 40,43 e 44 Updated GBD estimates were subsequently
produced for each year from 1999 to 2004 by the World Health
Organization, but these estimates were based on updated epidemi-
ology for only a subset of conditions. A major update to the GBD was
initiated in 2007, 45 led by a core team of researchers who also devel-
oped the methodology, while expert groups conducted systematic
reviews of incidence and prevalence of disease and disabling sequelae,
reporting their figures to core team members. This initiative
e
referred
e
to here as the 2010 GBD study
provided estimates for 2010, 2005,
and 1990, with key findings published in 2012. 46,47
Estimates for ascariasis in the 1990 study were based on analytical
work by Chan et al., 15 e 16 whereas the 2010 study estimates are based on
a central modeling approach, utilizing empirical estimates of infection
prevalence provided by an intestinal nematode expert group (led by the
current authors). There are a number of important methodological
differences between the 1990 study and the 2010 study, both in terms of
overall study design and in how ascariasis estimates were calculated,
including the following issues.
Project scope. The 1990 study provided estimates for 107 diseases and
injuries and for these conditions 483 sequelae were identified. The 2010
study identified 291 diseases and injuries which have 1160 sequelae and
provided estimates for 2010, 2005, and 1990. The 2010 study also provides
estimates for 20 age groups instead of eight; and 21 regions instead of the
14 used in the 1990 study.
e
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