Biology Reference
In-Depth Information
Age weighting and discounting. In the 1990 study, results were
computed using DALYs that included discounting and age-weighting.
This approach valued years lived as a young adult more highly than
years spent as a child or older adult, a reflection of the societal emphasis on
productivity. It also discounted future years at 3%, where each year in the
future is valued 3%more than the current year. Critics of these assumptions
argued that years lived by people of different ages and generations should
be valued equitably
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and consequently, based on a broad consensus,
the 2010 study omitted both discounting and age-weighting.
Disease sequelae and disability weights. The 1990 study defined six
broad disability classes and then mapped each sequela into the appro-
priate class, as judged by panels of healthcare professionals. Whereas the
2010 study derived disability weights from judgments of the general
public about the health loss associated with the health state related to each
disability sequela.
The disabling sequelae anddisabilityweights assigned to ascariasis in the
two GBD studies are presented in
Table 13. 3
. An important change in the
2010 study from 1990 study was the removal of cognitive impairment as
a sequela. This was justified by the perceived paucity of evidence on the
cognitive impact of intestinal nematodes. Many of the trials examining the
impact of A. lumbricoides and de-worming have small sample sizes or are
only quasi-randomized. In their 2012 Cochrane review of the available
randomized controlled trials of de-worming, Taylor-Robinson et al.
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found
contrasting evidence of nutritional benefits, and little support for cognitive
or educational benefits. There was, however, a difference in studies that
screened for intestinal nematode infections and mass de-worming studies,
with significant nutritional gains in infected individuals, but little impact in
unscreenedpopulation studieswhere any effectmay be diluted. It shouldbe
noted, however, that theCochrane reviewexcluded studies that treated both
intestinal nematodes and schistosomiasis. Furthermore, critics of the
Cochrane review
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have argued that many of the underlying trials of de-
worming suffer from a number of methodological challenges, including
non-assessment of treatment externalities, inadequate measurement of
cognitive outcomes and school attendance, and sample attrition, and that
further, better-designed studies are required. It is probable that, because of
the insidious nature of intestinal nematode infections and the complexity of
factors influencing cognition and education, it may be difficult to reach
a definitive conclusion on the impact of ascariasis on cognition.
Estimating prevalence of infection. In both GBD studies, DALY esti-
mates were based on extrapolating the population at risk of morbidity
(intensely infected individuals)
from empirical observations of
the
proportion of the population infected.
Chan et al.
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based their prevalence estimates on an extensive search
of the published literature for community-based studies (i.e. not hospital
