Biology Reference
In-Depth Information
Estimating
disability.
For many
diseases
and
injuries,
age
year estimates of incidence exist and can be used to
estimate disability in terms of YLDs. This is not the case for intestinal
nematodes and therefore estimates of disability need to be extrapolated
fromdataontheprevalenceofinfection.Todothis,Chanetal.
15
e
16
quantified relationships between prevalence of infection and worm
burden and between worm burden and potential disability, based on
the negative binomial distribution.
51
They further assumed that there is
some age-specific worm burden threshold above which morbidity is
likely to occur. Developmental and cognitive effects of infection in
childhood were assumed to occur at lower worm burdens, and there-
fore lower worm burden thresholds were assumed to correspond to
disability arising from cognitive and growth deficits.
Whereas Chan et al.
15
e
16
used worm burden to define the thresholds
for morbidity, the core team in the 2010 study related prevalence of
infection to the intensity of infection, as expressed by quantitative egg
counts. This was because most literature on the health impacts of intes-
tinal nematodes expresses results in these terms. Estimates of infection
prevalence provided by the expert group were related to the intensity of
infection using a negative binomial distribution and stratified by inten-
sity: light
country
e
e
¼
1
1999 eggs per gram (epg); medium
¼
2000
3999 epg; and
e
e
heavy
4000 epg. Subsequently, the core modeling team assumed that
mild abdominopelvic problems were associated with medium intensities
and mild infectious disease with heavy intensities (
Table 13.3
).
To estimate the prevalence of wasting attributable to heavy
A. lumbricoides infection, the core team adopted a two-stage approach.
First, the prevalence of wasting among children under five years old was
independently estimated using available data. Second, this wasting was
apportioned to two underlying causes: (1) heavy intestinal nematode
infection and (2) Protein Energy Malnutrition (PEM). The prevalence of
wasting due to the former was calculated by shifting the 2006 WHO
reference population weight-for-height distribution according to the
product of (1) the prevalence of heavy intestinal nematode infections and
(2) the average shift in weight-for-height per case of heavy intestinal
nematode infection, based on a meta-analysis of randomized controlled
trials of mass de-worming.
6
The pooled effect across identified studies
was a change in weight-for-height z-score per individual with heavy
intestinal nematode infection of 0.4938. Finally, the overall prevalence of
wasting attributable to heavy intestinal nematode infections due to
ascariasis versus trichuriasis versus hookworm was calculated by the
relative prevalence of heavy intestinal infection due to each individual
species. The prevalence of wasting due to PEM was calculated by simply
subtracting the prevalence of wasting due to heavy intestinal nematode
infections from the estimated overall prevalence of wasting.
¼
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