Biology Reference
In-Depth Information
inadequate water and sanitation facilities, and poor hygienic practices
promote fecal contamination of the environment and fecal
oral contacts.
Unsurprisingly, Ascaris infection is prevalent in many less developed
countries today and historically was endemic in some developed
countries.
The majority of individuals with A. lumbricoides will exhibit no signs
or symptoms. This is because pathology is strongly related to the
number of worms present (the intensity of infection),
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2
and most
individuals harbor only a few worms.
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4
In the minority (5%) of indi-
viduals who harbor large worm burdens, infection can result in clinical
disease, including intestinal, biliary, and pancreatic obstructions
(see
Box 13.1
). If left untreated, these complications can prove fatal.
5
In
addition to these acute manifestations, intervention studies have
demonstrated that moderate worm burdens are associated with revers-
ible growth deficits in children.
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Chronic ascariasis has also been
implicated in some studies with reduced cognitive performance and
school peformance.
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Reliably estimating the extent of the problem of A. lumbricoides and
ascariasis is difficult for a number of reasons. First, A. lumbricoides infec-
tion is typically diagnosed by the microscopic detection of the parasite's
eggs in stool samples, a procedure that misses light infection.
11
Second,
microscopy does not detect non-fecund infections (single worm or single
sex) which may represent an important minority of infections.
12
Thus, any
estimates of infection prevalence are likely to be conservative and an
underestimate. Third, much of the acute clinical morbidity caused by
ascariasis is non-specific, making attribution problematic. Fourth, the
chronic effects of infection on growth and cognition are subtle, making
their impact underappreciated and revealed only by well-designed
intervention studies.
7,13
These challenges have contributed to the debate
as to the true impact of A. lumbricoides and ascariasis. They have also
meant that estimates of the global distribution and disease burden of
A. lumbricoides and ascariasis have inevitably been based on informed
approximations, using the best available information.
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This chapter reviews efforts to define the global distribution and
disease burden of A. lumbricoides and ascariasis. It begins by evaluating
the biological limits imposed by climatic factors to A. lumbricoides trans-
mission and the influence of urbanization and control measures on
contemporary distributions. Next, past and current estimates of the global
population infected with A. lumbricoides are reviewed. The Global Burden
of Disease (GBD) study is then introduced, detailing how estimates of the
disease burden caused by ascariasis have been calculated, both in the
original 1990 GBD study and the recent 2010 study. Finally, the limitations
of the GBD approach to estimating the full extent of the problem of
ascariasis are discussed.
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