Biology Reference
In-Depth Information
children change rapidly over the reinfection period; behaviorally-
mediated exposure may change rapidly over the first three years of life
as children learn to walk and explore their environment, which will
increase their exposure to A. lumbricoides eggs. This changing “force of
infection” (the per host rate of parasite acquisition) has important
implications for control programs selecting cohorts of children to be fol-
lowed up for the purpose of monitoring and evaluation.
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An additional observation from reinfection studies that have
collected data on the size (weight/length) of individual worms
44,114
is
that the size of worms is host-age dependent, probably due to crowding
constraints exacerbated by the size of the gut lumen, and that after
treatment newly acquired worms appear to grow rapidly, possibly due
to the release of density-dependent constraints. Worm size is associated
with fecundity
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and so these observations have potentially important
consequences on transmission following anthelmintic treatment,
although the epidemiological significance of this is yet to be fully
elucidated.
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MATHEMATICAL APPROACHES
The seminal work of Crofton in 1971
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embedded the frequency
distribution of parasites among hosts at the heart of quantitative parasi-
tological research. Crofton revisited the words of Cassie,
117
who in 1963
wrote that, “The frequency distribution model may be applied at two
levels, the empirical and the fundamental. Empirically it is desirable to
condense the sample data, so that any given population may be described
by a few parameters, which are readily comparable with the corre-
sponding parameters of another population. [
] The fundamental model,
on the other hand is based on some hypothesis of some real biological
significance. [
.
] If it fits the data better than other possible models, it
provides some justification for the hypothesis concerned.”
Classical statistical inference is grounded in the “empirical,” and
a great deal of applied statistical analysis in the area has focused on the
adequacy of distributions for describing parasitological data and on the
best methods of estimating their parameters. Population dynamics
models are more associated with the fundamental, being defined in terms
of underlying population (biological) processes which determine the
number of parasites within a host. To what extent the frequency distri-
bution of worms among hosts is considered can vary greatly, from
a simple deterministic description of changes in the (population) mean
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.
e
which may include “empirical” components to account for the effect of
the frequency distribution on population processes (the so-called hybrid
structure, see “Dynamics models of infection and transmission,” below
