Biology Reference
In-Depth Information
collapse.
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Just as with constraining density dependencies, positive
density dependencies are enhanced by the overdispersed distribution of
worms among hosts since the majority of worms reside in large infra-
populations. Consequently, the threshold density of population
collapse, or breakpoint density (also known as the transmission break-
point or unstable equilibrium), is likely to be extremely low for
A. lumbricoides,
205
partly on account of the assumed polygamous nature of
male worms (one male can mate all females within a host), but also
because of the overdispersed worm distribution.
Age-structured Models
Many of the parameters included in population dynamics models are
likely to vary with host age. Indeed for A. lumbricoides, age-related
changes in exposure and (protective) acquired immunity have long
been considered as the principal drivers underlying the observed convex
age-infection profiles.
16,20,67
Age-related changes in contamination of the
environment are equally as important, but because of a lack of indepen-
dent epidemiological data, relative rates of contamination among age
groups must be either assumed, or if possible, inferred indirectly from
model output.
206
In their most general form, age-structured deterministic models
describe changes in the mean worm burden with respect to time and host
age, often using partial differential equations (see Chapter 9).
16,201
Consequently, they offer greater realism in two key ways. First, they can
capture adequately observed age-dependent epidemiological
patterns.
16,20
Second, the interplay between age-infection profiles and the
demography of the host population is modeled explicitly; the contribu-
tion to transmission of a particular age group depends on their average
worm burden, the net severity of any density-dependent constraints on
female worm fecundity, their contamination of the environment, and their
proportional representation in the host population.
16,201
Age-structured models may also be formulated in a less general
manner, tailored to the context of their application. This can ease
community-specific parameterizations, especially when limited epide-
miological data are available. For example, the EpiWorm
206,207
trans-
mission model for A. lumbricoides, T. trichiura and hookworm, which is
one of three models included in the EpiDynamics software suite of age-
structured helminth models (the others being EpiFil for lymphatic filari-
asis
208,209
and EpiSchisto for schistosomiasis
210
), is optionally divided into
age groups of children and adults, or school children, other children and
adults. This is because the chief aim of the model is to simulate the effect
of mass treatments targeted at (school-age) children, the predominant
strategy of global STH control.
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