Biology Reference
In-Depth Information
population processes can generate observed macro-epidemiological
trends, a point noted by Cassie in 1963
117
and revisited by Crofton in
1971
116
, both prior to any formal mathematical analysis. It is also apparent
that without simplifying assumptions or approximations to aid analytical
exploration,
62,192,195
models can become too intractable to yield useful
results.
Hybrid models
196
are essentially deterministic but include, in a
phenomenological manner, stochastic elements such as the distribution of
parasites among hosts or the observed host demography (Chapter 9). This
permits the population-level effects of non-linear density-dependent
processes, and demographic variability in infection and transmission, to
be accounted for. This approach has been used to great effect
16,20
and has
shaped our fundamental understanding of the population dynamics of
helminths in general, and of A. lumbricoides in particular.
Parasite Distribution and Density-dependent Processes
The size and stability of helminth populations is critically regulated
by the interaction between the overdispersed distribution of parasites
among hosts
119,197,198
and the predominant mechanism causing the
overdispersion.
199
In general, overdispersion enhances the severity of
density-dependent processes, highlighting the critical requirement for
population dynamics models to account for this ubiquitous characteristic
of helminth populations.
Negative density-dependent mechanisms restrict the rate of pop-
ulation growth at high densities and induce resilience to perturbation
resulting from the relaxation of constraints when densities
decrease.
16,64,200,201
The only directly observable density-dependent
process in A. lumbricoides infections is on female worm fecundity,
namely, the per capita egg output (eggs per gram of feces) declines with
increasing worm burden (
Figure 7.4
), although similar but unobservable
effects on mortality or establishment, possibly interacting with host
immune responses, are possible.
197
Regardless of the specific density
dependencies in operations, a worrying connotation for the success of
A. lumbricoides control is that increasing degrees of overdispersion,
which occur with decreasing average worm burdens,
57
enhance the
parasite's resilience and sustainability at very low population densities.
202
A positive, or facilitating, density-dependent effect common to all
dioecious (separate sexes) obligatory parasites is the female mating
probability;
203
the chance of a female worm infecting a host without a co-
infecting male worm decreases with increasing population density.
Facilitating density-dependent mechanisms (Allee effect
204
) act as
a destabilizing force, either promoting runaway growth (though at high
population densities constraining processes are likely to operate) or, at
low densities, failing to facilitate enough growth leading to population
