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by the World Health Organization (WHO), which still advocates the use
of prevalence as a marker of intervention success. 40,59 As illustrated in
Figure 9.2 , orders of magnitude change can occur in the mean intensity of
infection with little change in prevalence. More generally, efforts to refine
parameter estimates of key factors that control the rate at which infection
bounces back to pre-control levels following an intervention has been
very limited in the past two decades. This is despite the obvious impor-
tance of these parameters in the design of effective control programs
based on mass chemotherapy.
There are a number of important needs if progress in epidemiological
understanding is to be made and if the design of interventions is to be
based on quantitative calculation. In epidemiological domains, the key
unknowns are the importance of density-dependent constraints on
parasite establishment in the human host and adult parasite survival, the
patterns of mixing in infected communities with respect to exposure to
infective stages contributed to the environment by individuals perhaps of
different ages, and the importance of acquired immunity versus exposure
to infection as determinants of convex intensity profiles by age. 10,11,29 All
require more focused study. Mathematical model-based simulation
studies can help in the absence of hard information, to understand their
relative importance in shaping observed pattern and in parasite pop-
ulation responses post-treatment. More work of this nature needs to be
carried out, given the obvious difficulties in designing epidemiological
studies for measurement and the time scales over which such studies will
have to be performed.
Perhaps the most urgent need is for better estimates of the key
parameters. These include the severity of density dependence in fecun-
dity (the parameter Y), estimates of parasite life expectancy in the human
host, egg survival under different environmental conditions, age depen-
dence in aggregation, sex ratios in adult worm populations, and age-
specific forces of infection. Estimation of many of these parameters
requires worm expulsion studies with eggs per gram data taken before
expulsion and detailed follow-up during reinfection with appropriate
stratification by host age and gender. Model fitting to observed patterns of
reinfection is one approach using age and gender stratified models.
Stochastic individual-based models should ideally be used, with MCMC
methods employed in parameter estimation.
The final need relates to the design of intervention programs based on
school age or community-based chemotherapeutic treatment. Ascaris is
perhaps the most difficult STH to control by chemotherapy due to high
rates of reinfection and the robust egg infective stage which can persist in
the environment for many months under favorable environmental
conditions. Currently, the much increased volume of drug donations to
treat those infected with STH in developing countries combined with
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