Biology Reference
In-Depth Information
(scenario C) results in a higher worm burden in the children than in the
adults, as is seen in several settings (note this model does not include any
immunity). All models have the same mid-range R
0
value of 3. We have
simulated these scenarios for A. lumbricoides, with a life expectancy of 1
year (
Figure 9.11
).
Under these scenarios, and for these parameter settings, treatment of
childrenhasamodestimpactontransmission,evenathighlevelsof
efficacy (95%) and coverage (85%). There are benefits for children in
terms of period of low average worm burdens. The impact on the rest of
the community is limited, due to the proportion of the worm pop-
ulation actually reached by treatment. The proportion of the population
who are children being 30% is at the high end of school-attending
fraction of the population.
55
Increasing the frequency of treatments to
six monthly, there are some small additional benefits (bottom row in
Figure 9.11
).
When we divide the population into two groups who both interact
equally with the reservoir (scenario B), the direct effect of school-based
treatment on school-aged children is clearly shown, and is what would
be measured in a monitoring and evaluation program among the
children. The indirect effect on adults is much smaller. It should be
noted that the rate of bounce-back after treatment is slightly lower in
this model than in scenario A. This means that homogeneous mixing
models, such as those earlier in this chapter, when used to describe non-
uniform treatment regimens (targeted at some portion of the pop-
ulation) will always underestimate the time to recover to pre-treatment
levels.
When we assume that children overcontribute to infection (scenario C),
which is likely to be a more realistic scenario resulting in higher worm
burdens in school-aged children, the effect of treatment on the school-
aged group is quite pronounced. However, for these parameter values
the impact at the community level is only marginally improved, due again
to the small proportion of worms treated.
D
ISCUSSION AND FUTURE NEED
S
Much of the basic framework for the study of the transmission
dynamics of helminth infections was laid down in the 1960s and 1980s.
Rather little has been achieved since that time in model development and
parameter estimation. Concomitantly, little use has been made of the
insights gained from mathematical models in the design of public health
policy for the control of STH and schistosome infections. A good illus-
tration of the general ignorance of these insights among public health
policy makers is well illustrated by the recommendations for STH control
