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TABLE 9.1 Key issues in the control of soil-transmitted helminths by chemotherapy 42
1. For a given transmission level, how often should mass or targeted chemotherapy be
administered to sustain infection prevalence and intensity below defined levels?
2. In terms of cost effectiveness, is it best to target school children, those predisposed to
heavy infection, or the entire community?
3. As the prevalence and intensity fall after repeated rounds of treatment, can the interval
between treatments increase, and by how much?
4. How is the interval between treatments influenced by the species mix of STHs in the
community?
5. How do the demography of the population and the starting geographical distribution of
infection influence the structure of the optimum treatment programwhere resources are
finite?
6. What level of infection across a community should trigger mass chemotherapy to
minimize morbidity?
7. Is elimination in a defined area possible by chemotherapy alone?
8. How might repeated mass treatment influence the evolution and spread of drug
resistance
and how can this risk be minimized and how can it be best monitored?
9. What should be the target of STH control programs?
10. What are the best indicators of assessing the impact of STH control?
11. What is the optimal design and sampling requirements of programs for monitoring and
evaluation?
e
are an interesting problem. For the simplest model structures of parasite
infection
death, the time required to return to a fraction f of the pre-
treatment equilibrium worm burden, t f , from a zero worm burden in
the treated individuals is given by:
e
t f ¼lnð1
f
Þ=m 1
(9.11)
This result is revealing; it states that it is only the life expectancy (1/ m 1 )
of the adult worm that determines bounce-back time. In other words it is
the generation time that determines this key aspect of intervention.
Generation time is an ecological concept and it defines the average time
for a parasite from birth to achieve sexual maturity. It is an essential aspect
of the epidemiology of all infectious agents. It is not to be confused with
the sum of the development times around the life-cycle from birth of an
egg to maturation to an infective stage, and on to sexual maturity in the
human host. Ecological generation time is this plus the weightings
induced by mortality at all stages of the life-cycle. For the STH worms
these generation times are in the domain of a few months to many years.
Ascaris has one of the faster generation times for a helminth, and this
ensures rapid bounce-back times. A key indicator of this is the need to
treat every 3 months or so to bring Ascaris intensity to low levels in areas
of high transmission intensity. 45
The assumptions embedded in the model on which Eq. (9.11) is
calculated are far too simple to reflect reality
but they provide a rough
e
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