Biology Reference
In-Depth Information
Critical Treatment Coverage
One further application of models to the design of community-based
chemotherapy intervention studies is to use them to get insights into
what fraction of the host community needs to be treated per interval of
time to lower R
0
to below unity in value. For simple hybrid models
without age structure it is fairly straightforward to calculate the critical
fraction of the population, g
c
,thatmustbetreatedperunitoftimeto
lower the effective reproductive number (transmission success under
the influence of community-based chemotherapy), R, below unity in
value
16
:
g
c
¼ð1 exp½ð1
R
0
Þ
s
=
L
1
Þ=
h
(9.13)
Here, h defines drug efficacy as a fraction of worm expelled by treat-
ment, and
is the time interval between treatments. For the commonly
used drugs to treat Ascaris, h is usually around 0.9 to 0.95 in value.
47,48
The
parameter g
c
is defined in the same time units as used for parasite life
expectancy (fraction treated per unit of time). Note that it is transmission
success, R
0
, and parasite life expectancy, L
1
(representing the rate of
turnover of parasites in the human host in the absence of treatment), that
define how many and how often treatment should take place. The influ-
ence of R
0
on the impact of the treatment program and the value of g
c
are presented in
Figure 9.10
, which shows how changes in the fraction
treated influence the mean intensity of infection and the prevalence at
equilibrium. This figure again indicates how limited the measure of
prevalence is of the success of community-based treatment.
s
Targeting by Age
Community-based treatment can take different forms ranging from
the treatment of the entire population of all ages above 12 months of age,
the treatment of school-aged children (5
15 years of age), and targeted
treatment of only those with high worm loads. The last strategy benefits
from the observation of predisposition to heavy infection in STH first
recorded by Schad and Anderson
35
for hookworm, and subsequently
observed for Ascaris,
38,49
Trichuris,
50
and schistosomes
36
infections. Tar-
geting is rarely used in practice despite simulation studies recording its
effectiveness.
39
The reasons relate to the perceived cost of identifying those
with heavy infections. However, the evidence for predisposition suggests
that you may only need to identify them once, but this depends on how
predisposition varies over time for soil-transmitted helminths.
51,52
The
drivers of predisposition may be multifactorial, including household
factors and genetic factors.
49,53,54
Whatever the drivers, targeting by
intensity, whether at an individual or household level, may be a method
of getting maximum impact for lower numbers of treatments. More
e
