Biology Reference
In-Depth Information
“breakpoint” has been crossed. The simple rebound behavior is exactly
what is observed in reality, following the cessation of a community-based
chemotherapy program or in the intervals between rounds of treatment
(
Figure 9.6
). The existence of the breakpoint behavior is less readily
observed since the breakpoint is predicted to lie at such low mean worm
burdens (
Figure 9.5
).
Broadly speaking, by comparison with other infectious agents, such as
viral and bacterial pathogens that can induce immunity and have short
generation times of a few days, the dynamics of helminth parasites are
fairly predictable and simple in form.
14
This is largely due to two factors;
the inability of the human host to generate protective immunity to rein-
fection and the long life expectancy of helminth parasites in the human
host which range from many months to many years.
10
The insights from
these simple models have given many useful insights, and continue to
inform planning of intervention programs (see below).
0.0
0.2
0.4
0.6
0.8
1.0
Years since treatment
FIGURE 9.6
Rebound of
Ascaris
worm burden following treatment; data and fitted
model (Eqs.
(9.4) and (9.5)
). Fitted parameters: R
0
¼
5.5, treatment efficacy
¼
98%. Other
0.04. Data and parameter values from.
33
parameter values:
s ¼
1/yr, k
¼
0.57,
g ¼
