Biology Reference
In-Depth Information
published in the 1980s, but it is only recently for the STH group of
parasites that these approaches have come back into focus
10,16,39
and
questions raised on how best to improve upon these deterministic,
hybrid, and stochastic frameworks. Most of current activity is centered on
chemotherapeutic treatment, either targeted at children of school age,
those most at risk (predisposed to heavy infection
35
), or the entire
community. The donation of drugs in the 2012 London Declaration is
targeted at school children, following the school-based de-worming
targets published by the WHO.
40,43,44
Although effective and cheap drugs
are available (often donated free by the pharmaceutical companies that
manufacture them), it is widely recognized that, ultimately, treatment
alone will not eliminate Ascaris infection. Effective sewage disposal is
essential, as so clearly illustrated in South Korea, Japan, and the southern
states in the US in their elimination campaigns.
Mathematical models can be used to investigate different strategies of
treatment and other control options such as sanitation that reduces the
pool of infective stages, but they reveal how difficult it is to reduce the
population abundance of a parasite that evades creating protective
immunity in the human host. After chemotherapeutic treatment to expel
the worms, reinfection immediately starts to replenish the parasites in the
human host via the pool of infective stages in the habitat of the commu-
nity. As discussed earlier, bounce-back via reinfection to pre-treatment
levels is the norm, unless treatment is both intense across all age classes
in the population and takes place repeatedly.
The existence of a breakpoint in transmission (an unstable equilibrium,
as discussed above), resulting from adult wormmating probabilities, does
imply that repeated treatment could lower the average worm load to
a point where it falls below the breakpoint and hence the parasite pop-
ulation moves to extinction at the stable equilibrium of M
0. In practice,
this breakpoint is close to an average of zero worms per host due to the
high degrees of parasite aggregation observed in most
ΒΌ
if not all
communities.
An illustrative example of repeated treatment using the basic model
defined in Eqs.
(9.4) and (9.5)
is presented in
Figure 9.8
. The parameter
assignments are set to mimic Ascaris in a setting with moderate to high
levels of transmission. Note the jagged pattern of mean intensity over
time as each round of treatment reduces worm burdens followed by
a period of reinfection before the next treatment. In one example the
frequency of treatment is sufficient to cross the breakpoint and the mean
worm load moves to extinction. Elimination will persist provided no
immigration of infected persons occurs to repopulate the habitat with
infective stages.
Sanitation measures act to reduce R
0
due to the reduction in the pool of
infective stages in the community and, concomitantly, the average force of
