Interventions
Biogas
Environmental control
Chemotherapy
In the continuing
absence of a vaccine for schistosomiasis, it is necessary to rely on
various environmental and behavioral interventions to diminish risk of
infection. Since water
contact is the route of exposure of the vertebrate host, it is possible
to identify particularly hazardous aquatic environments and attempt to
control access to them by both humans and animals.
Alternatively, one may either reduce the chance of snail
infection by humans and animals or destroy the snails and their habitat.
In
China
the ancient and pervasive use of human excrement, termed night soil, is
mixed with that of animals and used for crop fertilization, and provides
a direct pathway for S. japonicum eggs to reach snails.
Hence the strategy of enhancing sanitation facilities and
conditions employed in other regions of the world to date has not been a
viable strategy in
China.
More commonly the large-scale use of chemotherapy for humans and
animals has been used and has the beneficial effect of both controlling
morbidity and subsequently reducing the concentration of eggs shed into
the environment.
While various
combinations of these control strategies have been used quite
successfully to reduce the incidence of disease in
China,
as recently as 1995 approximately 865,000 people and 100,000 water
buffalo were infected (Chen, 1999).
Control has been particularly difficult in certain regions
including the hill areas of
Sichuan
where our work has been focused. In order to move beyond morbidity
control to the elimination of schistosomiasis transmission, Chinese
policy-makers, as well as our research group, have recognized the need
for a more comprehensive approach, which incorporates environmental
interventions as well as traditional control methods. We evaluate the
efficacy of different intervention strategies using our
Stochastic Individual-Based
model.
Biogas
Our
group has studied the co-benefits of biogas digesters in sterilizing
potentially infected stool before it is spread onto fields while at the
same time providing a clean, economical energy source.
Biogas stoves use methane
produced by the anaerobic decomposition of organic waste material, which
can be used for cooking, heating and lighting. Potential benefits of
these systems include improved rural sanitation, reduced labor
requirements for wood collection (especially among women), reduced
greenhouse gas emissions, improved respiratory health in kitchens, and
increased agricultural productivity through improvements in soil
quality. Our research suggests that bio-gas digesters effectively remove
viable S. japonicum eggs
from infected stool, and could help control the continued application of
infected fertilizers into agricultural fields.
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Environmental control
A
widely employed method of schistosomiasis control is the application of
molluscicides to control the intermediary snail host population. The
application of the molluscicide niclosamide is a common method to
interrupt transmission. This method, especially in combination with
other control techniques, including lining irrigation ditches with
concrete, can be an effective tool for temporary snail control. However,
niclosamide has also been shown to be toxic to fish populations.
This characteristic forces us to question its feasibility for
routine use. Our current research hypothesizes that relying on
niclosamide alone may not be sufficient in preventing migratory snails
and diffusing parasites from reestablishing transmission.
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Chemotherapy
Chemotherapy is the main approach used by the Chinese government to
control morbidity.
Praziquantel is an effective and cheap single oral method to treat
mature schistosomes in the definitive host.
Recently, artemether, a compound used in
malaria control programs, has been found to have high potency in killing
immature schistosomes in vivo, and shows promising control effects in
combination with praziquantel (Xiao et al., 1997, 1999, 2000). Both
drugs reduce the risk to the community as a result of reducing egg
excretion to the environment by infected individuals.
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