Water Management for Disease Vector Control - Biological and Environmental Control of Disease Vectors - page 169

plant growth, thereby reducing opportunities
for larval breeding. Most of the experiments
focused on the technical and operational
feasibility of installing siphons in Sri Lanka, but
aspects of cost were also discussed for the various
types of dams and siphons tested. Unfortunately,
less emphasis was placed on the assessment of
the ef ectiveness of the interventions on the
actual breeding of mosquitoes and possible
implications for the transmission of malaria
(Matsuno et al ., 1999).
During the 1990s, work in Sri Lanka
assessed the options for control of malaria
vectors through dif erent water-management
practices in irrigation conveyance canals and
streams. The approach was based on the use of
existing irrigation structures regulating the
water levels in the waterways, and was aimed at
removing the principal breeding sites of the
most important malaria vector in the country,
An . culicifacies . Changes in irrigation and
stream-water levels from fortnightly releases of
water from upstream reservoirs was enough to
eliminate mosquito breeding sites and render
the habitat unsuitable for the vector for 1-2
weeks after the release. This approach did not
result in a loss of water since the water was
captured in reservoirs downstream. The
designated water management strategy was far
cheaper, at US$0.25 per protected person per
year, than the use of chemically based larvicides
(Konradsen et al ., 2004).
Aquatic snails require 1-2 months to grow
from hatching to maturity, so interrupting the
fl ow every month may potentially have some
impact upon snail populations. However, to have
an ef ect on mosquito larvae, the intervention
would have to be much more frequent. A
number of experiments have been undertaken
to identify the mean fl ow velocity needed to
dislodge and immobilize snails in canals systems.
This has been used to model dif erent water
management approaches for snail control in a
diversity of canal conditions (Oomen et al .,
1990). The fl ow velocity required for an impact
on the snails is likely to be greater than the fl ow
velocity needed to af ect mosquito immature
stages. However, more research and experiments
are needed to identify the required interval, fl ow
velocity and management practice to be ef ective
and feasible in managing both snails and
mosquito breeding in natural habitats and

environments such as drainage and irrigation
canals. Attention should also be given to the fact
that irrigation canals serve multiple purposes,
including bathing, laundry and drinking water
collection (Meinzen-Dick and van der Hoek,
2001) and are often used by children in which to
swim. Thus, increasing fl ow velocities too much
in irrigation canals could lead to hazardous
situations.

9.3.4 Water management and drainage of
fl ooded areas

Drainage is the water management method for
disease vector control with the longest history
(Konradsen et al ., 2004). As early as the 6th
century BC, the Greeks and the Romans were
aware of the association between fevers, swamps
and stagnant waters. This awareness led to
drainage interventions aimed at improving the
health of the nearby population and increasing
agricultural production.
Waterlogged conditions may create, or
increase, the breeding potential for vector
mosquitoes of malaria. The potential for
mosquito breeding is infl uenced by the extent of
groundwater pools, whereas the type of
mosquitoes that breeds in them is determined by
the water quality in these pools. Both of these
factors are closely related to fl uctuations in
depth of the groundwater table. In canal
irrigation systems in the semi-arid regions, the
number of potential mosquito breeding sites has
increased tremendously as a consequence of
poor drainage. However, this ef ect is partly of set
by the increase in salinity, associated with high
water tables. A recent retrospective analysis of
the period 1970-1999 in Pakistan and India
suggests that the increased abundance of rural
An . stephensi (a poor vector of malaria) relative
to An . culicifacies (the main malaria vector) may
have been the cause of the low malaria levels at
present (Klinkenberg et al ., 2004). The shift may
have been due to waterlogging with related
salinization that has created an environment
favourable for the more salt-tolerant An .
stephensi (Klinkenberg et al ., 2004). Overall, the
extensive ecological changes resulting from poor
drainage will have dif erent impacts on the
transmission of malaria in various regions of
the world, depending on the type of vector found