Water Management for Disease Vector Control - Biological and Environmental Control of Disease Vectors - page 166

anal cleansing was found to be an important
route of water contamination in northern
Senegal (Sow et al ., 2008).
In general, irrigation is most strongly linked
with Schistosoma mansoni (intestinal schistoso-
miasis); in some areas, water resources
development has even caused a shift from
Schistosoma haematobium (urinary schisto-
somiasis) to S . mansoni as the predominant
species. Bulinus and Biomphalaria snails are the
aquatic intermediate hosts of S . haematobium
and S . mansoni , respectively. They spend their
entire life in water, although the adult snail may
resist desiccation for long periods. The snails
have adapted to a wide range of environmental
conditions, including natural and man-made
habitats, but with distinct dif erences by genus
and species. In east Asia, another schistosome
species, Schistosoma japonicum , is prevalent. It
causes disease symptoms comparable to S .
mansoni , but is transmitted by amphibious
Oncomelania snails, which spend short periods of
time out of water during their immature stages.
Snail hosts of human schistosomiasis are
found in habitats including fl ood plains,
shorelines of rivers and lakes, marshland, in
irrigation canals and drainage ditches.
Reductions in snail populations can be achieved
by various engineering means, including proper
drainage, canal lining, removal of aquatic
vegetation from canals, regular fl ushing of
canals, increasing the fl ow velocity, drying of
irrigation systems and changing water levels in
reservoirs (Oomen et al ., 1990). In addition,
there are biological control measures such as
introduction of snail-eating fi sh, non-susceptible
snail species, or predatory snails (Chimbari et al .,
1996). Chemical molluscicides have also been
used on a large scale, especially in Egypt (Farooq
et al ., 1966; Lardans and Dissous, 1998). Finally,
the need for contact with infested water bodies
can be reduced by improved water supplies,
constructing latrines, laundry and shower
facilities and building footbridges (Esrey et al .,
1991). Some interventions show good results
under experimental conditions, but attempts to
bring them to scale under natural conditions
have been disappointing. For example,
increasing the velocity of water in irrigation
canals can result in dangerous situations for
children swimming in irrigation canals, while
snail-eating fi sh cannot be released in irrigation

canals that fall dry as result of a rotational
schedule. Rather than focusing on just one
intervention, an integrated approach is needed
(see Cameron, Chapter 10, this volume). For
example, in Japan, schistosomiasis was
ef ectively eliminated by a combination of water
management, molluscicides, improved sani-
tation, treatment of patients and community
involvement. Water management consisted of
cementing ditches around rice fi elds for
destruction of snail habitats, improved drainage
and reclamation of wetlands and river dredging
(Tanaka and Tsuji, 1997).

9.3 Water Management Interventions
for Disease Vector Control

9.3.1 Alternate wet-dry irrigation

Increasing the productivity of land and water by
producing more rice with less water is important
for maintaining food supply in the face of
impending water scarcity. Agriculture is the
largest consumer of freshwater resources
worldwide, and in some countries might
constitute 95% of all water used (Seckler et al .,
1998). Agriculture is often criticized for its poor
water use ei ciency, and one of the suggested
techniques of using water more ei ciently is the
alternate wet-dry irrigation method (AWDI),
which is practised in countries such as Japan,
China and India. In this method, water is not
kept continuously inundating rice fi elds, but is
supplied intermittently so that the fi elds are
alternately wetted and dried. There is no uniform
defi nition of AWDI, and consequently there is an
inconsistency in terminology. The term AWDI
has been adopted in preference to intermittent
irrigation, which could also apply to irrigation
practices that deliver water intermittently
without necessarily creating dry conditions.
Commonly used terminology for AWDI include
'water saving irrigation', 'wet irrigation',
'intermittent submerged irrigation' and 'non-
fl ooded rice irrigation'.
AWDI has potential health benefi ts as the
intermittent drying of rice fi elds could kill
immature stages of mosquitoes that transmit
diseases such as malaria and JE. Originally tested
in Portugal in the 1940s, the approach was later
tried in North America, the Philippines, Japan,