Biology Reference
In-Depth Information
canal irrigation system caused severe malaria
epidemics (Tyagi, 2004). In contrast, several
empirical studies from the same continents have
shown the counter-intuitive result of no increase
in malaria cases, hypothesized to be due to the
improved socio-economic status of people in
irrigated areas, allowing them to live in better
houses, purchase better treatment and apply
preventive and control interventions (Ijumba
and Lindsay, 2001). The Mwea irrigated villages
in Kenya exhibited a classic malaria 'paddies
paradox' with irrigated villages having from 30-
to 300-fold greater mosquito vector densities, but
two- to sixfold lower human malaria incidence
than non-irrigated villages. There, the major
vector,
Anopheles arabiensis
, fed mainly on cattle
and other non-human hosts (85-96% of meals)
in irrigated villages, in contrast to much heavier
human feeding rates (42-45% of meals) in the
non-irrigated villages (Mutero
et al
., 2004).
Another possible explanation was put forward
following studies in West Africa where more
vectors of malaria were found in villages in
irrigated areas than in adjacent non-irrigated
villages, but the overall malaria prevalence was
substantially less. Here, high anopheline densities
may have resulted in smaller adult mosquitoes
that did not live as long, and hence were less
ei cient in transmitting malaria parasites (Diuk-
Wasser
et al
., 2005).
Several species of anopheline malaria
vectors have been identifi ed in irrigated systems
and small dams, including
An
.
arabiensis
,
An
.
gambiae
sensu stricto
and An
.
culicifacies
. The
opportunities for malaria vector breeding are
often associated with faulty irrigation designs,
favouring the establishment of water pools for
extended periods of time, stagnant pools along
the water reservoir shoreline and seepage pools
below the dam wall of the many small earthen
water reservoirs. In addition, poor maintenance
of canals allowing seepage water to accumulate
and inappropriate water management practices,
favours the breeding of vectors in the irrigated
fi elds or in pools of water left behind in the canals
(Mather and That, 1984; Kibret
et al
., 2010).
Rice irrigation systems, in particular, present a
range of water habitats with a great diversity in
vegetation cover and freshwater ecological
parameters for the greater part of year.
Irrigation systems and water reservoirs
may increase the mosquito vector habitat
availability beyond the rainy season, potentially
impacting upon the seasonal vector abundance
and transmission patterns of disease. Studies
investigating the breeding of vectors of the
An
.
culicifacies
complex in Pakistan and India found
that
irrigation water releases were closely related
to population peaks of the malaria vector
(Sharma
et al
., 1991; Konradsen
et al
., 1998;
Tyagi, 2004). In Sri Lanka, the scheduling of
irrigation water releases between the two large
irrigation reservoirs contributed to the
maintenance of vector breeding pools within an
irrigation conveyance channel, thereby greatly
extending the period of vector presence in the
area (Amerasinghe
et al
., 1997, 1999, 2001;
Konradsen
et al
., 2000). Similarly, fi ndings from
the Ethiopian Rift Valley showed that even small
scale irrigation systems and dams may intensify
malaria during the dry season (Kibret
et al
.,
2010). In contrast, Sharma
et al
.
(2008) found
that the construction of small dams along
streams in Orissa, India, decreased malaria. The
small dams in the study villages altered the
water fl ow above and below the dam, thereby
making it unfavourable for the breeding of
An
.
fl uviatilis
, which in turn brought about a
signifi cant
negative
impact
on
malaria
transmission.
9.2.2 Filariasis
In parts of rural Africa,
Anopheles
mosquitoes
transmit lymphatic fi lariasis as well as malaria;
therefore the impact of irrigation and dam
development may potentially also infl uence the
seasonality and intensity of lymphatic fi lariasis
transmission. Few studies have been conducted
to assess the impact of water resources
development and water management practices
on lymphatic fi lariasis and no systematic
attempts to assess the impact of water manage-
ment as a control intervention have been
undertaken. One case study, studying an
irrigation system in northern Ghana, reported
that
An
.
gambiae
s.s. and
An
.
funestus
breeding in
the irrigation system created signifi cantly higher
annual lymphatic fi lariasis transmission
potential in the irrigated areas compared with
the non-irrigated areas (Appawu
et al
., 2001).
In urban areas, lymphatic fi lariasis is most often
associated with
Culex quinquefasciatus
, which
Search WWH ::
Custom Search