The Effect of Water Physical Quality and Water Level Changes on the Abundance and Occurrence of Anopheles Larvae (Diptera: Culicidae) Around the Shoreline of the Koka Reservoir, Central Ethiopia
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Date
2008-07
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Addis Ababa University
Abstract
Entomological studies on the effect of water physical quality and water level change on the
occurrence of Anopheles mosquito larvae and the formation of larval breeding habitats were
conducted in two villages (Ejersa and Kuma) at the Koka Reservoir in Central Ethiopia
between August and December 2007. Data on the type and number and physical characteristics
of Anopheles larval breeding habitats, species composition and densities were recorded.
Meteorological and reservoir water level data were compared with availability of Anopheles
larval breeding sites and densities. Four-year retrospective clinical data indicated that the
pattern of malaria transmission in the village at close proximity of the Koka reservoir is
strongly associated with reservoir water level change during the peak malaria transmission
season. Data from adult and larval collection showed that Anopheles pharoensis, An. gambiae
s.l., An. coustani and An. squamosus were found in the study area in different proportions. An.
pharoensis larvae were dominant at the village close by the reservoir while An. gambiae s.l.
more common at the second village away from reservoir in the short breeding interval during
the study periods. The total count of An. pharoensis larvae at the reservoir site was
significantly higher than at the control village (X2 = 942.8, df = 1, p < 0.05). This indicates that
this species prefers breeding sites created in association with shoreline puddles that provide
ideal turbid breeding pools with much floating aquatic vegetation. The total count of An.
gambiae s.l. at the reservoir site was also significantly higher (X2=200.5, df=1, p < 0.05) than
at a the nearby control village. Generally, mean larval density of An. gambiae s.l. was higher in
slightly turbid and shallow aquatic habitats (F=16.97, p<0.05 and F=6.03, p<0.05 respectively)
than that of turbid and deep aquatic habitats. The density of An. pharoensis in breeding habitats
with floating vegetation and with relatively shady condition was significantly higher than that
of the aquatic habitats with much light and greater emergent vegetation (F=15.75, p < 0.05 and
F=10.56, p < 0.05 respectively). There was also a positive correlation between the occurrence
of larvae, water temperature of the breeding habitats and daily minimum atmospheric
temperature (r= 0.541 and, r= .0.604, respectively p <0.05). Similar comparison indicated a
positive correlation between water level changes resulting in subsequent recession of the
reservoir and the number of positive breeding habitats during the sampling period in the
reservoir village (r = 0.605, p < 0.05). Results in this study clearly showed that water physical
characteristics such as water temperature, turbidity, depth and vegetation cover play an
important role in the species composition, total count and density of Anopheles mosquitoes in
the vicinity. Reservoir water level change is also associated with the proliferation of ideal
mosquito breeding habitats.
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Biology