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The distribution and ecology of larval malarial mosquitoes
in
Chapare Valley, Bolivia
Reema Paudel1, Gary N. Fritz1 and Roberto
Rodriguez2
1Department of Biological Sciences, Eastern Illinois
University
2Laboratorio de Entomolgia Medica, Escuela de Salud Publica,
Cochabamba, Bolivia
Introduction
Malaria is an important public problem, affecting approximately
270-488 million people and killing 1-3 million people each year (Target, 1991,
Savage et al, 1990); nearly 15 million of these cases occur in tropical America
(Sturchler, 1989). Although the vector species of malaria are well known in
Africa and some regions in Asia, significantly less is known about the
transmission of the disease in the Neotropics. Unlike in Africa, where two or
three species are responsible for most transmission throughout the continent,
the epidemiology of malaria in the Neotropics is more complex due, in part, to
the diversity of the potential vectors (Audho et al, 1995) and complex
ecological history (Vuilleumier, 1971). Thus one of the major obstacles in
studies attempting to determine the identity, behavior and ecology of primary
vectors has been the great difficulty in distinguishing species. The Chapare
Valley in Bolivia (Figures 1 and
2) is of particular interest for examining
the epidemiology of malaria since it exemplifies the changing nature of tropical
rainforest areas of South America. Much of the Valley is now disturbed, is
inhabited by subsistence farmers, is a patchwork of secondary forest and
agricultural lands, and has a large proportion of migrant workers. Human
disturbed rainforests typically have as much as a five-fold increase in
anopheline densities over undisturbed forests (Tadei et al, 1998) and probably
affect species diversity too. The Chapare Valley thus offers a good model system
for examining and understanding vector dynamics in a region of the Neotropics
where there is a great diversity of potential vector species and ecological
habitats, concomitant with the effects of human activity in the ecology of a
rainforest.
Purpose of this study
The purpose of the study is to address the ecological
differentiation of anopheline species at the level of breeding sites in a valley
region where species diversity is high and the ecology of the area is complex.
We hypothesize that environmental variables predict the distribution patterns of
larval mosquitoes in the Chapare Valley. The specific objectives include the
determination of malaria mosquito species and their distributions in breeding
habitats, the description of limnological and altitudinal profiles of breeding
sites and correlations with species distribution patterns.
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Methods |
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Sampling at breeding sites: Fifty-six
anopheline breeding sites were sampled for mosquito larvae and water
quality parameters. Measures of water quality included pH, TDS,
conductivity, turbidity, temperature, NO3, NH4-N,
COD, PO4, volatile solids, suspended solids and fixed
solids using standard limnological methods. Elevation of each site
was taken using an altimeter. |
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Figure 1. Study site in Bolivia |
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Figure 2. Sampling transect in the Chapare Valley,
Bolivia. |
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Polymerase Chain Reaction (PCR): Fifty random
larvae from each site were amplified using a multiplex PCR that
identified four species. Amplification products for these species
were at least 40bp different in length and thus easily resolved on
2% agarose gels.
Statistical analyses: Relationships among
environmental variables and the occurrence of the mosquito taxa were
investigated using Principal Component Analysis (PCA). A series of
correlations were completed for each species and the PCA factor.
One-way ANOVA was used to relate species distribution to elevation.
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Results and discussion
Using a multiplex PCR that identified only four species of
anopheline mosquitoes found in the Chapare Valley (Figures
3, 4,
5 and 6),
we accounted for nearly 50% of all mosquito larvae collected from fifty-six
aquatic breeding sites (Figure 7).
Anopheles rangeli and A. trinkae appear to be the most abundant
species (Figure 8), but A. triannulatus
is most widely dispersed (Figure 9). The
distributions of A. trinkae, A. rangeli and A. strodei are
limited to certain altitudinal zones, though interspecific competition cannot be
ruled out as a factor in addition to other variables (e.g. limnological
variables and other aspects of habitat including those that may affect adults).
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Figures 3, 4, 5 and 6. Multiplex PCR identification
of four anopheline species |
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Figure 7. Mosquito species identified. |
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Figure 8. Abundance of identified anopheline species |
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Figure 9. Species distribution |
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Environmental variables correlated significantly with
the distributions of species (Tables
1,
2 and
3). DO, pH, TDS and suspended
solids have a significant role in defining A. triannulatus
breeding sites. A. strodei breeding sites had a significant
relationship with COD, TDS, conductivity, pH and fixed solids. Most
of the limnological variables measured have significant
relationships with A. trinkae habitat. |
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Table 1. Pearson correlation of limnological
variables with PCA factors |
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Table 2. Pearson correlation of species with
limnological variables |
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Table 3. One way ANOVA (species vs. elevation) |
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Understanding species distribution patterns and their
relationships to environmental variables will elucidate the
epidemiology of malaria in the Chapare Valley and lead to more
efficient and cost effective measures of control. |
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Literature cited
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Science 173: 771-780. |
