Abstract:
In the past decades, the use and production of chemicals has been on the rise globally
due to increasing industrialization and intensive agriculture; resulting in the occurrence
and ecotoxicological risks of chemicals of emerging concern (CECs) in the aquatic
compartments. Risks include changes in community structure resulting in the dominance
of one species and ecosystem imbalance. When dominant disease-causing organisms
are in the environment, the disease transmission is increased. For example, host snails
for the schistosomiasis, a human trematode disease, are known to be tolerant to pesticide
exposure compared to the predators. This would therefore result in an increased
abundance of snails which consequently increase the disease transmission in the human
population.
Kenya, being a low income country faces a lot of challenges with provision of clean water,
diseases and sanitation facilities, and increasing population which results in intensive
agriculture coupled with pesticide use. Although a lot of research has been carried out on
the environmental occurrence and risk of CECs (Chapter 1), most of these studies have
been done in developed countries with limited information from Africa. Additionally,
research in Africa focused on urban areas with limited number of compounds analyzed
and mostly in the water phase, and inadequate information on the effects of CECs on the
aquatic organisms. In order to reduce this knowledge gap, this dissertation focused on
identification and quantification of CECs present in water, sediment and snails from
western Kenya, and the contribution of pesticides to the transmission of schistosomiasis.
Chapter 2 gives a summary of the results and discussion of the dissertation. In Chapter
3, a comprehensive chemical analysis was carried out on 48 water samples to identify
compounds, spatial patterns and associated risks for fish, crustacean and algae using
toxic unit (TU) approach. A total of 78 compounds were detected with pesticides and
biocides being the compounds most frequently detected. Spatial pattern analysis
revealed limited compound grouping based on land use. Acute risk for crustaceans and
algae were driven by one to three individual compounds. These compounds responsible
Abstract
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for toxicity were prioritized as candidate compounds for monitoring and regulation in
Kenya.
In Chapter 4, an extension of Chapter 3 was done to cover the CECs present in snails
and sediment from the 48 sites. A total of 30 compounds were found in snails and 78 in
sediments with 68 additional compounds being found which were not previously detected
in water. Higher contaminant concentrations were found in agricultural sites than in areas
without anthropogenic activities. The highest acute toxicity (TU 0.99) was determined for
crustaceans based on compounds in sediment samples. The risk was driven by diazinon
and pirimiphos-methyl. Acute and chronic risks to algae were driven by diuron whereas
fish were found to be at low to no acute risk.
In Chapter 5, the effect of pesticide contamination on schistosomiasis transmission was
evaluated by applying complimentary laboratory and field studies. In the field studies, the
ecological mechanisms through which pesticides and physical chemical parameters
affect host snails, predators and competitors were investigated. Pesticide data was
obtained from the results in chapter 3. The overall distribution of grazers and predators
was not affected by pesticide pollution. However, within the grazers, pesticide pollution
increased dominance of host snails. On the contrary, the host-snail competitors were
highly sensitive to pesticide exposure. For the laboratory studies, macroinvertebrates
including Schistosoma-host snails, competitors and predators were exposed to 6
concentrations levels of imidacloprid and diazinon. Snails showed higher insecticide
tolerance compared to competitors and predators.
Finally, Chapter 6 summarizes the conclusions of this dissertation, placing it in a broader
context. In this dissertation, a comprehensive chemical characterization and risk
assessment of CECs has been carried out in freshwater systems; together with the effects
of pesticides on schistosomiasis transmission in rural western Kenya. Results of this
dissertation showed that rural areas are contaminated posing a risk to aquatic organisms
which contribute to schistosomiasis transmission. This shows the need for regular
monitoring and policy formulation to reduce pollutant emissions which contributes
negatively to both ecological and human health effects