Understanding the ecological processes that impact parasite abundance and distribution is critically important for epidemiology and predicting how infectious disease dynamics may respond to future disturbance. Digenean trematodes (Platyhelminthes: Trematoda) are parasitic flatworms with complex, multi-host life cycles that include snail first-intermediate hosts and vertebrate definitive hosts. Trematodes cause numerous diseases of humans (e.g. schistosomiasis) and livestock (e.g. fascioliasis), and impact the ecology of wildlife systems. Identifying the ecological mechanisms that regulate these complex, multi-host interactions will advance both our understanding of parasitism and the dynamics of infectious disease. By examining patterns of infection in Elimia (= Oxytrema = Goniobasis) proxima snails, my dissertation research investigated the environmental factors and ecological processes that structure trematode communities in streams. First, I examined temporal variation in trematode infection of snails in five headwater streams. Over a three year period, I found no consistent seasonal patterns of trematode infection. There was consistency across sites in trematode prevalence, as sites with high prevalence at the beginning of the study tended to remain sites of high infection, relative to lower prevalence sites. Second, I examined landscape level variation in trematode infection by characterizing the regional distribution, abundance and diversity of E. proxima infections in 20 headwater streams. I found a broad scale spatial pattern in trematode communities due to regional turnover in dominant species. This pattern was correlated with elevation, but there were no significant relationships with other environmental variables. Additionally, molecular characterization of trematodes indicated the presence of cryptic (morphologically indistinguishable) species complexes within this system, and variation in genetic diversity among trematode types may reflect differences in host dispersal abilities. Third, I examined trematode infection within a single stream network across multiple headwaters and the mainstem. I found a decreasing downstream gradient of trematode prevalence related to several environmental variables including elevation, snail density, conductivity, and stream depth. Additionally, headwater communities were nested subsets of the communities found in the mainstem. By combining approaches at different temporal and spatial scales, my dissertation research increases our understanding of the processes that impact the abundance and distribution of parasites. / Ph. D. / Understanding the ecology of wildlife parasite infection is critical both for public health and the conservation of global biodiversity. Digenean trematodes (Phylum: Platyhelminthes, Class: Trematoda) are parasitic flatworms that cause numerous diseases of humans (e.g. schistosomiasis) and livestock (e.g. fascioliasis), and can impact wildlife ecology. Trematodes have complex life cycles that involve multiple hosts. A typical trematode life cycle includes a series of three hosts: (1) a snail first-intermediate host; (2) an aquatic invertebrate or vertebrate second-intermediate host; and (3) a vertebrate final host. By identifying the ecological processes that are important in these complex, multi-host interactions, we can advance our understanding of parasites and infectious disease. Freshwater ecosystems serve as the transmission channels for many types of parasites, including trematodes, but we know relatively little about the ecology of parasites in streams. My dissertation research investigated the ecology of parasites in streams by examining patterns of trematode infection in stream snails, <i>Elimia</i> (= <i>Oxytrema</i> = <i>Goniobasis</i>) <i>proxima</i>. First, I examined seasonal changes in trematode infection of snails in five headwater streams. Over a three year period, I found no consistent seasonal patterns of trematode infection. There was consistency across sites in the level of trematode infection (i.e. sites with high levels of infection at the beginning of the study tended to remain sites of high infection, relative to sites with lower levels of infection). Second, I examined variation in trematode infection across a regional spatial scale by examining the number and types of trematodes infecting snails in 20 headwater streams in southwestern Virginia and northwestern North Carolina. Across the region, I found a broad spatial pattern in the geographic distribution of trematodes due to changes in the dominant type of trematode infection. This pattern was related to elevation, but not to any other environmental variables we measured. Additionally, I obtained genetic sequences from these trematode samples, and this revealed the existence of additional trematode species that we could not distinguish based on visual examination of morphological features. Furthermore, differences in the genetic diversity of trematode species may be related to differences in the mobility of trematode host species. Third, I examined trematode infection at eight locations within a single stream. I found that trematode infection decreased from upstream to downstream, and that this pattern was related to several environmental variables including elevation, snail density, conductivity, and stream depth. By combining examinations of infection patterns over time and at different spatial scales, my dissertation research increases our understanding of the processes that impact parasite transmission in freshwater ecosystems.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/82832 |
| Date | 20 October 2016 |
| Creators | Zemmer, Sally A. |
| Contributors | Biological Sciences, Belden, Lisa K., Benfield, Ernest F., Zajac, Anne M., Hopkins, William A. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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