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Abstract
Catfish aquaculture is a significant economic driver in the southeastern United States. The industry is heavily impacted by infectious disease, including myxozoan parasitism. The most significant myxozoan, Henneguya ictaluri, is cited as the cause of proliferative gill disease (PGD) in channel (Ictalurus punctatus) and channel × blue (I. furcatus) hybrid catfish, resulting in high mortality in severe outbreaks. Although susceptible to infection and presporogonic development, there is reduced incidence of PGD in hybrid systems. Previous research implies incomplete sporogenesis in hybrids, suggesting hybrid catfish may be a dead-end host in the H. ictaluri life cycle. This dissertation used a polyphasic approach to investigate myxozoan community dynamics and pathology in channel and hybrid catfish systems. Metagenomic analysis of industry PGD cases (Chapter 2) revealed mixed infections, with H. ictaluri always present but not always the most abundant myxozoan. Myxozoan communities in channel and hybrid catfish were incongruous, composed of numerous known and unknown myxozoan taxa, with reduced abundance of H. ictaluri in hybrids. In situ hybridization (ISH) assays applied to laboratory-induced and natural infections (Chapter 3) confirmed H. ictaluri presporogonic stages were associated with PGD lesions in channel and hybrid catfish. However, mature H. ictaluri plasmodia failed to develop in hybrids, even as late as 20-weeks post-challenge, supporting assertions that hybrids are an aberrant, dead-end host to H. ictaluri. ISH assays were consistent with metagenomics findings, revealing mixed infections in PGD cases, with non-H. ictaluri myxozoans contributing secondary pathology, including rare PGD-like lesions. Histological examination and molecular confirmation of H. adiposa infections by laser capture microdissection (LCM) (Chapter 4) illustrate how non-H. ictaluri myxozoans contribute subclinical pathology and demonstrate antemortem release into the environment. Lastly, massive interlamellar H. exilis infections in hybrid catfish were defined by pathologic description and LCM (Chapter 5), as well as metagenomic analysis and ISH assays (Chapter 6). Collectively, these findings reveal fish host susceptibility dictates myxozoan community dynamics, which could be manipulated to reduce incidence of PGD on catfish farms. Optimized crop rotation strategies alternating channel and hybrid catfish monocultures can potentially prevent or mitigate PGD outbreaks in catfish aquaculture, significantly improving industry productivity and profitability.