Influence of Channel Catfish and Channel × Blue Catfish Hybrids on Myxozoan Community Composition in Catfish Aquaculture Ponds

Abstract

The myxozoan Henneguya ictaluri is cited as the cause of proliferative gill disease (PGD) in Channel Catfish Ictalurus punctatus and Channel × Blue Catfish I. furcatus hybrids. Differences in host susceptibility result in decreased transmission rates, reduced sporogenesis, and lesser incidence of PGD in hybrid catfish. In experimental pond trials, continuous monoculture of hybrid catfish suppressed H. ictaluri abundance in pond systems compared to Channel Catfish monoculture. The present study examined differences in myxozoan community structure in catfish ponds stocked with Channel Catfish or hybrid catfish. Experimental ponds (0.4 ha) dedicated to Channel Catfish or hybrid catfish monoculture were maintained over three production cycles. Pond water collected during the spring of each year was subjected to metagenomic analysis by targeted amplicon sequencing of a diagnostic hypervariable region (DVR3) of the myxozoan 18S ribosomal RNA gene. Relative abundance of H. ictaluri was significantly greater in Channel Catfish ponds than in hybrid catfish ponds during years 2 and 3. In hybrid catfish ponds, H. ictaluri never exceeded 20% average relative abundance. Hybrid catfish ponds also revealed decreased myxozoan diversity and species evenness compared to Channel Catfish ponds, suggesting that myxozoan taxa associated with Channel Catfish may not be well adapted to the hybridized host. Results corroborate previous research indicating that hybrid catfish monoculture can suppress H. ictaluri compared to ponds that are devoted to Channel Catfish. These data imply that alternating catfish culture species in ponds (crop rotation) has the potential to mitigate PGD by precluding H. ictaluri from accumulating within ponds to levels associated with disease and catastrophic loss. Further, this work identified numerous previously unidentified taxa present within these systems that clustered phylogenetically with both catfish and noncatfish hosts. The impacts that these other myxozoans have on catfish health and production are unknown, but their presence indicates that myxozoan diversity within catfish aquaculture ponds has been greatly underestimated and is significantly influenced by stocking choice.

Species‐specific in situ hybridization confirms arrested development of Henneguya ictaluri in hybrid catfish (Channel Catfish × Blue Catfish) under experimental conditions, with notes on mixed‐species infections in clinical cases of proliferative gill disease from Mississippi catfish aquaculture

“…disease and the presence of non-H. ictaluri myxozoan plasmodia in hybrids. This is consistent with the decreased myxozoan diversity previously demonstrated in hybrid catfish ponds compared to Channel Catfish ponds and supports observations that hybrid catfish monoculture may result in suppression of other myxozoan species in addition to H. ictaluri and disruption of the pond myxozoan community (Griffin et al 2020;Stilwell et al 2022Stilwell et al , 2023. Previous metagenomic analyses of clinical PGD cases from catfish industry samples reveal that H. ictaluri was always associated with outbreaks.…”

“…Furthermore, during PGD outbreaks the myxozoan community composition within infected tissues and pond water is highly variable. Although H. ictaluri is always present, dozens of known and unknown myxozoan sequences have been detected in ponds and PGD cases, suggesting that additional myxozoan species could be involved in the pathologic changes and pathogenesis of PGD (Stilwell et al 2022(Stilwell et al , 2023.…”

“…In combination with molecular techniques, experimental infectivity studies have illustrated decreased rates of infection and arrested parasite development of H. ictaluri in hybrid catfish compared to Channel Catfish (Griffin et al 2010;Rosser et al 2019). These challenges utilized water from ponds with active PGD outbreaks, potentially containing a mixed inoculum of myxozoans (Griffin et al 2010;Rosser et al 2019;Stilwell et al 2022Stilwell et al , 2023. Thus, although H. ictaluri DNA could be detected by PCR methods, species-level identification of presporogonic stages and developing plasmodia within lesions could not be confirmed histologically (Rosser et al 2019).…”

“…These observed differences in disease incidence indicate that catfish intermediate hosts exhibit varied susceptibilities to these closely related myxozoan species. Furthermore, variable susceptibilities to one myxozoan over another in a given catfish species may produce selective pressures on overall myxozoan diversity in pond monoculture systems (Griffin et al 2010(Griffin et al , 2020Rosser et al 2019;Stilwell et al 2019aStilwell et al , 2022Stilwell et al , 2023.…”