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...

Authors: STILWELL, JUSTIN - Mississippi State University; CAMUS, ALVIN - University Of Georgia; WOODYARD, ETHAN - Mississippi State University; WARE, CYNTHIA - Mississippi State University; ROSSER, THOMAS - Mississippi State University; GUNN, MACKENZIE - Mississippi State University; LOPEZ-PORRAS, ADRIAN - Mississippi State University; KHOO, LESTER - Mississippi State University; WISE, DAVID - Mississippi State University; GRIFFIN, MATT - Mississippi State University

Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/2023
Publication Date: 11/15/2023
Citation: Stilwell, J.M., Camus, A.C., Woodyard, E.T., Ware, C., Rosser, T.G., Gunn, M.A., Lopez-Porras, A., Khoo, L.H., Wise, D.J., Griffin, M.J. 2023. 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.... Journal of Aquatic Animal Health. 35(4)223-237. , https://doi.org/10.1002/aah.10196.
DOI: https://doi.org/10.1002/aah.10196

Interpretive Summary: Proliferative gill disease (PGD) is a significant health concern in catfish aquaculture, affecting both channel catfish (Ictalurus punctatus) and hybrid catfish (channel × blue catfish, I. furcatus). While hybrid catfish exhibit reduced Henneguya ictaluri transmission and parasite development, PGD remains a challenge in hybrid production systems. This study provides further molecular evidence that hybrid catfish likely serve as a dead-end host in the Henneguya ictaluri life cycle. Despite exposure to infectious pond water and development of mature H. ictaluri in concurrently exposed channel catfish, mature H. ictaluri plasmodia failed to develop in hybrids, suggesting impaired or delayed sporogenesis. These findings reinforce previous research indicating that H. ictaluri transmission is hindered in hybrids, limiting their role in sustaining the parasite's life cycle which could potentially be exploited using crop-rotations to minimize impact of PGD on the US farm-raised catfish industry.

Technical Abstract: Objective Proliferative gill disease (PGD) in Channel Catfish Ictalurus punctatus and hybrid catfish (Channel Catfish × Blue Catfish I. furcatus) is attributed to the myxozoan Henneguya ictaluri. Despite evidence of decreased H. ictaluri transmission and impaired parasite development in hybrid catfish, PGD still occurs in hybrid production systems. Previous metagenomic assessments of clinical PGD cases revealed numerous myxozoans within affected gill tissues in addition to H. ictaluri. The objective of this study was to investigate the development and pathologic contributions of H. ictaluri and other myxozoans in naturally and experimentally induced PGD. Methods Henneguya species-specific in situ hybridization (ISH) assays were developed using RNAscope technology. Natural infections were sourced from diagnostic case submissions in 2019. Experimental challenges involved Channel Catfish and hybrid catfish exposed to pond water from an active PGD outbreak, and the fish were sampled at 1, 7, 10, 12, 14, 16, 18, and 20'weeks postchallenge. Result Nine unique ISH probes were designed, targeting a diagnostic variable region of the 18S ribosomal RNA gene of select myxozoan taxa identified in clinical PGD cases. Partial validation from pure H. ictaluri, H. adiposa, H. postexilis, and H. exilis infections illustrated species-specific labeling and no cross-reactivity between different myxozoan species or the catfish hosts. After experimental challenge, mature plasmodia of H. ictaluri and H. postexilis formed in Channel Catfish but were not observed in hybrids, suggesting impaired or delayed sporogenesis in the hybridized host. These investigations also confirmed the presence of mixed infections in clinical PGD cases.