Activation of Henneguya ictaluri actinospores by non-ictalurid fish species, with implications for management of proliferative gill disease in catfish aquaculture

Authors: Richardson, Brad; MISCHKE, CHARLES - Mississippi State University; ROSSER, GRAHAM - Mississippi State University; WOODYARD, ETHAN - Mississippi State University; WARE, CYNTHIA - Mississippi State University; WISE, DAVID - Mississippi State University; GRIFFIN, MATT - Mississippi State University

Submitted to: North American Journal of Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/2022
Publication Date: 2/8/2022
Citation: Richardson, B.M., Mischke, C.C., Rosser, G.T., Woodyard, E.T., Ware, C., Wise, D.J., Griffin, M.J. 2022. Non-specific activation of Henneguya ictaluri actinospores. North American Journal of Aquaculture. https://doi.org/10.1002/naaq.10242.
DOI: https://doi.org/10.1002/naaq.10242

Interpretive Summary: Proliferative Gill Disease (PGD) is a leading parasitic disease in the US catfish aquaculture industry. Infection by a myxozoan parasite Henneguya ictaluri, leads to an immune response that destroys the gills and can result in large mortality events in commercial culture. While the parasite uses chemical cues from fish mucus to initially locate the host, the parasite successfully infects only a few fish species. To determine whether a parasite-resistant fish species could serve as a decoy host, researchers at Mississippi State University stocked Channel Catfish (Ictalurus punctatus) with Western Mosquitofish (Gambusia affinis) at various densities and exposed the fish to 3000 H. ictaluri actinospores. After 24 hours, the H. ictaluri infection rate was 3-fold lower when mosquitofish were stocked at a rate equivalent to 25-50% the biomass of the catfish. However, after 7 days parasite load did not differ between treatments, suggesting a trade-off between initial parasite density and competition within the host. The initial decrease in infections highlighted potential benefits of reducing PGD mortalities by co-stocking catfish aquaculture ponds with non-catfish species.

Technical Abstract: Proliferative Gill Disease (PGD), caused by the myxozoan Henneguya ictaluri, is an important parasitic disease in US catfish aquaculture. Continuous exposure of Channel Catfish Ictalurus punctatus to the H. ictaluri actinospore stage results in a severe inflammatory response at the gills, leading to morbidity and death. Previous work indicates chemical cues in fish mucus recognized by myxozoan actinospores are not host-specific. Building on these findings, the potential decoy effects of non-ictalurid fish to H. ictaluri actinospores were evaluated. Actinospores exposed to gill mucus from multiple non-ictalurid fishes resulted in actinospore activation for all fish tested. Based on these findings, experimental transmission trials with potential decoy fish were conducted. Channel Catfish were co-stocked with Western Mosquitofish Gambusia affinis at 0, 10, 25, or 50% mean Channel Catfish biomass and exposed to 3,000 H. ictaluri actinospores. Gill tissues were sampled 24 hours post-challenge and parasite burden was estimated by H. ictaluri-specific qPCR. Results revealed Western Mosquitofish stocked at 25 and 50% Channel Catfish biomass reduced H. ictaluri DNA in Channel Catfish gills >3-fold. In a second study, Channel Catfish were exposed to pond water collected from an active PGD outbreak with Western Mosquitofish stocked at 25% Channel Catfish biomass. Channel Catfish were sampled 24 hours and seven days after the last pond water exposure. Twenty-four hours after the last exposure, Channel Catfish co-stocked with Western Mosquitofish showed significantly lower H. ictaluri DNA than Channel Catfish stocked alone. This treatment effect was absent seven days later, as parasite quantities within tissues had increased >1000X, with marked variability. Still, results indicate chemical cues activating H. ictaluri actinospores are not specific to Channel Catfish. This work evinces a potential benefit of non-ictalurid fish in combating H. ictaluri, suggesting non-ictalurid interceptor fish in Channel Catfish ponds may reduce PGD severity.