Fingerling stocking size has no influence on proliferative gill disease severity in farm-raised Channel Catfish

Abstract

Objective: The myxozoan Henneguya ictaluri is the causative agent of proliferative gill disease (PGD) in Channel Catfish Ictalurus punctatus and hybrid catfish (Channel Catfish × Blue Catfish I. furcatus), which is a significant disease concern within the commercial catfish industry of the southeastern United States. Incidence of PGD occurs most frequently in fingerling-sized catfish when the fish are being transferred from nursery ponds to grow-out ponds. Mitigation strategies for PGD primarily involve the avoidance of stocking fish into ponds with existent lethal concentrations of the parasite, as determined through sentinel fish exposures or H. ictaluri-specific quantitative PCR. This study aimed to evaluate the potential of stocking larger fingerlings to improve survival and investigate the influence on three metrics of gill condition.

Methods: Two sizes of Channel Catfish fingerlings (∼12 and ∼20 cm) were stocked into nylon-mesh net-pens located in 19 commercial ponds with varying levels of H. ictaluri activity. After 1 week, fish were removed from the ponds and mortality was recorded. All survivors were euthanized for gross, histological, and molecular assessment. Gill biopsies from surviving fish were evaluated to estimate gill damage based on the presence of chondrolytic lesions in gill clip wet mounts. The number of characteristic PGD lesions and the number of presporogonic stages present were assessed histologically.

Results: Generalized linear regression showed no interaction between parasite burden in the pond water or gill tissues and fingerling size. In all regressions, only parasite concentrations in pond water or gill tissues were significant predictors of any gill condition metrics.

Conclusions: This study suggests that stocking of larger fingerlings provides no appreciable protection from PGD mortality or sublethal gill damage. Though smaller fingerlings regularly showed slightly better average gill condition compared to larger fingerlings, this occurred primarily in ponds with the highest parasite concentrations, which were likely influenced by survival bias.

Keywords: Henneguya ictaluri; aquaculture; disease; myxozoan; parasite; proliferative gill disease.

Figure 1.

Binomial dose–response curve based on Henneguya ictaluri actinospore concentrations in pond water. Response curves represent large (black circles with solid line) and small (red triangles with dashed line) Channel Catfish fingerlings. The quantitative PCR quantification cycle (Cq) values have an inverse relationship with estimated DNA concentration.

Figure 2.

Lesion criteria used in histologic scoring of proliferative gill disease lesions in Channel Catfish: (A) inflammatory cell infiltrates expanding the gill filament with or without cartilage breaks or presporogonic stages (arrows); (B) congestion within gill filaments (asterisks); (C) focal chondrolysis (cartilage breaks) of the gill filaments (asterisks); and (D) abnormal regeneration (chondrodysplasia) of filament cartilage (asterisks). Scale bars represent 50 µm.

Figure 3.

Images from the Henneguya ictaluri-specific in situ hybridization assay from select Channel Catfish gills. Presporogonic stages of H. ictaluri are represented by red and dark-colored aggregations within the cartilage and filaments of the gill tissues. Significant swelling of the primary and secondary lamellae is also evident in many of the infested tissues. Scale bars for panels A and B are 200 and 50 µm, respectively.

Figure 4.

Scatterplot of pond water quantification cycle (Cq) values and Channel Catfish gill tissue Cq values. Diagonal line represents a theoretical 1:1 relationship assuming uniform and proportional infection based on parasite density in the environment.

Figure 5.

Scatterplot of (A) gill filament damage, (B) average number of lesions, and (C) average number of presporogonic stages based on Henneguya ictaluri actinospore concentration in the pond water (as quantitative PCR quantification cycle [Cq]) and Channel Catfish size. Lines represent estimated means from a generalized linear model with a negative binomial distribution. The Cq values have an inverse relationship with estimated DNA concentration.

Figure 6.

Scatterplot of (A) gill filament damage, (B) average number of lesions, and (C) average number of presporogonic stages based on Henneguya ictaluri actinospore concentration in the gill tissue (as quantitative PCR quantification cycle [Cq]) and Channel Catfish size. Lines represent estimated means from a generalized linear model with a negative binomial distribution. The Cq values have an inverse relationship with estimated DNA concentration.