Location: Warmwater Aquaculture Research Unit
2024 Annual Report
Objectives
1. Identify emergent pathogens in catfish aquaculture and develop disease diagnostic methodologies for use in field surveillance studies.
1.1. Identify emergent pathogens in catfish aquaculture and develop disease diagnostic methodologies for field surveillance studies.
1.2. Characterize and evaluate pathogenesis of emergent diseases and fulfillment of Koch’s postulates or River’s postulate for newly recognized or emergent pathogens.
1.3. Develop rapid diagnostic tests and ELISA procedures to determine total and antigen specific antibody for epidemiological studies.
1.4. Develop primary catfish cell lines for identification and confirmation of fish viruses.
2. Optimize treatments and management strategies to minimize infectious diseases in catfish aquaculture.
2.1. Optimize Edwardsiella (E.) ictaluri vaccine delivery, evaluate cross protective potential of E. ictaluri vaccine against E. piscicida.
2.2. Role of other myxozoans (non-H. ictaluri) and intraspecific variability of oligochaete hosts in occurrence of proliferative gill disease (PGD) in channel and hybrid catfish.
2.3. Role of iron fortified diets and occurrence of bacterial infections in channel and hybrid catfish.
2.4. Evaluation of the pathophysiological effects of Bolbophorus damnificus (trematode) in hybrid and channel catfish.
3. Determine the epidemiology of infectious diseases in catfish aquaculture and conduct economic evaluations of disease management strategies.
3.1. Significance of genetic E. piscicida variants recovered from commercially cultured hybrid and channel catfish.
3.2. Epidemiology of A. hydrophila infections in catfish aquaculture; predictive modeling to determine risk factors.
3.3. Spatio-temporal survey of channel catfish virus (CCV) isolates and evaluation of trends in the occurrence and virulence of different genetic strains of CCV in channel and hybrid catfish.
3.4. Evaluate disease transmission of emergent Vibrio spp. infections in hatchery fry.
3.5. Economic evaluation of a live, attenuated E. ictaluri vaccine in commercial fingerling and foodfish production.
Approach
In the United States, pond production of catfish ranks as the leading aquaculture species in terms of farm gate value. Health management strategies, technologies, and bio-security plans that are environmentally safe are necessary to help mitigate disease-related losses. There is presently a lack of validated technologies for early and rapid detection of pathogens, disease prevention, and treatment of diseases in catfish aquaculture, which has hindered the growth and profitability of the industry. Validated diagnostic tools for use in production systems to detect the disease agents in a rapid fashion are needed. In addition to the need for diagnostics, developing effective control strategies to manage disease is a priority, given only a few drugs are available for the treatment of sick fish. Further research will develop molecular based diagnostic tools used in to monitor potential emergent pathogens, optimize vaccination strategies for control of bacterial infections, determine the epidemiology of priority infectious diseases and assess costs and benefits of disease management strategies in hybrid and channel catfish aquaculture.
We will identify emergent pathogens in catfish aquaculture and develop disease diagnostic methodologies for use in field surveillance studies. Specifically, we will characterize and evaluate pathogenesis of emergent diseases and fulfillment of Koch’s postulates or River’s postulate for newly recognized or emergent pathogens, develop rapid diagnostic tests and ELISA procedures to determine total and antigen specific antibody for epidemiological studies, and develop primary catfish cell lines for identification and confirmation of fish viruses. To improve disease management strategies in catfish aquaculture we will optimize Edwardsiella (E.) ictaluri vaccine delivery and evaluate cross protective potential of E. ictaluri vaccine against E. piscicida, determine the role of other myxozoans (non-H. ictaluri) and intraspecific variability of oligochaete hosts in occurrence of proliferative gill disease (PGD) in channel and hybrid catfish, determine the role of iron fortified diets and occurrence of bacterial infections in channel and hybrid catfish, and evaluate the pathophysiological effects of Bolbophorus damnificus (trematode) in hybrid and channel catfish.
In order to determine the epidemiology of infectious diseases in catfish aquaculture and conduct economic evaluations of disease management strategies we will determine the significance of genetic E. piscicida variants recovered from commercially cultured hybrid and channel catfish, determine the epidemiology of atypical Aeromonas hydrophila (aAh) infections in catfish aquaculture, perform a spatio-temporal survey of channel catfish virus (CCV) isolates and evaluate trends in the occurrence and virulence of different genetic strains of CCV in channel and hybrid catfish, evaluate disease transmission of emergent Vibrio spp. infections in hatchery fry, and evaluate economic impact of a live, attenuated E. ictaluri vaccine in commercial fingerling and foodfish production.
Progress Report
Research related to Objective 1 and 3. The Aquatic Research and Diagnostic Laboratory (ARDL) is a clinical program that provides diagnostic services to support research and the commercial catfish industry in the southeastern United States. This service offers valuable insights into disease trends and helps identify emerging pathogens. In 2023, 536 cases were submitted by producers and researchers for diagnostic and water quality analysis. The use of hybrid catfish in aquaculture has led to the emergence of pathogens not typically seen in channel catfish, most notably Edwardsiella piscicida, which has become a significant pathogen in hybrid catfish. In contrast, Proliferative Gill Disease (PGD) cases were less frequently observed in hybrid catfish, prompting research into the host specificity of the causative myxozoan parasites in both hybrid and channel catfish. This research demonstrated that hybrid catfish are a dead-end host for the myxozoan parasites, thereby reducing pathogen levels in the culture system. Additionally, there has been a continued decline in antibiotic resistance reported through the ARDL, likely due to the vaccination of catfish fingerlings against Edwardsiella ictaluri infection, which in turn has reduced antibiotic use. Group C Streptococcus dysgalactiae (subspecies dysgalactiae) was identified as a new disease agent in catfish broodstock. This potentially emerging pathogen could affect egg quality and subsequently reduce hatching rates in hybrid catfish eggs. We are allocating resources to develop control measures during the production of hybrid catfish to address this issue.
Research related to Objective 2. The Delta Select line of channel catfish was established in 2006 and has undergone five generations of genetic selection, specifically targeting increased growth rate and carcass yield. This line was released to U.S. catfish farmers in 2020. Growth rate and carcass yield have been improved but other important production traits have yet to be evaluated. Infectious diseases, particularly bacterial infections caused by Edwardsiella spp., result in significant economic losses for catfish producers in the southeastern United States. Given the economic impact of these bacterial pathogens, we conducted experimental infectivity trials to compare susceptibility to Edwardsiella spp. between the Delta Select line and a randomly bred line of channel catfish originating from the same base population (Delta Control line). The Delta Select catfish had higher survival rates compared to Delta Controls when exposed to Edwardsiella piscicida, suggesting higher disease resistance. Comparably, no differences were observed between the lines in response to challenge with E. ictaluri. While there were no observed improvements in terms of susceptibility to Edwardsiella ictaluri, Delta Selects were no more susceptible than the Delta Controls. This coupled with improved survival during E. piscicida challenge indicates that selection for faster growth did not increase susceptibility to Edwardsiella spp. and may provide improve resistance to E. piscicida infection in channel catfish.
Research related to Objective 2. Enteric septicemia of catfish (ESC), caused by the gram-negative enteric bacteria Edwardsiella ictaluri, poses a significant threat to catfish aquaculture in the southeastern United States. While antibiotic intervention can reduce mortality, it often causes imbalances in the gut microbe populations (dysbiosis), potentially increasing susceptibility of otherwise healthy fish to infections. The administration of the antibiotic florfenicol led to dysbiosis, characterized by inflated microbial diversity, which began to recover in both diversity and composition four days after cessation of florfenicol administration. Fish fed a probiotic diet after antibiotic administration showed higher survival rates in response to ESC challenges compared to those fed a prebiotic diet. These results suggest that probiotic therapy may offer host protection against E. ictaluri infection, particularly following antibiotic treatments, and is currently being evaluated in a commercial setting.
Research related to Objective 2. Iron supplements are used to mitigate idiopathic anemia and stimulate red blood cell production. We conducted trials to evaluate the effects of prolonged exposure to excess dietary iron on growth performance and disease resistance in hybrid catfish. Fish were fed graded levels of ferrous sulfate to deliver between 500 and 1500 mg of iron per kg of diet. At the end of the study, production performance, survival, condition indices, protein and iron retention were unaffected by the dietary treatments. Blood hematocrit and iron concentration in the whole body showed a linear increase with the increasing dietary iron. However, there was a trend towards increased mortality in fish fed supplemental iron. Results suggest that while iron supplementation increases red blood cell production, it may also increase the susceptibility of naïve fish to E. ictaluri infection. Therefore, iron supplements should not be used in fingerling production, where bacterial infections are most problematic.
Research related to Objective 3. Proliferative gill disease (PGD) is a parasitic disease affecting channel and hybrid catfish, caused by the myxozoan parasite Henneguya ictaluri. The parasite has a complex life cycle involving catfish and a worm species found in catfish ponds. Research trials have demonstrated that hybrid catfish are a likely dead-end host for the parasite, suggesting they could be used as a management strategy to disrupt the parasite’s life cycle in production systems, thereby minimizing pathogen levels in pond environments. This was validated in experimental ponds where parasite loading rates were significantly decreased in ponds stocked with hybrid catfish. Moreover, a metagenomic approach developed as part of this project indicates hybrid catfish select for different myxozoan species that also utilize the same worm species, indicating myxozoan community structures in catfish ponds are dictated by fish stocking choice. This research expands our understanding of how host susceptibility influences the disease process and may provide insights into controlling myxozoan populations and mitigating disease outbreaks using hybrid catfish. These findings support the idea that strategic crop rotations alternating between channel catfish and hybrid catfish could exploit differences in host susceptibility, preventing H. ictaluri and other myxozoan species from reaching levels that cause catastrophic PGD outbreaks. The concept of “crop rotation” between channel and hybrid catfish is being evaluated as a management strategy on commercial farms to minimize the impact of myxozoan parasites in catfish aquaculture.
Research related to Objective 2. The trematode worm Bolbophorus damnificus causes significant production losses in catfish aquaculture and has even led to farm closures. The parasite has a complex life cycle that sequentially involves the American White Pelican, snails, and catfish. Control strategies focus on breaking the parasite's life cycle by eliminating the snail host from the culture environment. We have previously shown that multiple low-dose treatments of copper are as effective as a single high-dose treatment, improving treatment safety while maintaining efficacy. However, preparation of a copper sulfate solution is labor-intensive, requiring significant time and resources to dissolve granular copper sulfate to saturation. This process also requires multiple passes around the pond perimeter to deliver the estimated volume of solution needed to achieve the target dose. The inherent inaccuracy of this manual process significantly increases the likelihood of erroneous application rates, potentially resulting in either under-application or over-application. To improve treatment accuracy and facilitate application, a mechanized granular copper sulfate application system was designed and evaluated in both experimental and commercial ponds. This system delivers copper sulfate crystals consistently and uniformly along the littoral zone of commercial catfish production ponds in a single pass. This innovation aims to enhance the precision and reliability of copper sulfate applications, thereby improving treatment efficacy and reducing the risks associated with improper dosages.
Research related to Objective 2. Copper sulfate is used for algae control, the control of external fish parasites, and the elimination of snails that serve as disease vectors for trematode infestations. The active component of copper sulfate is the cupric ion, which readily binds with inorganic and organic ligands, resulting in inactive complexes. Although copper sulfate is a commonly used chemical treatment essential in aquaculture, little information is available on the effects of copper on pond microflora and the overall impacts on fish production. To address this issue, multiple low-dose copper treatments on zooplankton, phytoplankton, and water quality variables were evaluated in experimental ponds. Copper treatments significantly influenced phytoplankton communities, with dose-dependent reductions in Cyanophyta and increases in Charophyta in copper-treated ponds compared to controls. Phytoplankton populations persisted throughout the study, even in the highest treatments, and oxygen depletions were not observed. At harvest, there were no differences among treatments in production variables. Additionally, there were no treatment effects on water quality variables or zooplankton communities. Furthermore, copper sulfate appeared to stabilize microbial communities. Control and 0.5 mg/L treatments exhibited temporary dynamic fluctuations, while 1.0 and 1.5 mg/L treatments were relatively constant.
Accomplishments
1. Emergent disease agents and diagnostic trends. ARS researcher at Stoneville, Mississippi, provide a clinical program that provides diagnostic services to support research and the commercial catfish industry. This service offers insights into disease trends, identifies emerging pathogens, and directs resources to forming research projects. Antibiotic resistance continues to be a growing threat to the utility of approved medicated feeds. However, compared to previous years, there has been a dramatic reduction in antibiotic resistance, likely related to the recent adoption of an Edwardsiella ictaluri vaccine, developed from the previous NACA. The number of antibiotic cases dropped from 111 in 2019 to 7 in 2023. Yersinia ruckeri and Vibrio cholerae have been identified as potential emergent pathogens. Y. ruckeri was isolated from hybrid catfish. Infectivity trials demonstrated a low potential for waterborne transmission, and it is not currently expected to be a significant pathogen. Vibrio cholerae has been isolated from catfish during the hatchery phase of production and raised concerns not only for catfish fry production but also as a zoonotic agent. Research demonstrated that outbreaks were associated with overcrowding and have been resolved through good management practices. Additionally, isolates archived from diagnostic submissions show that the V. cholerae strains lack the cholera toxin genes (CTX) and consistent with other non-toxigenic strains found in aquatic environments and are part of the normal water flora. As such, V. cholerae affecting hatchery catfish is not considered a contagion or to have zoonotic potential. Recently, Group C Streptococcus dysgalactiae has been identified as a new disease agent in catfish brood stock. Molecular analysis has tied these isolates to a discrete fish associated group attributed to disease outbreaks in both marine and freshwater environments, indicating a potential emergence of this fish associated pathogen in US catfish aquaculture.
2. Implementation of a Proliferative Gill Disease (PGD) risk assessment model. Proliferative gill disease (PGD) is caused by a ubiquitous myxozoan parasite causing substantial losses in commercially raised catfish. Most losses occur when catfish fingerlings are introduced in grow-out ponds for food fish production. Related to the current and previous research agreement (Project No. 6066-31320-004-D), researchers at Mississippi State University developed a PGD risk assessment model was to determine the likelihood of fish losses in newly stocked production ponds or when fish are understocked for food fish production. The risk assessment model relies on comprehensive water analysis, utilizing environmental DNA methodologies and a quantitative PCR assay specifically developed and validated through this project. By determining the levels of infectious life stages present in the pond water, correlations between parasite levels and mortality events have been established. This program has been implemented as a demonstration project on farms where water samples are collected concurrently with sentinel fish exposures to identify ponds that can be safely stocked with minimal risk to fish health. Under this program, producers submit their pond water samples for analysis, and the risk of fish loss is determined based on the parasite levels detected. Ponds with high parasite levels are closely monitored and re-evaluated until the levels fall below a predetermined threshold, thus minimizing losses associated with PGD. The cost of PGD is not known but feedback from one participating producer indicates the program saves between 25-50k annually by avoiding stocking fish into ponds with lethal parasite levels. By accurately evaluating the parasite levels in pond water, producers can make informed decisions regarding stocking strategies, ultimately minimizing losses, and increasing profitability.
3. Epidemiology of myxozoan infections in catfish aquaculture using metagenomic analysis. Myxozoan parasites have multiple life stages involving both fish and a host-specific aquatic oligochaete, with numerous species affecting cultured and wild fish globally. Until recently, Henneguya ictaluri, the cause of proliferative gill disease (PGD), was the only significant pathogen identified in cultured catfish, causing severe gill inflammation, impaired function, and death. Reseachers at Mississippi State University determined the susceptibility of hybrid catfish to H. ictaluri, revealing that while both channel and hybrid catfish are susceptible to initial infection and gill pathology, myxospore sporogenesis was not observed in hybrid catfish, suggesting they are a dead-end host that could potentially disrupt the parasite's life cycle. A three-year experimental pond study showed that ponds stocked with hybrid catfish had marked decreases in the waterborne life stage infecting channel catfish. Metagenomic analysis from this study indicated that fish type dictates myxozoan community composition, as channel and hybrid catfish exert different selective pressures on the pond ecosystem, providing insights into how management practices influence myxozoan community dynamics in catfish aquaculture. Metagenomic assessments revealed hybrid catfish select for increased abundance of Henneguya exilis in the pond system while suppressing other channel catfish associated myxozoans, namely the PGD organism Henneguya ictaluri. In line with these findings, a new myxozoan gill disease in hybrid catfish was characterized as “Massive Branchial Henneguyosis of Catfish (MBHC)”; a distinct, myxozoan-induced gill disease caused by severe interlamellar Henneguya exilis infection in hybrid and channel catfish. Outbreaks of MBHC resultsin high morbidity due to Outbreaks of MBHC result in high morbidity, due to the mechanical obstruction of gill function due to high numbers of large plasmodia leading to respiratory compromise.
4. Influence of fish size on Proliferative Gill Disease (PGD) susceptibility. Incidence of PGD occurs most frequently in fingerling sized catfish, often during spring-time stocking, although stocker and market-size fish can also be affected. In response to stakeholder queries, researchers at Mississippi State University evaluated the potential benefit of stocking larger fingerlings to improve survival during spring-time stocking and investigate the influence of parasite burden in pond water and fish size on three metrics of gill condition. Two sizes of fingerlings (~12 cm and ~20 cm), which correlate to industry-relevant fingerling weights of 40lbs/1000 and 120lb/1000, were stocked into nylon-mesh net pens located in 19 commercial ponds with varying levels of H. ictaluri activity. After one week, fish were removed from ponds, mortality was recorded and all survivors subjected to gross examinations, as well as histologic and molecular assessment. Gill biopsies from surviving fish were evaluated with a previously established lesion scoring system to estimate gill damage based on the presence of chondrolytic lesions in gill clip wet mounts. Histologically, the number of characteristic PGD lesions was assessed as well as the number of presporogonic stages present. There was no significant difference in mortality rates between the small and large fingerlings. 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. Results from this study suggest that stocking of larger fingerlings provides no appreciable protection from PGD mortality or sub-lethal gill damage.
Review Publications
Heckman, T.I., Yazdi, Z., Older, C.E., Griffin, M.J., Waldbieser, G.C., Chow, A.M., Medina Silva, I., Anenson, K.M., Garcia, J.C., Lafrentz, B.R. 2024. Redefining Piscine Lactococcosis. Applied and Environmental Microbiology. 90:e02349-23. https://doi.org/10.1128/aem.02349-23.
Older, C.E., Griffin, M.J., Richardson, B.M., Waldbieser, G.C., Reifers, J.G., Goodman, P.M., Ware, C., Gatlin Iii, D.M., Wise, D.J., Yamamoto, F.Y. 2023. Influence of probiotic and prebiotic supplementation on intestinal microbiota and resistance to Edwardsiella ictaluri infection in channel catfish (Ictalurus punctatus) following florfenicol administration. Journal of Fish Diseases. 47(4):e13910. https://doi.org/10.1111/jfd.13910.
Ott, B.D., Torrans, E.L., Allen, P.J. 2022. Design of a Vacuum Degassing Apparatus to Reduce Nitrogen Supersaturation and Maintain Hypoxia in Well-Water. North American Journal of Aquaculture. https://doi.org/10.1002/naaq.10263.
Quiniou, S., Bengten, E., Boudinot, P. 2024. Costimulatory receptors in the Channel catfish: CD28 family members and their ligands . Immunogenetics. 76:51-67. https://doi.org/10.1007/s00251-023-01327-3.
Ott, B.D., Chisolm, D.O., Griffin, M.J., Torrans, E.L., Allen, P.J. 2023. Effect of hypoxia duration and pattern on channel catfish (Ictalurus punctatus) neuropeptide gene expression and hematology. Journal of Comparative Physiology. 193:631-645. https://doi.org/10.1007/s00360-023-01521-5.
Pfeiffer, T.J., Baptiste, R.M., Wills, P.S. 2024. Fine solids removal by foam fractionation in a low-salinity recirculating aquaculture system for red drum juveniles, Sciaenops ocellatus. North American Journal of Aquaculture. https://doi.org/10.1002/naaq.10345.