Location: Warmwater Aquaculture Research Unit
2022 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
Diagnostic Surveillance: The Aquatic Research and Diagnostic Laboratory is a clinical program providing diagnostic services in support of research and the commercial catfish industry in the southeastern United States. This service provides insight into disease trends and identifies emergent pathogens. In 2021, 733 cases were submitted for diagnostic and water quality analysis by producers and researchers. The utilization of hybrid catfish as a culture fish has led to the emergence of pathogens typically not seen in channel catfish. Most noticeable is the emergence of E. piscicida as a significant pathogen of hybrid catfish. In 2021, 95% of cases diagnosed with E. piscicida were from hybrid catfish. Conversely proliferative gill disease (PGD) appears to affect channel catfish more frequently, as channel catfish comprise ~70% of PGD diagnoses. Another emergent issue is the accumulation of ammonia levels in intensive production systems utilizing hybrid catfish. Nutrient loading caused by high feeding and stocking rates appears to be the limiting factor in hybrid catfish production. This information is being utilized to redirect resources to develop mitigation strategies to address changes in disease trends with changes in production practices.
Virology: Blue catfish alloherpesvirus (BCAHV) is a novel virus isolated from the blue catfish which could pose a threat to hybrid catfish production. Neutralization assays revealed BCAHV is refractive to neutralizing monoclonal antibodies against channel catfish virus (CCV) suggesting sufficient variation in the targeted antigenic epitope. Challenge studies showed increased mortality in blue catfish with intermediate mortality in hybrid catfish and negligible mortality in channel catfish. Fish that survived BCAHV challenge were refractive to subsequent CCV challenges, suggesting shared protective antigens between BCAHV and CCV. Host susceptibility differed with age and elevated temperature appeared to inactivate BCAHV which, is in contrast to temperature ranges permissive for CCV. These data, along with genomic sequence data, provide evidence that BCAHV should be given consideration as a new species of Ictalurivirus. The prevalence of BCAHV is being evaluated in commercial hatcheries. In support of this work new immortal fish cell lines have been developed for prevalence screening and in-vitro clinical studies.
Bacterial Diseases: Edwardsiella piscicida is an emergent bacterial pathogen diagnosed primarily in hybrid catfish. Experimental exposures resulted in significantly higher mortality in blue and hybrid catfish, compared to channel catfish. Pathology associated with E. piscicida included disseminated hemorrhage and necrosis often with numerous bacteria accompanied by gastric and intestinal submucosal edema, which transitions to a granulomatous response with reduced bacterial numbers with disease progression. Exocrine pancreatic degeneration occurred sporadically. Fish surviving E. piscicida infection were shown to be protected against subsequent exposure to E. piscicida and E. ictaluri (closely related pathogen in channel catfish). Further work showed E. piscicida can be subdivided in 5 genetic clads which reflect host specificity. Immunization with a live-attenuated E. ictaluri vaccine offers protection to channel and hybrid catfish against infection from all genetic E. piscicida variants. Protection was more pronounced in channel catfish and work is being conducted to develop an attenuated E. piscicida vaccine which can used in the current E. ictaluri vaccination platform to potentiate the immune response against E. piscicida infection in hybrid catfish. Preliminary work shows a combinatory vaccine increases protection against E. piscicida infection in hybrid catfish.
An atypical strain of Aeromonas hydrophila (aAh) has been associated with catastrophic losses within the catfish industry. A rapid quantitative PCR based method for detecting aAh in fish tissue and environmental samples was developed and used to monitor active infections and carrier states in commercially raised catfish. Results showed the state of the pond has a significant effect on which swab location yields the highest proportion of PCR positive results. During an active outbreak, gill swabs identified more fish as aAh-positive. Conversely, in apparently healthy populations, swabs taken from the lower intestine identified significantly more aAh-positive fish than gill swabs. Data suggests the presence of carrier states related to epizootics and indicates the pathogen may pose minimal risk unless the host is compromised or a mechanical portal of entry into the blood stream is available. This provides an important direction for future research in understanding the mechanisms of aAh outbreaks in catfish aquaculture ponds.
Vibrio cholerae has been identified as the cause of mortality in hatchery catfish. A comparative analysis was performed on 60+ suspected Vibrio spp. isolated from fish health laboratories in North America. Molecular analysis revealed multiple V. cholerae variants formed multiple two discrete heterologous groups within catfish and ornamental fish. Regardless of host origin, all isolates from fish were negative for the toxin production genes which are associated with human illness. This work lays the foundation for future studies investigating the origins of V. cholerae outbreaks in catfish hatcheries in the catfish farming region of the Mississippi Delta.
Parasitic Diseases: The myxozoan Henneguya ictaluri causes proliferative gill disease in channel and hybrid catfish and is responsible for significant losses in catfish aquaculture. The parasite has a complex life cycle utilizing fish as an intermediate host and an aquatic oligochaete, Dero digitata, as the final host. Life cycle studies indicated hybrid catfish are a dead-end host for the parasite. Monoculture of hybrid catfish following culture with channel catfish was shown to suppress H. ictaluri in the culture system. These data suggest a crop rotation strategy should be effective in minimizing PGD related losses. This approach is being monitored on commercial operations. In support of this work Henneguya species-specific in situ hybridization assays were developed, which identified mixed Henneguya infections in channel and hybrid catfish suffering from PGD. Metagenomic analysis of pond water showed a shift in myxozoan communities in response to hybrid or channel monoculture, indicating fish choice influences pond myxozoan communities. Optimized protocols for rearing laboratory populations of D. digitata have been developed, which have been critical to establishing laboratory models to assess developmental timelines and evaluate host–parasite interactions of catfish associated myxozoa.
The trematode Bolbophorus damnificus is significant cause of lost production in catfish aquaculture. Copper sulfate is widely used to control snails, which serve as intermediate hosts in several trematode life cycles. However, copper can be toxic to fish. Four weekly, low-dose copper sulfate treatments were shown to be just as effective as a single, high-dose application, although with less risk to fish. To improve treatment accuracy and increase application efficiency, a mechanized, powder copper sulfate delivery system was developed utilizing a radar groundspeed sensor and a logic-based control system to evenly distribute granular copper sulfate along the pond margins.
Accomplishments
1. Development of an effective oral enteric septicemia of catfish vaccination platform. Enteric septicemia of catfish is considered the most problematic bacterial disease affecting the production of catfish fingerlings. Historically, management strategies relied on the use of medicated feed and feed restrictions to limit the oral route of infection. While both strategies can be effective they are not without limitations. Overuse of medicated results in the development of antibiotic resistance rendering the medication useless and feed restrictions severely limits growth. In efforts to develop more proactive management strategies, ARS researchers in Stoneville, Mississippi, developed a live attenuated vaccine along with a mechanized delivery system allowing for the in-pond vaccination fish during the early stages of fingerling production. The oral vaccine is currently available by veterinarian prescription and has dramatically increased survival and profitability of fingerling catfish production. To date, over 90% of catfish produced in Mississippi and Alabama are vaccinated with the delivery platform being applicable to other live attenuated vaccines. The vaccine also provides cross protection against E. piscicida which is an emergent pathogen in hybrid catfish production. A patent for the vaccine isolate was granted in 2015 (Patent No. 8,999,319) and the patent for the delivery system granted in 2022 (Patent No. 11,330,833 B2). These technologies were developed and validated with funding from the current and previous appropriated USDA/ARS projects.
2. Development of technologies for controlling snail populations in catfish production ponds. Trematode infestations have been linked to significant production losses and farm closures. Since recognition as an emergent disease in the late 1990’s, management strategies have been developed to break the trematode life cycle by eradicating the snail intermediate host in the pond environment. Copper sulfate is the most widely used treatment option and is highly effective against snails with a single application of 3 ppm but this treatment level can result in increased mortality in fish, especially when water temperatures are elevated. These losses are associated with direct toxicity to fish and algacidal properties of copper, causing algal bloom die offs that result in oxygen depletion within the pond ecosystem. Recent research by ARS researchers in Stoneville, Mississippi, generated from this this project has demonstrated that weekly low-dose copper treatments (1.0-1.5 ppm) spread across four weeks are as effective in killing snails and treatment rates <0.1 ppm can halt snail reproduction and kill snail embryos. While this approach will greatly improve treatment safety with respect to fish health, it increases the time and labor requirements for treatment application. As a result, a mechanized copper sulfate delivery system was designed to consistently and uniformly apply copper sulfate crystals along the littoral zone of the pond environment. The system was designed in a three-point hitch configuration, for tractor attachment, and driven in a single pass in a 2 to 5 mph range around pond margins. The system utilizes a radar groundspeed sensor and a logic-based control system to evenly and accurately distribute granular copper sulfate along the pond margins in a single pass.
Review Publications
Kumar, G., Hegde, S., Wise, D., Mischke, C.C., Dorr, B. 2021. Economic losses of catfish to Avian Predation: A case report. North American Journal of Aquaculture. 83:127-137. https://doi.org/10.1002/naaq.10170.
Mischke, C.C., Wise, D.J., Griffin, M.J., Rosser, T.G., Tiwari, A., Rehman, J.U., Ashfaq, M.K., Khan, I.A. 2021. Effects of multiple, low-dose copper sulfate treatments on the marsh Rams-Horn Snail. North American Journal of Aquaculture. 83:363-371. https://doi.org/10.1002/naaq.10207.
