Project : USDA ARS

ARS Home » Southeast Area » Auburn, Alabama » Aquatic Animal Health Research » Research » Research Project #437658

Research Project: Integrated Research to Improve Aquatic Animal Health in Warmwater Aquaculture

Location: Aquatic Animal Health Research

2025 Annual Report

Objectives
1. Identify virulence factors critical for pathogenesis of major catfish pathogens to guide the development of novel and cost-effective disease interventions. 1.A. Identify the genes (or their protein products) governing the virulence of Aeromonas hydrophila in catfish. 1.B. Characterize the environmental conditions of perturbations that influence the expression of virulence determinants in Aeromonas hydrophila. 1.C. Elucidate the capsular polysaccharide (CPS) antigenic diversity in Flavobacterium columnare and determine its role in pathogenesis. 2. Improve prevention and control strategies for bacterial and parasitic diseases of catfish and shrimp. 2.A. Evaluate the efficacy of next generation Flavobacterium columnare vaccines and identify the host immune responses that govern protection. 2.B. Determine the extent to which various feed additives (e.g. immunostimulants, toxin binders, etc.) modulate susceptibility of fish and shrimp to industry relevant pathogens. 2.C. Investigate host pathophysiology and performance following parasitic insult.

Approach
The catfish industry is the largest sector of U.S. aquaculture and shrimp production represents a growing and important sector. Improving the health of catfish, shrimp, and other warmwater species is important for long-term sustainability of these industries because losses due to disease are a significant impact to production. This project will take a multifaceted approach to accomplish two objectives that address the host, pathogen, and environmental interactions that are critical for improving aquatic animal health in aquaculture. Although Aeromonas (A.) hydrophila and Flavobacterium (F.) columnare have been studied for years, there are still gaps in our knowledge regarding the virulence factors of these pathogens and how environmental conditions alter their virulence. Therefore, Objective 1 will identify the genes governing the virulence of A. hydrophila, characterize environmental conditions that impact virulence amd elucidate the antigenic diversity of the capsular polysaccharide of F. columnare. Furthermore, prevention and control strategies for bacterial and parasitic diseases are limited and there are gaps in knowledge regarding host immune responses against pathogens. Research conducted under Objective 2 will develop new vaccines for F. columnare, determine the effect of feed additives on the susceptibility of fish and shrimp to disease, investigate the effect of parasitic insult on catfish performance and disease susceptibility, and determine the host immune mechanisms involved in protective immunity. The overall impact of this research is a reduction in disease related losses thereby increasing the profitability and production efficiency in the catfish, shrimp and other warmwater aquaculture industries.

Progress Report
This is the final report for project 6010-32000-027-000D, Integrated Research to Improve Aquatic Animal Health in Warmwater Aquaculture, which has been replaced by new project 6010-10600-002-000D, Improving Warmwater Finfish Health through Pathogen Characterization, Vaccination, and Natural Feed Additives. For additional information, see the new project report. Below is a summary of the significant accomplishments. A multiplex polymerase chain reaction assay was developed for genotyping Flavobacterium columnare, the causative agent of columnaris disease. The assay is being used by stakeholders and university partners to determine which genetic group(s) are responsible for disease losses in different aquaculture industries impacted by columnaris disease. Research identified inclusion of frass, a by-product of the black soldier fly larval meal industry, into the diets of channel catfish resulted in greater weight gain and improved resistance to disease in catfish and tilapia. Thus, frass has the potential as an alternative source of protein in fish feeds or as an ingredient for enhancing palatability, growth, and disease resistance. Research demonstrated conditions of iron scarcity (common in ponds in some catfish farming regions), can trigger release of potent virulence factors of virulent Aeromonas hydrophila and lead to significantly elevated catfish mortality. These findings are being applied in the ongoing development of an efficacious vaccine to protect against this pathogen and in practical pond applications/management strategies that seek to stabilize available iron levels. Research demonstrated channel catfish fingerlings vaccinated with a live attenuated Flavobacterium columnare vaccine stimulated immune responses and improved the growth and food conversion ratio compared to non-vaccinated fingerlings. Partial budget analysis demonstrated use of vaccinated fingerlings resulted in a net benefit of US $600 per acre, thus showing an economic benefit to producers. Research demonstrated that selectively breeding Nile tilapia for disease resistance against Streptococcus spp. does not negatively impact harvest weight. Thus, multitrait selection is recommended to balance growth and disease resistance. The goal of this research is to provide fish farmers with a robust stock of tilapia that are resistant to disease and exhibit fast growth. Research confirmed the toxicity of Vibrio parahaemolyticus PirA and PirB toxins and demonstrated they contribute to acute hepatopancreatic necrosis disease in shrimp (AHPND). The two toxins were produced recombinantly and were shown to cause mortality in shrimp. Use of the recombinant proteins rPirA and rPirB may aid the search for antitoxin strategies to prevent losses in shrimp aquaculture due to AHPND. ARS scientists in Auburn, Alabama previously demonstrated the presence of four discrete genetic groups within the fish pathogen Flavobacterium columnare and reported associated host and virulence differences. In collaborative research, the scientists completed a suite of laboratory experiments and showed each group is a discrete species. The names F. covae, F. davisii, and F. oreochromis were proposed to represent genetic groups 2, 3, and 4, respectively, while genetic group 1 isolates remain recognized as F. columnare. Since these pathogens are globally distributed and have significant impacts on wild and cultured fish species, research needs to target the correct bacterial species. ARS scientists in Auburn, Alabama previously developed a recombinant DnaK protein vaccine for catfish that provided protection against columnaris disease. To improve the effectiveness of the vaccine, the scientists tested the use of an immersion adjuvant to boost the immune system. The results demonstrated significant protection of channel catfish at 6 to 8 weeks post vaccination and an overall benefit of using the immersion adjuvant to reduce the time required to stimulate a protective immune response. This research further confirmed the potential for developing an immersion vaccine based on the recombinant protein for the prevention of columnaris disease which is greatly needed for the catfish industry. In collaboration with industry partners, ARS scientists in Auburn, Alabama identified genetic markers for selectively breeding Nile tilapia for resistance to disease caused by Streptococcus iniae. The effectiveness of using the genetic markers for improving resistance of tilapia to S. iniae was confirmed experimentally. This was a significant milestone and demonstrates that tilapia families no longer need to be routinely infected with S. iniae to assess their level of resistance to this pathogen. Rather, their DNA can be analyzed and if they carry the favorable genetic marker, they will produce the more disease resistant offspring. This is the first genetic marker associated with resistance to a bacterial pathogen in Nile tilapia and will greatly contribute to breeding of fish with resistance to S. iniae and reduce losses associated with this pathogen in US and global tilapia aquaculture. ARS scientists in Auburn, Alabama and collaborators at Auburn University developed a system to recombinantly express murine monoclonal antibodies (mAb). The system was used to generate a recombinant 9E1 mAb which specifically recognizes channel catfish immunoglobulin. The recombinant 9E1 mAb was shown to effectively bind to soluble catfish IgM and was effective in a suite of commonly used immunological assays. The recombinant 9E1 mAb will be a valuable research tool for catfish immunology and the developed system provides a means to develop additional recombinant mAbs to safeguard these important research tools. ARS scientists in Auburn, Alabama and collaborating scientists from Bergen, Norway and Miami, FL conducted research to determine the potential to selectively breed tilapia for resistance to Francisella orientalis and to examine the relationship between other traits of economic importance. The results demonstrated moderate additive genetic variation in resistance to F. orientalis across four generations and families bred for resistance or susceptibility performed as anticipated following bacterial challenge (i.e., resistant families had high survival and susceptible families had poor survival). No unfavorable genetic correlations were found between commercial traits such as resistance to other pathogens and growth. This research has led to the development of a robust strain of tilapia that are resistant to disease and exhibit fast growth, important traits for the tilapia industry. Columnaris disease is one of the largest contributors to disease losses for catfish farmers in the southeastern US. The term ‘columnaris-causing bacteria’ (CCB) has been coined in reference to the four recently described species that cause columnaris disease, Flavobacterium columnare, F. covae, F. davisii, and F. oreochromis. The species of CCBs most severely impacting the industry, as well as their virulence in channel catfish were not known. ARS scientists in Auburn, AL and university collaborators demonstrated F. covae is the predominant CCB impacting the catfish industry and this data was supported by laboratory infection experiments. These results demonstrate that research aimed at developing new prevention and control strategies should target this species of CCB. ARS scientists in Auburn, Alabama fed channel catfish with different dietary amounts of frass, a by-product of the black soldier fly larval meal industry, and examined the expression of growth and immune-related genes. The results demonstrated that a series of metabolic and immune-related genes were differentially regulated after being fed either a low- or high-frass diet for 10 weeks. This data supports a beneficial role of frass when included into catfish feed and potential to boost the catfish immune system and enhance disease resistance. Auburn University partners in collaboration with ARS scientists in Auburn, Alabama, assessed the effects of bacterial coinfections of Flavobacterium covae and Aeromonas hydrophila in juvenile channel catfish. The results demonstrated an additive effect on mortality (i.e., increased mortality) when these two pathogens were used to infect catfish simultaneously, thus illustrating a negative impact of coinfections for the industry. Reducing disease outbreaks in catfish farming is critical to enhancing production yields and quality products, and an increased understanding of coinfection dynamics will provide more insight into targeted control measures for catfish health. Auburn University partners in collaboration with ARS scientists in Auburn, Alabama conducted a six-month, replicated, whole pond experiment at a farm in west Alabama where catfish production ponds were not or were treated with 500 mg/L gypsum (calcium sulfate). The results showed large increases in soluble reactive phosphorus (7x) and cyanobacteria (3.5x) after gypsum addition, which was unexpected. The “new” phosphorus appears to be released from the sediment once gypsum is applied. Although gypsum is a common additive to ponds to elevate water hardness, the negative impacts of gypsum on water quality could lead to algal blooms and fish death.

Accomplishments

U.S. DEPARTMENT OF AGRICULTURE

Aquatic Animal Health Research: Auburn, AL