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United States Department of Agriculture

Agricultural Research Service

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Location: Warmwater Aquaculture Research Unit

2011 Annual Report

1a. Objectives (from AD-416)
Objective 1: Monitoring and surveillance of biotoxin, pathogen, pathogen/host interactions in aquatic environment. Objective 2: Develop methods to protect channel catfish and its hybrids with vaccines, antibiotics and other therapeutics against enteric septicemia of catfish, columnaris, proliferative gill disease, botulism, Bolbophorus and anemia. Objective 3: Effects of chemical and mycotoxin feed contaminants on growth and disease resistance of catfish.

1b. Approach (from AD-416)
Despite successful growth and prosperity of the past few decades, the U.S. catfish industry is threatened by increasing disease losses, low fish prices, high feed costs and foreign competition. Fish losses due to disease are estimated to cost the U.S. catfish industry $100 million in direct sales annually and are considered the largest impediment to increasing production efficiencies. Objective 1 will provide catfish farmers a better method to monitor biotoxins, pathogens, and pathogen/host interaction in the aquatic environment using new and better surveillance and monitoring procedures coupled with the development of experimental vaccines. Case submissions will also document the prevalence and the emergence of new diseases in the catfish industry. Objective 2 will develop new methods to protect catfish against known disease organisms including antibiotics and vaccines and evaluate the effectiveness of these products to improve disease resistance. New and improved on-farm management programs for the control of trematode infections will be developed. Objective 3 will investigate the prevalence of chemical and mycotoxin feed contaminants in fish feeds and develop methods to detect and control feed contaminants that affect disease resistance of catfish. The overall benefits of this project will be to improve fish health reduce losses due to infectious and non-infectious diseases. The reduction of losses to disease will make catfish farming a more profitable endeavor and increase the competitiveness of U.S. aquaculture.

3. Progress Report
Oral delivery of a newly developed Edwardsiella (E.) ictaluri vaccine was shown to protect fish against infection and significantly increase production efficiencies. A U.S. patent application was filed for the developed vaccine and method of delivery. Research demonstrated the combined use of vaccines, medicated feeds, and restricted feeding practices will greatly reduce the impact of bacterial infections channel catfish. Anticoccidal drugs were evaluated palatability and treatment the myxozoan parasite responsible for proliferative gill disease (PGD). Efficacy and pharmacokinetic studies lead to Federal Drug Administration approval of Aquaflor® medicated feed for treatment of columnaris infection in channel catfish. Amprolium was shown palatable at levels therapeutic in other fish. Treatments did not reduce initial infection levels but did reduce plasmodial development indicating the amprolium may be effective in breaking the parasite life cycle. Pond and tank trials indicate small mouth buffalo are effective in reducing the benthic oligocheate serving as the intermediate host of the myxozoan parasite causing proliferative gill disease. In a 2 year pond study, initial sampling showed a significant decrease in benthic oligocheates in ponds stocked with 300 Small Mouth Buffello (SMB) per acre, indicating SMB may be effective in reducing parasite loads in the pond environment. An experimental vaccine against botulinum type-E toxicosis, the cause of visceral toxicosis of catfish (VTC), has been acquired and is currently being evaluated in laboratory trials. The Aquatic Research & Diagnostic Laboratory published an annual report summarizing diagnostic cases submission, frequency of reported diseases, and identification of emerging diseases. The 2010 annual report has been posted on the National Warmwater Aquaculture Center website. Primer and probe combinations have been developed for specific amplification for Edwardsiella ictaluri, Flavobacterium columnare, Bolbophorus damnificus, Edwardsiella tarda and Henneguya ictaluri. Assays for H. ictaluri, B. damnificus and E. ictaluri have been validated and are being used to evaluate pathogen levels in fish tissue and environmental samples. Surface Plasmon Resonance (SPR), an immunologically based portable assay, has been designed for the detection of E. ictaluri in pond water. Validation trials are in progress. Corn naturally contaminated with Deoxynivalenol (DON) toxin has been purchased and used to prepare toxin contaminated feeds. Fish are currently being acclimated in aquaria and will be fed experimental diets following the acclimation period. These studies will be used to establish tolerance levels for mycotoxin contaminated corn suitable for use in channel catfish feeds. Several commercially formulated catfish diets was shown to cause severe anemia in catfish. Diet are being evaluated for chemical and mycotoxin contamination.

4. Accomplishments
1. Enteric septicemia of catfish (ESC) is the most significant bacterial disease of cultured channel catfish. The exact economic impact of this disease is unknown but is estimated to cost the industry 30 million dollars per year. Aside from morbidity and mortality resulting from E. ictaluri infection, the disease has an indirect effect on growth due to the implementation of restricted feeding practices designed to prevent or slow infection rates. A new vaccine and an “in-pond" vaccine delivery platform has been developed resulting in exceptional protection against ESC. Based on experimental pond studies conducted under conditions similar to the commercial production setting, vaccination increased survival, fish size, and feed consumption while decreasing the feed conversion ratio by approximately 40.4%. This improved feed conversion ratio dramatically decreased feed costs which is the largest variable cost of production. At a given feed cost of $350/ton, the cost of feed to produce 1 lb of vaccinated fish was $0.23 compared to a feed cost of $0.36 to produce the same weight of non-vaccinated fish. Improved production efficiencies in vaccinated fish increased gross sales from $1,294 to $3,726 acre, representing a 187.9% increase. On a commercial scale, vaccination could increase gross sales to over $6,000/acre.

2. Proliferative gill disease (PGD), or hamburger gill, is the most prevalent parasitic disease associated with the commercial production of channel catfish. The complex parasitic life cycle involves a myxospore stage in the channel catfish and an actinospore stage in the ubiquitous benthic oligochaete Dero digitata. Prolonged exposure of catfish to this actinospore stage results in a severe inflammatory response at the gills with mortality rates approaching 100% in severe outbreaks. To aide in management and diagnostics a quantitative molecular assay has been validated for the dectection and quantification of actinospore levels in pond water. Actinospore levels have been correlated to disease severity and the assay is currently being used to predict production losses in newly stocked fish. The assay has also been used as a supplemental diagnostic tool to verify the etiology of production losses when traditional diagnostic techniques prove inconclusive. In efforts to break the parasite life cycle and minimize production losses associated with PGD the antiparasitic drug amprolium was shown to reduce parasite levels in the fish host 60 days post-infection. In addition smallmouth buffalo were shown to reduce populations of benthic oligocheates in pond sediments reducing the source of the actinospore stage which is infective to fish. These approaches are being evaluated as potential management strategies for the control of PGD on commercial farms. To aid in on-going disease monitoring programs we have developed quantitative polyerase chain reaction (QPCR) protocols for the detection of primary bacterial pathogens (Edwardsiella ictaluri, Edwardsiella tarda, Flavobacterium columnare and Aeromonas hydrophyla) and for two species of trematodes (Bolbophorus damnificus type-1, Bolbophorus damnificus type-2).

3. The National Warmwater Aquaculture Center’s Aquatic Research and Diagnostic Laboratory (ARDL), provides a comprehensive disease diagnostic service to catfish producers. The ARDL received a total of 523 fish diagnostic cases from 52 commercial farms and provided water quality analysis for 1,341 samples in 2010. A highly virulent strain of Aeromonas hydrophila has been identified as the cause of significant losses in Alabama and East Mississippi. As a result, disease monitoring and biosecurity measures have been implemented in collaboration with Arkansas and Alabama to track and prevent the spread of this disease. To date the disease has been primarily limited to west Alabama and east Mississippi, although an isolated case was reported in Arkansas. A video conference, broadcast at 5 regional locations, was conducted to inform producers about disease management options and the implementation of biosecurity measures. Anemia was diagnosed as the single cause of mortality and morbidity in 19 case submissions. The disease primarily affects high value production size fish and is a primary cause of decreased production efficiencies in harvestable size fish. Oral supplementation with iron dextran and ferrous sulfate were shown to significantly elevate pack cell volumes of anemic fish in laboratory studies. As a result, ferrous sulfate concentrations have been elevated in commercial diets on a limited bases as supportive therapy for this disease. A large amphistome type cercariae, collected from rams horn snails was shown to in infect channel catfish with developing metacercaria localized in the vessels of the branchial arches. Molecular analysis identified this trematode as Drepanocephalus spathans which was shown to cause significant gill pathology adversely affecting gill function.

Review Publications
Griffin, M.J., Wise, D.J., Yost, M.C., Doffitt, C.M., Pote, L.M., Greenway, T.E., Khoo, L.H. 2010. A multiplex real-time polymerase chain reaction assay differentiates between Bolbphorus damnificus and Bolbophorus type II sp. Journal of Veterinary Diagnostic Investigation. 22:615-622.

Gaunt, P., Endris, R., Mcginnis, A., Baumgartner, W., Camus, A., Steadman, J., Sweeney, D., Sun, F. 2010. Determination of florfenicol dose rate in feed for control of mortality in nile tilapia Oreochromis nilotica infected with streptococcus iniae. Journal of Aquatic Animal Health. 22:158-166.

Griffin, M.J., Goodwin, A.E. 2011. Thelohanellus toyamai infecting the gills of koi Cyprinus carpio in the eastern United States. Journal of Parasitology. 97(3):495-502.

Mischke, C.C., Griffin, M.J. 2011. Laboratory mass culture of the freshwater oligochaete Dero digitata. North American Journal of Aquaculture. 73:13-16.

Gaunt, P.S., Gao, D., Sun, F., Endris, R. 2010. Efficacy of florfenicol for control of mortality caused by Flavobacterium columnare infection in channel catfish Ictalurus punctatus. Journal of Aquatic Animal Health. 22:115-122.

Gaunt, P.S., Gao, D., Wills, R. 2011. Preparation of ormetoprim sulfadimethoxine medicated discs for disc diffusion assay. North American Journal of Aquaculture. 73:17-20.

Last Modified: 06/21/2017
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