Efficacy of Sarafloxacin Hydrochloride in Treating Naturally-Induced Edwardsiella ictaluri Infections in Channel Catfish,Ictalurus punctatus

Sarafin (sarafloxacin hydrochloride), a new antibacterial, was evaluated in the field on a naturally induced infection of Edwardsiella ictaluri in channel catfish, -Ictalurus punctatus. Healthy channel catfish (mean weight = 50 g) were stocked into nine cages at 200 fish per cage in a pond with an undergoing E. ictaluri infection. Seven days after stocking, dead fish were observed in the cages with clinical signs of enteric septicemia of channel catfish (ESC). After E. ictaluri was confirmed through isolative biochemical tests, medicated feed was applied for five consecutive days. During this period, fish in three control cages received a commercial 32% protein floating feed, three other cages of fish served as positive controls and were fed Romet, and three cages received the test feed with Sarafin. Both medicated feeds reduced the increase in cumulative percent mortality. In the control cages, cumulative percent mortalities continued to increase throughout the study period. Average daily mortality rates were significantly lower following both treatments of medicated feed, and treatments receiving Sarafin showed the greatest reduction in average daily mortality rates. Average daily mortality rates in the control did not change after the medicated feeding period. Toward the end of the study, temperatures reached 30°C, above the active range of ESC infections, and all mortalities ceased.     

Efficacy of a Live‐attenuated Edwardsiella ictaluri Oral Vaccine in Channel and Hybrid Catfish

This study evaluated the efficacy of an oral live‐attenuated Edwardsiella ictaluri vaccine against enteric septicemia of catfish (ESC) in channel and hybrid catfish. The vaccine was delivered one time orally by feeding fish a diet coated with an attenuated E. ictaluri isolate at four doses to deliver between 4 × 10⁶ to 3.2 × 10⁷ viable vaccine cells/g wet feed. Thirty‐five days postvaccination, control and vaccinated fish were challenged with virulent E. ictaluri and mortality was examined for 30 d postchallenge. Mortality of nonvaccinated hybrids (85%) and nonvaccinated channel catfish (73%) was similar but significantly greater than all groups of vaccinated fish. In channel catfish, a trend toward increasing mortality with decreasing dose was observed. Mortality of channel catfish vaccinated with the lowest dose (26.6%) was significantly higher than fish vaccinated with the highest dose (14.1%) but similar to fish vaccinated at the intermediate doses (17.5 and 19.4%). In contrast, mortality of four doses of vaccinated hybrid catfish was similar and ranged between 10.4 and 14.0%. The data demonstrate that the attenuated E. ictaluri vaccine at all four doses tested is effective at reducing ESC‐related mortalities in hybrid and channel catfish.     

Studies on Vaccination of Channel Catfish,Ictalurus punctatus,Against Edwardsiella ictaluri

Enteric septicemia of cattish (ESC), caused by the bacterium Edwardsiella ictaluri, has become the most significant disease problem affecting the commercial channel catfish, Ictalurus punctatus, industry in the United States. Although antibiotics are used extensively for the control of ESC, there are inherent problems associated with their use. Consequently, experiments were initiated to evaluate the effectiveness of vaccination program that used immersion and oral delivery methods to administer a killed E. ictaluri vaccine to fry and fingerling channel cafish. In a preliminary pond study with laboratory challenge, mortality in a group vaccinated with a combination of immersion and oral procedures was only 5.0% in both high- and low-dose challenges. This was significantly different (P c 0.01) from non-vaccinated controls, which had 46.7%mortality in the lowdose challenge and the 6 1.7% mortality in the highdose challenge. This corresponds to relative percent survival (RPS) values of 89.3 and 91.9 respectively. Subsequent field trials further indicated the efficacy of a vaccination program for the prevention of ESC in channel catfish. In 1987-1988, a field study was conducted using 12 commercial ponds, with three replicates of four treatments. The four treatments included vaccination by immersion only, oral only, a combination of both immersion and oral procedures, and non-vaccinated conwols. Relative percent survival was 57.4 for the immersion only treatment, 50.3 for the oral only treatment, and 53.5 for the combination immersion and oral treatment. In 1989-1990, no significant difference was found between vaccinated and non-vaccinated fish. However, in 1989-1990, a vaccine-oil emulsion was topcoated on a floating feed, rather than incorporating vaccine in a sinking pellet. In 1990-1991, overall mortality in vaccinated fish was significantly less (P < 0.05) than non-vaccinated fish, with 41.2% mortality in vaccinates compared to 63.5% in non-vaccinated fish, for an RPS of 35.1. In examining RPS values for individual farms, two farms had excellent results, with RPS values of 81.3 and 76.9; two farms had only moderate success, with RPS values of 26.6 and 15.4; and one location had greater mortality in the vaccinated fish than in the non-vaccinated fish. However, that farm had only two ponds in the study and experienced significant losses to proliferative gill disease in the pond with vaccinated fish.     

A Non-Nutritive,Ambient Temperature-Processed Floating Pellet as a Carrier of Antibiotics for Channel Catfish

A non-nutritive, ambient temperature-processed floating pellet (ATFP) was used as a medium to deliver apramycin (Eli Lilly and Co., Greenfield, Indiana), 52% active, and oxytetracycline hydrochloride (Pfizer, Inc., Lee's Summit, Missouri) (91% active) to channel catfish, Ictalurus punctatus, experimentally infected with Edwardsiella ictaluri. The drugs were fed at 50 mg of active antibiotic per kilogram of fish per day. Two medicated feeding trials were run for 10 consecutive days in troughs with channel catfish, beginning 24 hours after exposure to E. ictaluri. Mortality in the non-infected control population in Trial 1 was 7.6%, significantly (P < 0.05) lower than the 88.3% of the infected, non-medicated fish. Fish fed diets containing apramycin experienced 62.5% mortality, while fish fed diets containing oxytetracycline hydrochloride had 63.3% mortality. Mortality of the two medicated groups were significantly (P < 0.05) different from either control group. In Trial II, the 23% mortality in the non-infected group was significantly lower than the 48% mortality in the infected, non-medicated group and the 41% mortality in the oxytetricycline hydrochloride-medicated group, but not different than fish fed diets containing apramycin (28%). Mortality of the two medicated groups in Trial II were significantly different from each other. The presence of thc apramycin and oxytetracycline hydrochloride in the ATFP was demonstrated by the antibiotic diffusion method on agar plates swabbed with E. ictaluri.     

Determination of Dose Rate of Florfenicol in Feed for Control of Mortality in Channel Catfish Ictalurus punctatus (Rafinesque) Infected with Edwardsiella ictaluri, Etiological Agent of Enteric Septicemia

–A dose titration study was conducted to determine the appropriate dosage of florfenicol in feed to control mortality in channel catfish Ictalurus punctatus associated with enteric septicemia of catfish caused by Edwardsiella ictaluri. Six tanks (20 fish/ tank) were assigned to each of the following treatment: 1) not challenged with E. ictaluri and fed unmedicated feed; 2) challenged with E. ictaluri and fed unmedicated feed; 3) challenged and fed 5-mg florfenicol/kg body weight (kg bw); 4) challenged and fed 10-mg florfenicol/kg bw; or 5) challenged and fed 15-mg florfenicol/kg bw. Treatment was initiated the day after inoculation, and feed was administered by hand at 2.5% body weight for 10 consecutive days. Feeding activity was scored for all groups and was noted to be significantly less than the challenged, unmedicated group. Cumulative mortality in the challenged untreated group was 60%. The mortality in the unchallenged untreated group was 0%, and in die 5-, 10-, 15-mg florfenicol/kg bw group was 2.5%, 0.8%, and 2.5%, respectively. The mortality in each challenged, treated group and the non-challenged control group was significantly less than the challenged, unmedicated controls (P < 0.0001 for each contrast). There were no pairwise statistically significant contrasts among the florfenicol treated groups and the non-challenged control group. All 600 fish in the study were necropsied, cultured for bacteria, and examined by gross pathology. No specific lesions that could be associated with the antibiotic were observed. The efficacy of the 10 mg/kg dosage was confirmed in a separate dose confirmation study. In this study, fish in 30 tanks (20 fish/ tank) were infected with E. icraluri by immersion. Two days post-inoculation, fish in 15 tanks were hand-fed unmedicated feed, and 15 tanks were hand-fed medicated feed at a dosage of 10-mg florfenicol/kg bw at 2.5% body weight for 10 d. Feeding activity was scored and was noted to be significantly less than the challenged, unmedicated group. Cumulative mortality in the florfenicol group (14%) was significantly less than cumulative mortality in the untreated group (87.3%) (P < 0.0001). All 600 fish were submitted for bacterial culture, necropsied. and examined for gross pathology, and once again, no specific lesions that could be associated with the antibiotic were observed. The minimum inhibitory concentration of florfenicol against E. ictaluri in both studies was 0.25 ug/mL. Florfenicol was palatable, safe, and efficacious for control of mortality due to infection by E. ictaluri in catfish.     

Effects of Co‐stocking Smallmouth Buffalo,Ictiobus bubalus,with Channel Catfish,Ictalurus punctatus

Proliferative gill disease (PGD) in catfish is caused by the myxozoan Henneguya ictaluri. The complex life cycle requires Dero digitata as the oligochaete host. Efforts to control PGD by eradicating D. digitata have been unsuccessful. Smallmouth buffalo, Ictiobus bubalus, (SMB) are opportunistic bottom feeders and a putative option for controlling D. digitata. In 2011, 15 ponds (0.4 ha) were stocked with 5000 channel catfish, Ictalurus punctatus; 7 of these 15 ponds were also stocked with 300 SMB fingerlings. There were no differences in benthic invertebrate numbers or water quality variables between ponds with or without SMB. At harvest, there were no differences in percent survival, total weight, or catfish feed conversion ratio. In the second year, 18 ponds (0.4 ha) were stocked with 6000 channel catfish. Half the ponds were also stocked with 300 SMB. Sentinel fish were used to estimate disease severity, and pond water was collected for molecular estimation of H. ictaluri actinospore concentrations. Similar to the first year, there were no differences between treatments in any variable tested, including PGD severity in sentinel fish and parasite concentrations in pond water. Under these study conditions, presence of SMB did not have a measureable effect on PGD incidence, parasite density, or overall catfish production.     

Vaccination of Full‐sib Channel Catfish Families Against Enteric Septicemia of Catfish with an Oral Live Attenuated Edwardsiella ictaluri Vaccine

This study evaluated the efficacy of an oral live attenuated Edwardsiella ictaluri vaccine against enteric septicemia of catfish in 20 full‐sib fingerling channel catfish families. The vaccine was delivered orally by feeding fish a diet coated with an attenuated E. ictaluri isolate. Sixty‐nine days postvaccination, control and vaccinated fish were challenged with virulent E. ictaluri and mortality was examined for 21 d postchallenge. Vaccinated fish had significantly lower mortality than nonvaccinated fish following challenge (P < 0.001). Mortality of vaccinated fish was 1.7 ± 1.4% as opposed to 47.8 ± 28.7% in nonvaccinated fish. Relative percent survival ranged from 87.7 to 100% with an average of 95.2 ± 4.0% (±SE) among the 20 families of fish. There were significant differences in mortality among families in nonvaccinated fish (P < 0.01) while there were no differences among vaccinated families of fish. Results indicate that the live attenuated E. ictaluri vaccine is effective at reducing mortality in channel catfish exposed to virulent E. ictaluri. These data demonstrate that genetic differences among healthy families of channel catfish are not major considerations in developing an effective vaccination program utilizing the oral vaccination platform described in this study.     

Mortality and pathology of hybrid catfish,Clarias macrocephalus (Günther) × Clarias gariepinus (Burchell), associated with Edwardsiella ictaluri infection in southern Thailand

Enteric septicaemia of catfish (ESC) caused by Edwardsiella ictaluri is becoming an increasing problem in aquaculture and has been reported worldwide in a variety of fish species. This study reports ESC in hybrid catfish, Clarias macrocephalus (Günther) × Clarias gariepinus (Burchell), cultured in southern Thailand. The bacteria were identified as E. ictaluri by conventional and rapid identification systems, as well as by genetic and phylogenetic characterization. Analysis of 16S rRNA indicated 100% homology to the 16S rRNA sequence of several E. ictaluri strains in GenBank. Plasmid profiles demonstrated 4.0‐ and 5.6‐kb plasmids, compared with the 4.8‐ and 5.6‐kb plasmids in the US isolates, and representative genes of three of the four known pathogenicity islands of US isolates were present. Serologically, lipopolysaccharide (LPS) purified from the Thai isolates was not recognized by a monoclonal antibody against the LPS of US isolates. Fish experimentally infected with E. ictaluri showed 23–100% mortality within 14 days with a 168‐h LD₅₀ of 6.92 × 10⁷ CFU mL^?1 by immersion and a 96‐h LD₅₀ of 1.58 × 10⁶ CFU fish^?1 by intraperitoneal injection. Examination of tissue sections obtained from both naturally and experimentally infected fish indicated that infection of hybrid catfish with E. ictaluri produced lesions in several organs including liver, kidney, spleen, heart and brain. Histopathology findings included cellular necrosis, focal haemorrhage, infiltration of lymphocytes and multifocal granulomatous inflammation in the infected organs.     

Establishing bacterial infectivity models in striped Catfish Pangasianodon hypophthalmus (Sauvage) with Edwardsiella ictaluri

A bacterial infectivity challenge model of Edwardsiella ictaluri in striped catfish was developed. All experiments were conducted using a bacterial isolate of E. ictaluri that had been recovered during a natural outbreak of bacillary necrosis of Pangasianodon (BNP) in farmed striped catfish Pangasianodon hypophthalmus in Vietnam. Time of immersion in 10⁷ CFU.ml⁻¹ had a significant effect on mortality. The immersion bacterial dose of 10⁷ CFU/ml for 30 s resulted in a cumulative percentage mortality of 63%. Three to four days post-bacterial challenge, fish showed gross clinical signs of natural BNP and E. ictaluri was recovered and identified from these fish. Moreover, a cohabitation challenge was evaluated as an alternative challenge method, although the mortalities among the infected fish were lower at around 15%–40%. This study confirmed the horizontal transmission of E. ictaluri in striped catfish and elucidated that cohabitation challenge could be used in reproducing the disease under controlled conditions.     

Pond‐raised Hybrid Catfish, ♀ Ictalurus punctatus × ♂ Ictalurus furcatus,Do Not Respond to Microbial Phytase Superdosing

Two experiments were conducted in consecutive years to evaluate the responses of hybrid catfish, ♀ Ictalurus punctatus × ♂ Ictalurus furcatus, to “superdosing” of 6‐phytase added to existing commercial catfish feeds. In each experiment, two diets with or without a phytase superdose (2500 and 5000 phytase units/kg, respectively) were compared. In Experiment 1, fingerlings (mean weight: 59 g/fish) were stocked in 17 0.4‐ha earthen ponds at 17,290 fish/ha and were fed once daily to apparent satiation for 198 d. In Experiment 2, fingerlings (mean weight: 47 g/fish) were stocked in 10 0.4‐ha ponds at 24,710 fish/ha and were fed for 128 d. In both experiments, there were no significant differences in total feed fed, gross yield, final fish weight, survival, or Blood packed cell volume between fish fed diets with or without phytase. The diets also had no significant effects on pond water column total phosphorus or chlorophyll a concentrations, but soluble reactive phosphorus concentrations were significantly higher in ponds receiving the phytase diet in Experiment 2. Phytase superdosing of nutritionally complete feeds does not appear to have additional benefits beyond the standard phytase dose on production characteristics or packed cell volume of pond‐raised hybrid catfish and had no beneficial effects on water quality.     

Environmental Assessment of Channel Catfish Ictalurus punctatus Farming in Alabama

An environmental assessment was made of Alabama channel catfish Ictalurus punctatus farming which is concentrated in the west‐central region of the state. There are about 10,000 ha of production ponds with 10.7% of the area for fry and fingerlings and 89.3% for food fish. Food fish production was about 40,800 tons in 1997. Watershed ponds filled by rainfall and runoff make up 76% of total pond area. Water levels in many of these ponds are maintained in dry weather with well water. The other ponds are embankment ponds supplied by well water. Harvest is primarily by seine‐through procedures and ponds are not drained frequently. The main points related to Alabama catfish farming and environment issues are as follows: 1) catfish farming in Alabama is conservative of water, and excluding storm overflow, about two pond volumes are intentionally discharged from each pond in 15 yr; 2) overflow from ponds following rains occurs mostly in winter and early spring when pond water quality is good and stream discharge volume is high; 3) total suspended solids concentrations in pond effluents were high, and the main sources of total suspended solids were erosion of embankments, pond bottoms, and discharge ditches; 4) concentrations of nitrogen and phosphorus in effluents were not high, but annual effluent loads of these two nutrients were greater than for typical row crops in Alabama; 5) ground water use by the industry is about 86,000 m³/d, but seepage from ponds returns water to aquifers; 6) there is little use of medicated feeds; 7) copper sulfate is used to control blue‐green algae and off‐flavor in ponds, but copper is rapidly lost from pond water; 8) although sodium chloride is applied to ponds to control nitrite toxicity, stream or ground water salinization has not resulted from this practice; 9) fertilizers are applied two or three times annually to fry and fingerling ponds and occasionally to grow‐out ponds; 10) hydrated lime is applied occasionally at 50 to 100 kg/ha but this does not cause high pH in pond waters or effluents; 11) accumulated sediment removed from pond bottoms is used to repair embankments and not discarded outside ponds; 12) sampling above and below catfish pond outfalls on eight streams revealed few differences in stream water quality; 13) electricity used for pumping water and mechanical aeration is only 0.90 kW h/kg of production; 14) each metric ton of fish meal used in feeds yields about 10 tons of dressed catfish. Reduction in effluent volume through water reuse and effluent treatment in settling basins or wetlands does not appear feasible on most farms. However, some management practices are recommended for reducing the volume and improving the quality of channel catfish pond effluents.     

A rapid bioassay for bactericides against the catfish pathogens Edwardsiella ictaluri and Flavobacterium columnare

The most common bacterial diseases in pond‐raised channel catfish Ictalurus punctatus (Rafinesque) are enteric septicemia of catfish and columnaris, caused by Edwardsiella ictaluri and Flavobacterium columnare respectively. Medicated feed containing antibiotics is one management approach that catfish producers use in the treatment of bacterial diseases. However, the future use of all types of medicated feed in catfish aquaculture is uncertain. To discover effective alternatives to antibiotics, a rapid 96‐well microplate bioassay utilizing E. ictaluri and F. columnare to evaluate natural compounds and extracts was developed. In this bioassay, bacterial growth is determined by absorbance measurements of microplate wells after 24 h incubation and then confirmed by detecting cell viability after the addition of 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyl tetrazolium bromide with additional incubation for 24 h. The minimum inhibitory concentration, minimum bactericidal concentration and 50% inhibition concentration (IC50) are determined by graphing the absorbance data. The 24 h IC50 results of test compounds are compared with the 24 h IC50 results of the drug controls oxytetracycline and florfenicol. Among the antibiotics evaluated, doxycycline and tetracycline appear more effective against E. ictaluri and F. columnare than either drug control. This bioassay is rapid, reproducible and economical for evaluating a large number of compounds and extracts.     

Edwardsiella ictaluri infection in Pangasius catfish imported from West Bengal into the Southern Caribbean

In response to a mortality event, seven Pangasius catfish (Pangasianodon hypophthalmus) were submitted to the University of the West Indies, School of Veterinary Medicine, Trinidad and Tobago, for diagnostic evaluation. These fish were part of a consignment that arrived from Kolkata two weeks earlier. Fish presented with perianal haemorrhage and blister‐like swellings on the skin which ruptured to leave ulcers. Edwardsiella ictaluri was consistently recovered from the brain and skin. Repetitive sequence‐mediated PCR analysis revealed genetic fingerprints consistent with E. ictaluri isolates from farm‐raised channel catfish in Mississippi, USA. Plasmid analysis of the case isolates identified two unique plasmids that differ slightly in conformation and content from the pEI1 and pEI2 plasmids described for E. ictaluri from other fish hosts. The case isolates were also PCR negative for several E. ictaluri virulence factors. The biological implications of these genetic differences are unclear and warrant further study. This is the first report and documentation of E. ictaluri infection in Trinidad and Tobago, suggesting the pathogen may have been introduced concurrently with the importation of fish. This report emphasizes the importance of adequate health screenings of imported lots to minimize the threat of introducing E. ictaluri to non‐endemic areas.     

Response of Pond‐raised Fingerling Hybrid Catfish, ♀ Ictalurus punctatus × ♂ Ictalurus furcatus,to Dietary Protein Concentrations and Sources

This study examined four experimental diets with different protein concentrations and sources for pond‐raised fingerling hybrid catfish, ♀ Ictalurus punctatus × ♂ Ictalurus furcatus. A 35% protein diet with fishmeal was used as the control diet. Test diets were 32 and 28% all‐plant‐protein diets and a 28% protein diet with porcine meat, bone, and blood meal. Small fingerlings with a mean initial weight of 2.9 g/fish were stocked into 20 earthen ponds (0.04 ha) at a density of 172,970 fish/ha. They were fed once daily to apparent satiation for 107 d. No significant differences were observed for total diet fed, gross yield, final weight, survival, or condition factor among dietary treatments. However, fish fed the 28 and 32% all‐plant‐protein diets had a significantly higher feed conversion ratio than fish fed the 35% protein diet with fishmeal. There were no significant differences in chlorophyll a and nitrite concentrations in the pond water, but ponds receiving the 35% protein diet had significantly higher ammonia than those receiving 28% protein diets. Economic analysis suggested potential cost savings by using low‐protein and all‐plant‐protein diets for hybrid catfish fingerling production.     

Effect of Sublethal Hypoxia on the Immune Response and Susceptibility of Channel Catfish, Ictalurus punctatus, to Enteric Septicemia

The effect of sublethal hypoxia exposure on stress and immune responses and susceptibility to Edwardsiella ictaluri infection in juvenile channel catfish, Ictalurus punctatus, was investigated. Fish were monitored for temporal changes in glucose and cortisol concentrations before, during, and after 2 h exposure to sublethal hypoxia (<2 mg/L dissolved oxygen [DO]) and when maintained under normoxic conditions (6.0 ± 0.3 mg/L DO). Both blood glucose and plasma cortisol increased significantly in response to hypoxic conditions. Fish exposed to hypoxic or normoxic conditions were challenged with a high dose (1.3 × 10⁷ colony‐forming units [CFU]/mL) or a low dose (1.3 × 10⁵ CFU/mL) of E. ictaluri or sterile culture broth by 30‐min immersion bath. Approximately 1% of fish in both the normoxic and the hypoxic groups died when challenged with the low dose of E. ictaluri. However, when challenged with the high dose of E. ictaluri, catfish exposed to hypoxic conditions had significantly higher cumulative mortality (36 ± 12.1%) than those maintained under normoxic conditions (12 ± 1.1%). Total hemolytic complement and bactericidal activities and antibody response were lower in hypoxia‐exposed channel catfish, indicating that increased susceptibility of channel catfish to E. ictaluri may be the result of the immunosuppressive effects of the stress response to hypoxia.     

Costs and Risk of Catfish Split‐pond Systems

Split ponds are recently developed pond‐based aquaculture systems that allow intensification of catfish aquaculture. Successful industry‐wide adoption of newly developing technologies like split‐pond systems will depend upon their productivity and cost efficiencies. Costs and production performance of the following three split‐pond design scenarios were monitored in Arkansas and Mississippi: (1) research design developed at the Thad Cochran National Warmwater Aquaculture Center, Stoneville, Mississippi; (2) waterwheel design tested on commercial catfish ponds; and (3) screw‐pump design tested on commercial catfish ponds. An economic engineering approach using standard enterprise budget analysis was used to develop estimates of breakeven prices (BEPs) ($/kg) for producing foodsize hybrid catfish (♂Ictalurus furcatus × ♀Ictalurus punctatus) for each scenario. Estimates of BEPs of hybrid catfish raised in split ponds ranged from $1.72 to $2.05/kg. The cost of catfish production in split ponds was sensitive to yield, fish prices, and feed prices. Annual net cash flows from both commercial split‐pond systems were high and sufficient to make the investment profitable in the long run. Feed price, feed conversion ratio, and yield contributed the most to downside risk of split ponds.     

The Relationship Between Vaccine Dose and Efficacy in Channel Catfish Ictalurus punctatus Vaccinated as Fry with a Live Attenuated Strain of Edwardsiella ictaluri (RE‐33)1

Channel catfish Ictalurus punctatus were vaccinated at 12 d of age (post‐hatch) by a 2‐min bath immersion with attenuated Edwardsiella ictaluri RE‐33 at doses of 2.5 × 10⁵, 2.5 × 10⁶, and 2.4 × 10⁷ colony‐forming units CFU/mL of water. Following vaccination, RE‐33 was recovered from a greater percentage of fry that were vaccinated at the high and intermediate doses compared to fry vaccinated at the lowest dose. Independent of dose, the greatest percentage of RE‐33 positive fry occurred between 1 and 6 d post‐vaccination with a significant decrease in positive fry observed on day 12. A significant increase in mortality occurred 6 to 12 d post‐vaccination in fry vaccinated at the highest dose. No differences in post‐vaccination mortalities occurred between the other treatments. Following virulent E. ictaluri challenge, mortalities of fish vaccinated at doses of 2.5 × 10⁶ and 2.4 × 10⁷ CFU/mL were significantly less than those of fish vaccinated at 2.5 × 10⁵ CFU/mL and sham‐vaccinated control fish. These data show that vaccination with RE‐33 can offer protection against subsequent virulent E. ictaluri infection.     

Enhanced Disease Resistance to Enteric Speticemia in Channel Catfish,Ictalurus punctatus,Administered Lytic Peptide

Lytic peptides have been shown in vitro to be toxic to a wide range of fish bacterial pathogens, including Edwardsiella ictaluri, the causative agent for enteric septicemia. Fingerling channel catfish, Ictalurus punctatus, were challenged with an injection of Edwardsiella ictaluri, and lytic peptide was administered with a single intraperitoneal injection 24 hours later. In a second group, lyptic peptide was also administered by osmotic pump 7 days before the bacterial challenge. A range of peptide concentrations (20-400µg/g fish) was used. Mortalities were recorded for 7 days after injection of bacteria, and E. icraluri was isolated and quantified by standard plate counts from livers. Various tissues were examined for abnormalities due to bacteria and peptide treatment. Fish injected with lytic peptide at 400 µg/g had up to a 95% reduction in bacteria and a 65% reduction in mortality compared to complete mortality in bacteria-injected controls. Fish implanted with osmotic pumps had 99.3% reduction in bacterial counts and had 6% mortality. Tissue samples from infected fish showed changes typical of reaction to immunologic stimuli and infection, while control fish injected with saline or peptide had normal gills, spleens, and kidneys. This study shows the potential for enhancing channel catfish disease resistance to E. ictaluri and other diseases by transferring the gene coding for the lytic peptide into channel catfish.     

Isolation and characterization of Edwardsiella ictaluri strains as pathogens from diseased yellow catfish Pelteobagrus fulvidraco (Richardson) cultured in China

Yellow catfish Pelteobagrus fulvidraco (Richardson) is a commercially important fish generally distributed in Southeast Asian countries. The well‐known aetiological agent of enteric septicaemia of catfish, Edwardsiella ictaluri, was isolated from diseased yellow catfish P. fulvidraco (Richardson) reared at two commercial fisheries in China. The economic losses due to the high mortalities (about 50%) caused by this bacterium have been increasing annually. The affected fish presented two different, typical symptoms: pale gills, slight exophthalmia and a ‘hole in the head’, and haemorrhage on the opercula, in the skin under the jaw, creating a ‘hole under the jaw’. These diseases were found frequently in cultured yellow catfish throughout China. The isolates from both outbreaks were all Gram negative, facultatively anaerobic and short rod. Morphological and biochemical tests and phylogenetic analysis based on the 16S rDNA sequences all strongly indicated that these yellow catfish isolates were highly identical to the known E. ictaluri. In addition, the isolates possessed the typical plasmid profile of E. ictaluri. Experimental infection assays were conducted and pathogenicity (by an intraperitoneal injection) was demonstrated in yellow catfish and channel catfish Ictalurus punctatus. The results showed that yellow catfish isolates were quite conservative phenotypically and genetically, and were able to cause two different, typical symptoms in this fish under unknown conditions and mechanism.     

Evaluation of a Barrier Confinement System for Channel Catfish,Ictalurus punctatus,Production

An in‐pond confinement system to separate channel catfish, Ictalurus punctatus, by size within a single pond provides an opportunity for improved growth of understocked fish in ponds with larger market‐sized fish. A barrier of polyvinyl chloride–coated galvanized wire mesh was constructed in five 0.10‐ha earthen ponds to partition the pond into one‐third and two‐third sections, while five other 0.10‐ha ponds were left as traditional open ponds for a control. To evaluate catfish performance in this confinement system, fingerlings (25 g) were stocked at 14,820/ha into the smaller one‐third section of the barrier and carryover fish (408 g) at 2580 kg/ha into the larger two‐third section of the barrier. The control ponds were stocked with the same sizes and numbers of fish in a traditional earthen pond without a barrier. Yield, survival, feed conversion ratio (FCR), growth, and economics were compared between treatments. Fingerling yields were greater in the barrier system that allowed fingerlings to be separated physically from larger carryover fish. There were no differences in yield of carryover fish, survival, FCR, or growth between the control and the barrier ponds. Partial budget analysis revealed a positive net change of $367/ha or $38,125 for a 104‐ha catfish farm (at a market price of $1.54/kg of additional stockers produced). The value of the greater weight of understocked fish produced in the barrier system was greater than the annualized cost of installing the barrier, for farmers raising fish in multiple batch. Thus, on an experimental basis, the confinement system was economically profitable; however, trials on commercial farms are needed to evaluate performance on a larger scale.