Introduction

The hybrid striped bass (Morone saxatilis) x (Morone chrysops) is becoming an increasingly popular aquaculture species in the southeastern U.S. because of the high market value, declining natural stocks of striped bass, fast growth rate and the ability to over winter in ponds. Louisiana and Mississippi currently have commercial scale culture facilities as well as pilot scale facilities in operation with plans for expansion of production in the near future. Methods for the culture of hybrid striped bass vary from semi-intensive pond culture to highly intensive recirculating systems. The fish is adaptable to culture in fresh water as well as salinities up to 25 ppt. Although a great deal of information has been published on the diseases of striped bass little is known about the disease susceptibility of hybrid striped bass under the stressful conditions of high density aquaculture.

The Louisiana Aquatic Animal Diagnostic Laboratory (LAADL) of the School of Veterinary Medicine, Louisiana State University, received 235 hybrid striped bass cases from January, 1988 through December, 1990. Bacterial disease was found to be the cause of mortality 98 cases (41.7%). Non-hemolytic Group B. type Ib Streptococcus sp. was the primary etiological agent in 10.2% of the bacterial cases and was isolated from hybrid striped bass cultured in cages in salt water. Enterococcus sp. was the primary agent in 14.2% of the bacterial cases and was isolated from hybrid striped bass cultured in semi-closed systems in fresh water.

Streptococcosis has been described as one of the most economically important diseases of fish in Japanese mariculture and excellent reviews have been published by Bullock, 1981 and Austin and Austin, 1987. Non-hemolytic Group B type Ib Streptococcus sp. infections in fish were first reported from marine fish populations along the U.S. gulf coast by Plumb et al. 1972 and except for a subsequent outbreak in Lake Pontchartrain in Louisiana in 1979, no additional natural fish kills have been documented. Group B Streptococcus sp. has recently been reported as causing disease in wild populations of striped bass in Chesapeake Bay and its tributaries (Baya et al. 1990). Group B Streptococcus sp. has also been described as a pathogen of cultured golden shiners (Notemigonus crysoleucas) in fresh water Robinson and Meyer, 1966. There are no records of this streptococcus causing disease in any other cultured species of fish in the U.S. other than in experimentally cultured (Fundulus grandis) Rasheed, 1983. There is also no record of fish disease caused by Enterococcus sp. from the U.S. however some Japanese strains are biochemically similar to E. faecalis and E. faecium (Kitao, 1982).

Necropsy and Bacteriology Techniques

Moribund specimens from the culture facilities were placed on ice voluntarily submitted to the LAADL for necropsy. Samples of trunk kidney, liver, spleen and eye were aseptically removed and streaked on brain heart infusion (BHI) agar with 2% NaCl or blood agar (BA) 5% bovine and fluid thioglycollate. Imprints of tissues were stained using the gram's method. Selected tissues were examined microscopically for parasitic organisms. Tissues were fixed for histopathology in Bouin's solution or 10% neutral buffered formalin.

Bacteria were identified using the API Rapid Strep system and standard tube methods. Basal media contained 1% NaCl to insure proper growth of the marine streptococcus isolates. Serological typing was done by commercially available kits and confirmed by the streptococcus laboratory of the Centers for Disease Control, Atlanta, Georgia, Dr. Richard Facklam director. Antibiotic susceptibility was determined on pure cultures of the strains by the disc diffusion method on Blood Mueller-Hinton agar using commercially available paper discs.

Description of Group B Streptococcal Infections

Hybrid striped bass were cultured at two separate commercial facilities in the salt marsh of coastal Louisiana in either cages or flow through raceways. Water quality conditions were basically identical at the two locations and in each case the fish were reared in natural estuarine water. Water temperature ranged from 28-32°C and salinities from 11-14 ppt. Dissolved oxygen, unionized ammonia, nitrites and other parameters were all within acceptable limits. Mortalities occurred primarily during the months of August and September and multiple specimen submissions occurred during this period.

Moribund hybrid striped bass submitted to the LAADL exhibited a variety of external clinical signs which included exophthalmia, corneal opacity, hemorrhage in the eye and shallow hemorrhagic lesions in the skin. Internally the liver was pale or mottled and the spleen was dark red and enlarged The intestine occasionally contained a bloody fluid but otherwise was empty. Microscopic analysis of sections revealed panophthalmitis, multifocal necrosis of the trunk kidney, liver and spleen and acute peritonitis with multifocal inflammatory infiltrates.

Gram positive cocci were observed in infected tissue and essentially pure cultures were obtained from the liver, kidney, spleen and eye. The bacterial isolates were found to be sensitive to Terramycin and moderately sensitive to Romet (Table 1). Floating pelleted feed medicated with Terramycin was fed at the rate of 80 mg/kg fish/day for ten days pending approval from the Center for Veterinary Medicine (CVM) USFDA.

Due to early diagnosis and treatment, losses were kept to a minimum at both facilities. At one facility the disease was diagnosed in 5 of 14 cages and approximately 6% of the marketable size fish were lost. In one cage where the infection was more advanced and the feeding rate of the fish reduced, approximately 47% of the population was lost. Some of the physical and biochemical characteristics of the Group B streptococcus are listed in Table 2.

Description of Enterococcus sp. Infections

Hybrid striped bass were cultured at an individual facility with multiple culture units utilizing semi-closed intensive fresh water culture systems. Water temperatures ranged from 26-28°C during periods of mortality. Dissolved oxygen was maintained at saturation and other water quality parameters were closely monitored and within acceptable limits.

Mortalities were chronic in nature with greatest losses occurring after partial harvesting or grading. Tilapia (Sarotherodon mossambicus) reared at the facility were also susceptible to infection. Moribund fish submitted for analysis possessed clinical signs very similar to Group B streptococcal infections. Bilateral exophthalmia, corneal opacity, and extensive hemorrhage in the skin were common external clinical signs. Affected fish were found swimming listlessly near the surface and exhibited lighter or darker coloration than healthy individuals. Internal lesions were also similar to those in fish infected with Group B Streptococcus sp. Microscopic lesions included multifocal necrosis of the kidney, liver and spleen. Panophthalmitis and mild to moderate diffuse pericarditis was observed in some specimens. Gram positive cocci were observed in stained imprints of infected tissue and pure cultures were obtained from internal organs and eyes of infected fish.

The Enterococcus sp. was resistant to most antibiotics tested and only Erythromycin and Ampicillin were considered for treatment (Table 1), Erythromycin was chosen because of greater availability. Special permission was granted for its use by the CVM, USFDA. Erythromycin was fed at a rate of 25 mg/kg fish/day for ten days and was apparently effective in controlling infections. The physical and biochemical characteristics of Enterococcus sp. are listed in Table 2.

Table 1. Antibiotic Susceptibility of Enterococcus sp. and Group B Streptococcus sp.

Table 2. Physical and Biochemical Characteristics of Enterococcus sp. and Group B Streptococcus sp.

References

1.  Austin, B. and D. A. Austin. 1987. Gram positive cocci, Chapter 8 pages 97-110 in Bacterial Fish Pathogens: Disease in Farmed and Wild Fish. Ellis Horwood Ltd. publishers, Chichester, England.

2.  Baya, A. M., B. Lupiani, F. M. Hetrick, B. S. Roberson, R. Lukacovic, E. May and C. Poukish. 1990. Association of Streptococcus sp. with fish mortalities in the Chesapeake Bay and its tributaries. Journal of Fish Diseases 13: 251-253.

3.  Bullock, G. L. 1981. Streptococcal infections of fishes. United States Department of the Interior, Fish and Wildlife Service, Fish Disease Leaflet 63:1-7.

4.  Kitao, T. 1982. The methods for detection of Streptococcus sp. causative bacteria of streptococcal disease of cultured yellowtail (Seriola quinquiradiata). Fish Pathology:17-26

5.  Plumb, J. A., J. H. Schachte, J. L. Gaines, W. Peltier and B. Carroll. 1974. Streptococcus sp. from marine fishes long the Alabama and northwest Florida coast of the Gulf of Mexico. Transactions of the American Fisheries Society. 103(2): 358-361.

6.  Rasheed, V. M. 1983 Streptococcus sp. infection in bullminnows (Fundulus grandis). PhD. dissertation. Department of Fisheries and Allied Aquacultures, Auburn University, Auburn, Alabama. 71 pp.

7.  Robinson, J. A. and F. P. Meyer. 1966. Streptococcal fish pathogen. Journal of Bacteriology. 92(2):512.