Streptococcal infections cause significant morbidity and mortality in a number of economically important freshwater and marine fish. Susceptible food fish species include yellowtail (Seriola quinqueradiata), Japanese flounder, (Paralichthys olivaceus), and red drum, (Sciaenops ocellatus) as well as tilapia (Oreochromis spp.), hybrid striped bass (M. chrysops male x M. saxatilis female), and rainbow trout (Oncorhynchus mykiss) (Austin and Austin, 1999).
A number of ornamental fish have also been reported to be susceptible, including zebra danio (Danio rerio), clown loach (Chromobotia macracanthus), rosy barb (Puntius conchonius), tiger barb (Puntius tetrazona), rainbow shark (Epalzeorhynchos frenatum) and redtail black shark (Epalzeorhynchos bicolor) (Yanong, 1995; Russo, Mitchell, and Yanong, 2006; unpublished data). In ornamental species, streptococcosis can cause rapid morbidity and mortality in a population and disease appears to be linked to certain stressors, including higher temperatures, induced spawning, harvest, and poor water quality. In addition to moderate to heavy mortalities, clinical signs often include loss of appetite, darkening, aberrant swimming (spinning), hemorrhages, and exophthalmia (Yanong, 1995; Russo, Mitchell, and Yanong, 2006).
Husbandry changes to alleviate some stressors can help to some degree. Antibiotics that have been used successfully during outbreaks in rainbow and redtail black broodstock, when given by injection, include erythromycin and florfenicol. Inappetance often occurs rapidly in a population during a disease outbreak; consequently, orally administered medications have significant limitations. However, if the disease is caught early enough, use of orally administered antibiotics including erythromycin and amoxicillin may be effective in reducing mortalities.
Because of the rapidity of disease onset and mortalities, more proactive methods for prevention including use of autogenous vaccines and immunostimulants have been studied over the past several years at the University of Florida for use in Florida's industry (Russo, Yanong, and Mitchell, 2006). An autogenous vaccine administered by injection intracoelomically proved effective and practical for rainbow and redtail black shark broodstock. Use of nucleotides or beta-glucans in feed also was shown to enhance resistance of redtail black sharks to streptococcosis. Autogenous vaccines are currently being used by one facility and educational and extension efforts are underway to increase use of these preventative approaches in the industry as a whole.
Finally, additional experiments are currently in development to examine effectiveness and feasibility of other commercially available oral and bath vaccines, as well as other preventative methodologies.
1. Austin B, DA Austin. 1999. Bacterial Fish Pathogens: Disease of Farmed and Wild Fish. Praxis Publishing Ltd., Chichester, UK. 457 pp.
2. Russo R, H Mitchell, RPE Yanong. 2006. Characterization of Streptococcus iniae isolated from ornamental cyprinid fishes and development of challenge models. Aquaculture 256:105-110.
3. Russo R, RPE Yanong, H Mitchell. 2006. Dietary beta-glucans and nucleotides enhance resistance of red-tail black shark (Epalzeorhynchos bicolor, fam. Cyprinidae) to Streptococcus iniae infection. Journal of the World Aquaculture Society 37:298-306.
4. Yanong RPE. 1995. Streptococcal infections in ornamental fish: a review. Proceedings, 26th Annual International Association for Aquatic Animal Medicine Conference.