Bacterial diseases occur in cultured fish and are responsible for heavy stock mortality in Korea. Diseases such as edwardsiellosis, streptococosis and vibriosis currently prevalent in cultured eel, flounder, rockfish and sea bass.
Edwardsiellosis due to Edwardsiella tarda occurs in marine and freshwater fish, including tilapia, catfish, carp, eel, yellowtail, seabream and flounder. E. tarda is an economically important problem for flounder production in Korea. Edwardsiellosis frequently occurs at high water temperature and it is also associated with organic pollution.
Edwardsiellosis, caused by E. tarda, is a mild to severe condition, but clinical signs of infection differ slightly between species of fish. Eels with acute infection of E. tarda develop severe hyperaemia with bloody congestion of fins. In flounders, external gross lesions of edwardsiellosis are darkening, abdominal distention, prolapsed rectum, exophthalmia, lens opacities and abscesses around eyes, and internal gross lesions of edwardsiellosis are milky or bloody ascites, abscess formation (liver, spleen and kidney), peritonitis and pale coloration of liver. Pathologically, the liver, kidney and spleen are severely affected organs in flounder. In addition to the lesions in the musculature there may be involvement of the Liver, kidney and spleen. Macrophages are prominent in all lesions and neutrophils are seen in large numbers in abscesses. Bacteria are present in large numbers in all lesions except long-standing granulomas.
Therapy of edwardsiellosis is by oral application of drugs, such as oxytetracycline, in feeds of cultured fish. National Fisheries Research & Development Institute (NFRDI) scientists created a safe and effective formalin killed E. tarda vaccine that controls E. tarda infections in flounder. The vaccine can be administered by bath immersion.
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The typical signs of flounder infected with E. tarda.
A: A hernia and ascites, B: The arrow indicate granulomas at the liver.
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Vibriosis is one of the most prevalent fish disease caused by bacteria belonging to the genus Vibrio. Vibriosis is the most significant disease of cultured and wild marine fish, but outbreaks have infrequently been recorded in freshwater.
Vibriosis seem to frequently occurs from early summer to late fall and to be taken ill regardless of age in flounder. Vibrios are facultative pathogens that can readily survive and multiply in the environment, although the relative pathogenicity of environmental versus fish isolates is uncertain. Vibrio species bacteria, such as Vibrio (Listonella) anguillarum, V. icthyoenteri and Vibrio (Photobacterium) damselae have been routinely associated with vibriosis in Korea.
According to National Fisheries Research & Development Institute (NFRDI) studies, the incidence of vibriosis in 2000's was high in the coastal flounder aquarium.
The clinical sings of vibriosis include anorexia, darkened body color and sudden death. In older fish, the subsequent development of the outbreak goes through an acute and often chronic phase. Acutely affected fish show swollen, dark, skin lesions that ulcerate and release blood-colored exudate. The ulcer is very deep and necrotic. Internally, the main disease feature is enlargement of the spleen and kidney. Focal hemorrhages also have been seen on the surface of the heart, and the gills are usually pale.
V. anguillarum is the most common fish-pathogenic vibrio, but it have been infrequently isolated from marine fish in Korea. Recent phylogenetic studies of ribosomal RNA suggest that this organism should belong in a new genus, Listonella.
Bacterial enteritis caused by Vibrio ichthyoenteri is a devastating disease characterized by an opaque intestine (intestinal necrosis) in olive flounder larvae. Although no reports have yet been published on bacterial enteritis in Korea, it is widely recognized that the disease has caused major losses in marine hatcheries.
Vibrio damsela causes skin ulcers or systemic disease in a wide range of fish. It is now considered a member of the genus Photobacterium.
In most cases, antibiotic therapy of clinical outbreaks is still the main method of controlling this disease. Oxytetracycline, erythromycin are the most commonly used drugs, but since they are usually given orally, anorectic fish are not treated with antibiotics.
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Streptococcicosis occurs frequently in recent years. beta-hemolytic streptococcus is the causative bacterium, which possesses the same biochemical and serological characteristics as ет-streptococci isolated from some marine and freshwater fish, is seemed to related to Streptococcus iniae. Streptococcus iniae is one of the major bacterial pathogens of cultured oliver flounder in Korea.
It is hemolytic, gram positive coccus and causes meningoencephalitis in flounder.
External gross lesions of streptococcus are hemorrhagic and congestive lesions, petechia on the inside of operculum, Congestion at the pectoral and caudal fins and mouth, exophthalmia, abdominal distension, pus formation around orbits and operculum.
Internal gross lesions streptococcus are ascites or peritonitis, often tinged with red or fibrinous, in association with peritonitis, hemorrhage often involved in enteritis, focal necrosis of fatty or pale-coloured liver, congestion and hemorrhage of the liver, spleen, kidney, brain and intestine, with characteristic hemorrhagic enteritis in enterococcosis.
To confirm the diagnosis, it is necessary to biochemically characterize the bacterium. Recently, Polymerase Chain Reaction (PCR) technic was successfully utilized to differentiate among Streptococcaceae.
Oral erythromycin is more effective than either oxytetracycline or ampicillin when treating the disease.
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