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Chapter: Health Management in Aquaculture: Bacterial diseases

Edwardsiella Septicaemia or Edwardsiellosis - Bacterial Diseases of Fish

SPECIES AFFECTED: Tilapia (Oreochromis niloticus), channel catfish (Ictalurus punctatus), mullet (Mugil cephalus), carp (Cyprinus carpio)

Edwardsiella Septicaemia or Edwardsiellosis

CAUSATIVE AGENT:

Edwardsiella tarda

SPECIES AFFECTED:

Tilapia (Oreochromis niloticus), channel catfish (Ictalurus punctatus), mullet (Mugil cephalus), carp (Cyprinus carpio)

GROSS SIGNS:

Edwardsiella tarda infection manifests itself by the presence of small, 3-5 mm cutaneous or skin lesions located dorso-laterally (from along the back to the side) on the body. These lesions progress into abscesses and develop obvious convex swollen areas. The skin loses pigmen- tation. A foul smelling gas is emitted when the skin is incised. The lesion contains large amounts of necrotized or dead tissue. Inter- nally, there is generalized hyperemia (Fig. 3-2) and enlargement of the liver and kidney. Histologically, the lesion is characterized by focal necrosis, often extending from muscle, haemopoietic tissue and liver parenchyma (Fig. 3-3) to perforate the abdominal wall.



EFFECTS ON HOST:

E. tarda infection usually occurs during the warm, summermonths. The source of E. tarda is presumably the intestinal con-tents of carrier animals such as snakes, fish (eel and catfish), and some amphibians and reptiles. High temperature, poor water qual-ity and crowding may contribute to the onset and severity of the disease. Affected fish lose mobility of the caudal or tail portion of the body.E. tarda infection may cause lesions in the dermis, mus-culature and visceral organs of the host. Skin lesions when incised emit a foul smelling gas. The lesion contains large amount of necro-tized or dead tissue.

DIAGNOSIS:

The bacterium is easily isolated from muscle and internal organs of clinically diseased fish on most general-purpose media such as brain heart infusion agar (BHIA) and tryptic soy agar (TSA). Small punctate colonies develop in 24-48 h on inoculated media.

 

PREVENTION AND CONTROL:

      Improve water quality.

 

      Reduce stocking density.

 

      Apply oxytetracycline at 55 mg/kg fish for 10 days.

 

CAUSATIVE AGENT:

Vibrio alginolyticus, V. anguillarum and V. vulnificus

SPECIES AFFECTED:

Grouper  (Epinephelus  sp.),  rabbitfish  (Siganus  sp.),  milkfish(Chanos chanos), seabass (Lates calcarifer), sea bream (Sparus aurata)

GROSS SIGNS:

The first signs of the disease are usually anorexia or loss of appetite, with darkening either of the whole fish or of particular areas of the dorsum or back. Other common signs of vibriosis are hemorrhagic spot on different parts of the body including necrotic fins (Fig. 3-4), eye opacity (Fig. 3-5) and exophthalmia (Fig. 3-6). The perachute condition results in death without any other clinical signs except oc-casional periorbital or abdominal oedema. Chronically infected fish generally exhibit very pale gills and large granulating lesions deep in the muscle (Fig. 3-7a; 3-7b). In hatcheries, the presence of red spots in tanks is a sign of Vibrio infection.


EFFECTS ON HOSTS:

Vibriosis usually occurs in the warm summer months, especially when the stocking densities are high, and the salinities and organic loads are also high. Stressed fish are more prone to Vibrio infection. When an outbreak occurs, mortalities of 50% or higher can be ob-served in young fish. In older fish, losses may be lower, but infected fish do not feed or grow. When harvested, fish may have large ne-crotic lesions in the middle of the muscle mass.


DIAGNOSIS:

Squash preparations of kidney, liver, spleen, necrotic muscle tissue and other organs reveal the bacterium. The pathogen can usually be isolated from infected organs in pure culture using standard bacterio-logical media, such as BHIA, nutrient agar (NA) and TSA, provided they contain 1-2% sodium chloride. Thiosulphate citrate bile salt agar (TCBS) is a medium that selectively promotes growth of patho-genic vibrios while inhibiting other bacteria.


PREVENTION AND CONTROL:

      Maintain good water quality, good husbandry procedures and lower stocking densities.

 

      Apply oxytetracycline at 77 mg/kg of fish or nitrofurazone at 56 mg/kg of fish for 10 days.

 

      Vaccinate.


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