Fish Diseases and Management

5.1.8.2 Streptococcosis

Streptococcus is a large, complex genus of Gram-positive spherical or oval bacteria less than 2 m m in diameter. Outbreaks of streptococcal septicemia has been different in cultured freshwater fish in different parts of the world. Some host specificity exists and trouts suffer from heavy mortalities while carp and largemouth bass are generally not affected.

It varies with the species of affected fish. Erratic swimming, darkening of body color, unilateral or bilateral exophthalmia, corneal opacity, hemorrhages on the opercular and the base of the fins, and ulceration of body surface are the most common symptoms. The haemorrhagic lesions, gradually extend and ulcerate to release necrotic material. They are more superficial than the lesions of furunculosis or vibriosis. In many species, the eyes are affected with high frequency. There may be hyperemia of branchial vessels, infiltration with macrophages and necrosis leading to massive haemorrhage and death. The intestinal tract may be hyperemic with sloughing of mucosa.

The causative organism streptococcus is present in seawater and mud around fish farms throughout the year, but in summer month their number increases. The presence of typical clinical signs and demonstration of Gram-positive cocci from internal organs constitutes a presumptive diagnosis. Confirmative diagnosis is by determining cultural characteristics and serological identification of the isolates. Fluorescent antibody technique is the most rapid and effective method for diagnosis of streptococcal infection. Mode of transmission of streptococcosis in fish is horizontal with infection occuring from direct contact with infected fish or contaminated fish food.

Pathogenicity is due to an exotoxin. Different strains of Streptococcus were lethal to different types of fishes like golden shiners, blue gill, green sunfish and american toads. b -hemolytic streptococcus spp. isolated from ayu in Japan were very pathogenic for tilapia, yellow tail and sea bream.

Erythromycin is effective against streptococcal infections in cultured yellow tail and rainbow trout at does of 25-50 mg / kg body cut of fish / day for 4-7 days. Other antibiotics like doxycyclin, kitasamycin, alkyl-trimtethyl-ammonium-calcium, oxytetracycline, josamycin, oleandomycin, lincomycin have also been used to control streptococcosis in cultured yellow tail in Japan. Reducing over crowding, overfeeding, unnecessary handling or transportation and the prompt removal and slaughter of all moribund fish in ponds or net cages at an early stage of infection may prevent outbreaks or reduce their severity.