Streptococcal Infection - Bacterial Diseases of Fish

Streptococcal Infection

CAUSATIVE AGENT:

Streptococcus sp.

Species affected:

Seabass (Lates calcarifer), tilapia (Oreochromis niloticus), rabbitfish (Siganusguttatus), ayu (Plecoglossus altivelis)

GROSS SIGNS:

Clinical signs vary among species of affected fish. However, erratic swimming, darkening of body color, unilateral or bilateral exopthalmia, corneal opacity, hemorrhages on the opercula and the bases of the fins and ulceration of body surface are the most common clinical signs. The hemorrhagic lesions, which gradually extend and ulcerate to release decayed material, are generally raised and have a darkened zone around them. The lesions are more superficial than in furunculosis or vibriosis.

EFFECTS ON HOST:

Infected fish have difficulty ventilating, and lose the ability to orient them-selves in the water. The eye becomes opaque and necrotic, conditions that can result to blindness. Fish swim in a spiralling motion. Fish are able to respond to stimuli, but have little control over movements. The spleen and kidney become enlarged. Dysfunction of the damaged internal organs may lead to mortalities.

DIAGNOSIS:

The pathogen grows easily on tryptic soy agar supplemented with 0.5% glu-cose, brain heart infusion agar, Todd-Hewitt broth agar and horse agar. Colo-nies develop after 24-48 h of incubation at 20-30°C. The colonies on agar plates appear small (0.5-1mm diameter), yellowish, translucent, rounded and slightly raised.

PREVENTION AND CONTROL:

•               Avoid overcrowding, overfeeding and unnecessary handling or transport.

•               Remove and slaughter promptly all moribund fish in ponds or net cages at early stage of infection to prevent outbreak or reduce severity of disease.

•               Apply erythromycin at 25-50mg/kg body weight of fish for 4-7 days.