Although most important bacterial pathogens of fish are Cram-negative, Gram-positive Streptococcus spp., considered primary pathogens, have been isolated from a number of food fish species including rainbow trout, eel, striped bass, and tilapia. In Japan, streptococcal infections of cultured yellowtail (Seriola quinqueradiuia) result in heavy losses.

Many ornamental fish species are also susceptible to streptococcal infections. Susceptible species observed at our facility include members of at least five families of fish. In the Cyprinidae, these include the Rosey Barb and High-Fin Rosey Barb (Barbus conchonius), Red Lace Barb, Tiger Barb and Green Tiger Barb(Barus tetrazona), Pearl Danio (Brachydanio albolinealus), High-Fin Leopard Danio (Brachydanio rerio), Red-Tailed Black Shark (Epalzeorhynchus bicolor), Rainbow Shark and Albino Rainbow Shark (Epalzeorhynchus frenatus). In the Characidae, these include the Red-Eye Tetra (Moenkhausia sanctuefilomenae), Blood Fin Tetra (Aphyocharax anisitsi), and Blue Tetra (Boehlkeafredcochui). In the Cichlidae, Cyrtocara venusta and Pelvicachromis kribensis. In the Pangasidae, the Iridescent Shark (Pangasius sutch) was affected. In the Monodactylidae, Monod~tylus argenteus was affected.

Clinical signs observed included darkening, aberrant swimming (either hanging close to the surface and/or whirling) exophthalmia, hemorrhages in the eye. The most common sign, however, was acute and moderate to heavy mortality, with many of the fish still appearing normal 24 hours before death. Positive identification was made through bacterial culture of brain and/or kidney tissue.

Sensitivities on selected cultures showed susceptibility to erythromycin, oxytetracycline, nalidixic acid, ampicillin, and Enrofloxacin.. Treatment of choice is oral erythromycin at 1.5 grams per pound of feed for 10 to 21 days. We have been able to control this disease through recognition and immediate prophylactic treatment of most commonly affected species.