Experimental streptococcal meningo-encephalitis in cultured fish

In 1984 a disease of fish appeared in Israel which spread rapidly in cultured fishponds. The disease affected tilapia (Oreochromis aura × Oreochromis nilotica hybrids) and trout (Oncorhynchus mykiss). Common carp (Cyprinus carpus), although reared in community with tilapia were not susceptible to the disease. Various species of ornamental cyprinids and cichlids were also affected. Morbidity was high and mortality ranged between 50% (in trout) and 30% (in tilapia). Clinical and pathological findings indicated that the tilapia and trout suffered from meningitis and meningo-encephalitis. Two new streptococcal species, Streptococcus shiloi and Streptococcus difficile were isolated from diseased fish. The disease was reproduced experimentally in both trout and tilapia with the two streptococcal species. The LD_50s of S. shiloi and S. difficile strains cultured in vitro (two to three passages on BHI medium) were 10⁷-10⁸ cfu. The virulence of these strains was increased (LD₅₀: 10²-10⁵ cfu) after three passages in vivo (brain to brain passage in fish without culture on agar plates). Highly virulent strains did not differ from low virulent strains by any identifiable extrachro-mosomal elements.