Experimental Streptococcus iniae infection of hybrid striped bass (Morone chrysops X Morone saxatilus) and tilapia (Oreochromis niloticus) by nares inoculation

Streptococcus iniae infection was evaluated in hybrid striped bass and tilapia following experimental, bilateral inoculation of a virulent isolate of S. iniae onto their eyes or into their nares. Inoculation of eyes did not result in mortality or disease signs in either species. In contrast, mortality and disease signs were observed in both species following nares inoculation Hybrid striped bass were susceptible to 4.8 x 10(3) S. iniae colony forming units (CEU), an inoculum 100 times lower than that affecting tilapia. We observed decreased mortality of hybrid striped bass administrated 4.8 x 10(5) CFU dose of S. iniae. Following S. iniae infection, the disease signs observed in hybrid striped bass and tilapia were erratic, serpentine, rising and falling, and head-up swimming, slow acceptance or refusal of food, lethargy and darkened skin. We did not observe exophthalmia, eye opacity or body curvature. We were able to recover S. iniae from either nares, eyes, brains, or head kidney of 87.5% and 66.7% of the morbid hybrid striped bass and tilapia, respectively. Both hybrid striped bass and tilapia produced significant anti-streptococcal titers 14 days post nares inoculation with 4.8 x 10(3) CFU of S. iniae. Successful experimental S. inaie infection of fish following nares inoculation suggest that nares may be a potential route of S. iniae infection in cultured fish species. This route of experimental infection for S. iniae has not been previously described in fish. (C) 2000 Elsevier Science B.V. All rights reserved.