Streptococcus iniae infection and tissue distribution in hybrid striped bass (Morone chrysops×Morone saxatilis) following inoculation of the gills

This study was designed to test the possibility that Streptococcus iniae enters through the gills and causes infection in hybrid striped bass. To determine the dose response, four groups of fish were inoculated with S. iniae via the gills with a dose of 5.0×105, 2.6×106, 5.0×106, or 1.0×108 CFU/fish. One group of fish was inoculated with tryptic soy broth (TSB) via the gills to serve as controls. The cumulative percent mortality was 13%, 27%, 100% and 100% for 5.0×105, 2.6×106, 5.0×106 and 1.0×108 CFU/fish, respectively. We also examined the tissue dissemination of S. iniae at 0.5, 4, 8, 12, 24, 48 and 72 h after experimental gill inoculation. Fish were inoculated with 2.6×106 or 5.0×106 CFU/fish, which caused low and high mortality, respectively. Within 48 h, fish inoculated with the 2.6×106 dose were culture positive on the gill surface, blood of the first and second gill arches, blood of the third and fourth gill arches and the nares. However, for the dose of 5.0×106 CFU/fish, S. iniae was also isolated from the olfactory, optic and cerebellum regions of the brain, eye, head kidney, trunk kidney, spleen and liver at 48 h. For the 2.6×106 dose, S. iniae was not isolated until 48 h post-inoculation, but was isolated at 12 h for the 5.0×106 dose. The results of this study indicate that S. iniae can enter hybrid striped bass through the gills. However, mortality at similar S. iniae doses was lower than we previously observed by inoculation of the nares. [Copyright &y& Elsevier]