Infection of barramundi Lates calcarifer with Streptococcus iniae: effects of different routes of exposure

The use of various challenge techniques has allowed the formation of a hypothesis for the mode of infection of Streptococcus iniae in barramundi. A bacterial dose of 1 x 10(3) colony forming units (cfu), corresponding to the LD50, delivered orally to barramundi could initiate the sub-acute form of the disease observed at the farms. The acute form of the disease could be initiated through bath exposure to the pathogen. S. iniae was equally as infective in freshwater, saltwater or when fish were subject to skin trauma prior to exposure, with LD50 values of 3.2 x 10(4), 2.0 x 10(4), 3.2 x 10(4) cfu, respectively, when observed over a 10 d period. It is suggested that sub-acute infection occurs orally, with mass mortalities occurring through the increased presence of the bacterium in the environment.