Experimental Streptococcus agalactiae infection in Nile tilapia (Oreochromis niloticus) via different routes

Abstract

Streptococcus agalactiae is one of the most pathogenic bacteria responsible for causing mortality in Nile tilapia and the routes of infection have not been clear. To determine the mortality, 320 Nile tilapias were assigned to 4 treatments (80 fish/treatment). Each fish was inoculated with 0.1 ml of S. agalactiae concentration of 7.25x107 CFU/ml via oral, nares, gills or eyes routes and was then maintained out of water for 30 seconds. Control fish were inoculated with sterile 0.9% normal saline solution using a similar manner. Samples from anterior kidney, spleen, eyes and/or brain from dead fish were sampled by bacteriological technique for confirming S. agalactiae infection. The results showed clinical signs of fish inoculated with S. agalactiae including lethargy, spiral swimming, corneal opacity and/or darkening of the skin as early as day 1 after inoculation. Cumulative mortality of S. agalactiae infection was 31.67%, 18.33%, 0% and 0% via oral, nares, gills and eyes inoculations, respectively. The mortality of fish between oral and nares inoculations was not significant (P > 0.05). Death among fish occurred from day 2 to day 8 post-inoculation. The results of this study indicate that S. agalactiae could enter through oral and nares routes and consequently led to mortality in Nile tilapia.