Streptococcus iniae is an invasive pathogen causing meningitis and other lesions in various fish species. Furthermore,
S. iniae is an emerging zoonotic agent that causes cellulitis in man. The aims of this study were to establish an intraperitoneal
infection model for S. iniae in Nile tilapia (Oreochromis niloticus) and to develop a new histopathological
scoring system to reflect the degree and extent of inflammation as well as the presence of necrosis in the brain
and eye. Intraperitoneal administration of 106 colony-forming units (CFU) led to 80% mortality and numerous
fish developing clinical signs of central nervous system dysfunction. Microscopical examination of four regions of
the brain (olfactory bulb, cerebellum, cerebrum and optical lobe) and the eye revealed the presence of lymphohistiocytic
leptomeningitis, meningoencephalitis and endophthalmitis. Lesions were dominated by macrophages
that often contained intracellular bacteria. Necrosis was recorded in some cases. Bacteriological screening revealed
that multiple organs, including brain and eye, were infected with S. iniae and S. iniae colonized the scales and gills in
high number. S. iniae was detected in tank water during the first week post infection, suggesting that infected tilapia
might shed up to 3 107 CFU of S. iniae within 24 h. A multiplex polymerase chain reaction allowed con-
firmation of the challenge strain