Streptococcus agalactiae is an emerging pathogen in Nile tilapia (Oreochromis niloticus)
worldwide. To investigate aspects of the epidemiology, transmission and virulence of S.
agalactiae infections, nine outbreaks of meningoencephalitis and septicemia in Nile tilapia
farms in Brazil were analyzed. Records from the outbreaks revealed large variation in the
weight of fish affected, high mortality, and disease occurrence at water temperatures
above 26 8C. S. agalactiae was isolated from diseased fish from all farms, and 29 strains
were identified by phenotypic tests and 16S rRNA gene sequencing. Five strains from
different geographic origins were selected to determine the 50% lethal dose (LD50). All
strains were highly virulent; for example, strain SA 20-06 had an LD50 of 90 bacteria. To
investigate S. agalactiae transmission, we conducted cohabitation assays with diseased
and healthy fish and fish challenges using an immersion bath or gill inoculation. Strain SA
20-06 was used in all assays. The disease was reproduced with characteristic clinical signs
and S. agalactiae was reisolated in all trials. The infection route studies were identified as
by direct contact or through the water. In conclusion, S. agalactiae, a major pathogen of Nile
tilapia in Brazil, exhibited high virulence, regardless of the geographic origin of the isolated
strains.
2008 Elsevier B.V. All rights reserved.