PATHOGENICITY AND PATHOLOGY OF Streptococcus agalactiae IN CHALLENGED MOZAMBIQUE TILAPIA Oreochromis mossambicus (PETERS, 1852)
The Mozambique tilapia Oreochromis mossambicus is the second most important farmed tilapia species in the world, after the Nile tilapia. Streptococcosis is one of the most significant diseases of tilapia, which contributed severe economic losses worldwide. Streptococcus iniae, S. agalactiae and other species of streptococci are the major bacterial species that affect the global tilapia production. Most outbreaks of streptococcosis in tilapia are caused by S. agalactiae that are influenced by the high water temperatures above 31°C. This communication reports the pathogenicity and pathology of S. agalactiae, isolated from O. niloticus with meningoencephalitis, in challenged O. mossambicus juveniles.
The pathogenicity of two Streptococcus agalactiae strains on 200 healthy Oreochromis mossambicus (10.09±1.06 g) divided into 10 groups in duplicate was tested by intraperitoneal challenge. The tilapia of groups 1-4 were injected intraperitoneally with S. agalactiae TKT1 at approximately 1.00×108, 1.00×107, 1.00×106 and 1.00×105 cells/fish, respectively. Similarly, the tilapia of groups 5-8 were injected intraperitoneally with S. agalactiae TBT2 at similar doses as above. The positive control fish (group 9) were injected with 0.1 mL of sterile physiological saline. The negative control tilapia (group 10) received no injection.
The intraperitoneal challenge with S. agalactiae TKT1 at 108 and 107 cells/fish caused 100% and 70% mortalities, respectively; while, the S. agalactiae TBT2 caused 90% and 40% mortalities. No mortalities were noted at the lower doses. The LD50 values of TKT1 and TBT2 strains were 1.60×107 and 7.33×107 cells/fish, respectively. The histological sections of the challenged O. mossambicus brain exhibited meningoencephalitis, marginated haemocytes, oedema and neurons with marginated nuclei. The kidney tissues had glomerulopathy, dilated Bowman's capsule, nephritis, melanization and granulomatous lesions. The spleen showed melanomacrophage aggregation and vasodilation. Ruptured blood capillary, blood vessel devoid of blood cells and disintegration of intrahepatic exocrine pancreatic tissues were noted in the liver. The results revealed S. agalactiae can cause systemic infection and meningoencephalitis in O. mossambicus.
