An investigation into the effects of Streptococcus agalactiae on the 5-HT system and the behavior of GIFT tilapia (Oreochromis niloticus)

Abstract

Meningitis is a characteristic clinical manifestation in GIFT tilapia (Oreochromis niloticus) infected with Streptococcus agalactiae. Fish that are infected with Streptococcus agalactiae and survive often show neurological disorders, such as constant, aberrant swimming. In the current study, the critical swimming speed (Ucrit) of farmed fish was investigated. The Ucrit of tilapia infected with S. agalactiae decreased to 2.78 BL s−1, 3.05 BL s−1 and 2.94 BL s−1 at 4, 8 and 16 h after challenge, respectively (P < 0.05). Meanwhile, the S. agalactiae quantification of the brain and intestine and the activities of serum lysozyme (LZM) and serotonin (5-HT) in the brain, intestine and stomach were evaluated. The copies of S. agalactiae in the brain and intestine of tilapia increased immediately after pathogen infection. The copies of S. agalactiae in the brain of infected fish increased sharply from the 8th hour to the 16th hour (P < 0.05), reaching 538 copies mg−1 at the 16th hour (P < 0.01). In the intestine, the copies of S. agalactiae in pathogen-infected fish increased rapidly from 1316.0 to 24,295.7 copies mg−1 (from the 4th hour to 16th hour, respectively), and the levels of the infected fish were higher than control fish during the whole experiment (P < 0.05). Similarly, the activities of LZM were increased immediately after infection of the fish (P < 0.05). The enzyme activities of the infected fish were significantly higher than those of control fish at 4 h, 12 h and 16 h (P < 0.01). The 5-HT levels in the intestine, brain and stomach of the tilapia also decreased with time after S. agalactiae infection and reached the lowest level at the 16th hour (P < 0.05). These results indicate that S. agalactiae infection affects the swimming behavior of tilapia, which may be connected to the 5-HT system.