Disease development in red hybrid tilapia following single and co-infection with tilapia lake virus and Streptococcus agalactiae

Tilapia lake virus (TiLV) has recently been reported to cause infection in cultured and wild tilapia globally, and co-infection between TiLV and other bacterial pathogens contributes to higher mortality and economic losses. Understanding the pathology and pathogenesis of the co-infection could help in controlling the outbreaks. This study determines and compares the severity of single and co-infection of TiLV and Streptococcus agalactiae in red hybrid tilapia. The tilapias were challenged intra-peritoneal with 106 TCID50/mL of TiLV and 103 CFU/mL of S. agalactiae either as single or as co-infection. Fish were divided into five experimental groups with four replicates (30 fish/replicate). Group 1 received a single TiLV intra-peritoneal infection at 106 TCID50/mL of TiLV, Group 2 received a single S. agalactiae intra-peritoneal infection at 103 CFU/mL of S. agalactiae, Group 3 was co-infected intra-peritoneal with TiLV and S. agalactiae, Group 4 was co-infected with S. agalactiae and TiLV, and Group 5 was the control unchallenged. Co-infection by S. agalactiae in Group 3 was done at 13-days post infection (dpi), while co-infection by TiLV in Group 4 was at 2-dpi. Following challenge, fish were monitored daily for 20 days, while samples of brain and liver were collected at 2-dpi intervals for viral and bacterial load analyses. Co-infected fish recorded higher cumulative mortalities (60% for TiLV-S. agalactiae and 73% for S. agalactiae-TiLV) compared to single infection (40% for TiLV and 20% for S. agalactiae). Both single and co-infected fish showed similar clinical signs and gross lesions, respective to the exposed pathogen. Notable histopathological changes included the presence of multinucleated syncytial giant cells and intracytoplasmic inclusion bodies following TiLV infection either in single or co-infection, but not in S. agalactiae single infection. The mean of histopathological score for brain and liver, and pathogen loads were significantly (p < 0.05) higher in the co-infected fish. This study describes the disease development following TiLV and S. agalactiae co-infection and concluded that co-infection of TiLV and S. agalactiae increases the mortality and enhances the disease severity in tilapia.