Co-infection with Tilapia Lake Virus, Aeromonas hydrophila, and Streptococcus agalactiae in Nile Tilapia: Mortality Investigation and Antibacterial Evaluation of Tinospora cordifolia

Abstract

The present study investigated mass mortality events in Nile tilapia (Oreochromis niloticus), with daily mortalities ranging from 70 to 100 fish/day a period of 15 days, reported during March 2024 from two commercial tilapia farms in the Krishna district of Andhra Pradesh, India. Infected tilapia exhibited lethargy, exophthalmia, skin ulceration, scale loss, anorexia, and fin rot. Sample were screened for tilapia viruses and bacterial pathogens using standard procedures. Phytochemical analysis and in vitro antibacterial activity of Tinospora cordifolia was evaluated. TiLV confirmed by RT-PCR, while Aeromonas hydrophila and Streptococcus agalactiae were confirmed by conventional microbiological methods and species-specific PCR. A. hydrophila shown resistance to oxytetracycline (30 µg), doxycycline (30 µg), co-trimoxazole (25 µg) whereas S. agalactiae resistant to oxytetracycline (30 µg), doxycycline (30 µg) and multiple antibiotic resistance index was 0.4 and 0.2, respectively. Regarding virulence genes, A. hydrophila harbored aerA, alt, act, and enol, and S. agalactiae harbored cpsG genes. T. cordifolia extract showed a strong antibacterial activity in 50 mg ml-1 , 75 mg ml-1, 100 mg ml-1, 125 mg ml-1 concentration and zone of inhibition ranging from 8±0.19 to 25±1.2 mm for A. hydrophila and 12±0.32 to 32±1.4 mm for S. agalactiae. These findings underscore the need for stringent biosecurity measures, adoption of best management practices, use of high-quality seed and feed, and incorporation of phytochemicals to mitigate disease outbreaks, reduce antibiotic dependency and promote sustainable tilapia aquaculture.