Microbiological investigation of Tilapia lake virus–associated mortalities in cage-farmed Oreochromis niloticus in India

Abstract

Tilapia lake virus (TiLV) is a serious pathogen of farmed Nile tilapia (Oreochromis niloticus) responsible for significant mortalities. In this study, we investigated a disease outbreak in cage-farmed Nile tilapia in India. The infected fish exhibited clinical signs such as severe scale loss, haemorrhage, exophthalmia, and fin and tail rot. The samples were screened for Tilapia lake virus (TiLV) by reverse-transcriptase PCR and also subjected to detailed bacteriological investigation to understand the association between TiLV and co-infecting bacterial pathogens. Bacteria were isolated from TiLV-infected and apparently healthy fish, and identified by conventional microbiological methods, followed by 16SrRNA gene sequencing. TiLV was detected by PCR in all the samples exhibiting clinical signs, while apparently healthy fish were negative for the virus. A total of 34 bacterial isolates belonging to the genera Aeromonas, Staphylococcus, Enterococcus, Plesiomonas, Enterobacter, Bacillus, Lysinibacillus, Solibacillus and Exiguobacterium were isolated from the virus-infected tilapia. However, Aeromonas veronii was found to be the most dominant bacterium isolated from the surface lesions and the internal organs of all infected fish. Antibiotic susceptibility testing revealed that A. veronii, by far, was susceptible to cephalosporins (ceftriaxone, cefotaxime, cefpodoxime), chloramphenicol, aminoglycosides (amikacin, gentamicin), ciprofloxacin and chloramphenicol. Experimental infection using intraperitoneally injected A. veronii reproduced the clinical signs of naturally infected Nile tilapia, and a lethal dose 50 (LD50) mortality was observed by day 7 post-challenge. Furthermore, A. veronii could be re-isolated from the experimentally infected fish. Based on this evidence, we propose that virulent A. veronii as a co-infecting bacterium can have an important role in the severity and outcome of the disease in Nile tilapia infected by TiLV.