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.

罗非鱼湖病毒（TiLV）是养殖尼罗罗非鱼（Oreochromis niloticus）造成重大死亡。在这项研究中，我们调查了印度网箱养殖的尼罗罗非鱼的疾病暴发。被感染的鱼表现出严重的鳞屑，出血，眼球突出，鳍和尾腐等临床症状。通过逆转录酶PCR筛选样品中的罗非鱼湖病毒（TiLV），并进行详细的细菌学研究，以了解TiLV与共感染细菌病原体之间的关联。从受TiLV感染且显然健康的鱼中分离细菌，并通过常规微生物学方法进行鉴定，然后进行16SrRNA基因测序。通过PCR在所有表现出临床体征的样品中检测到TiLV，而显然健康的鱼类对该病毒呈阴性。属于该属的34种细菌分离株气单胞菌，金黄色葡萄球菌，肠球菌属，邻单胞菌属，肠杆菌属，芽孢杆菌，Lysinibacillus，Solibacillus和微小杆菌属是从病毒感染的罗非鱼隔离。然而，发现从所有受感染鱼的表面病变和内部器官中分离出的维罗纳气单胞菌是最主要的细菌。抗生素药敏试验表明维氏曲霉迄今为止，对头孢菌素（头孢曲松，头孢噻肟，头孢泊肟），氯霉素，氨基糖苷（阿米卡星，庆大霉素），环丙沙星和氯霉素敏感。使用腹膜内注射维氏假单胞菌进行的实验性感染重现了自然感染的尼罗罗非鱼的临床体征，在攻击后第7天观察到了致命剂量50（LD ₅₀）的死亡率。此外，可以从实验感染的鱼中重新分离出ver.ii。基于此证据，我们提出有毒力的维氏假单胞菌作为共感染细菌在TiLV感染的尼罗罗非鱼中，在疾病的严重程度和预后方面可能具有重要作用。