Anatomopathological characterization and etiology of lesions on Nile tilapia fillets (Oreochromis niloticus) caused by bacterial pathogens

Abstract Fish farming represents an important and healthy source of protein for human nutrition. Currently, Nile tilapia (Oreochromis niloticus) is the second most raised fish species worldwide, and its fillet plays an important role in the international trade of fish meat. In Brazil, fish slaughterhouses submitted to the Federal Meat Inspection Service detected lesions on tilapia fillets, which cause condemnation and economic losses to farmers and companies. The main etiologic agents associated with these fillet lesions have never been addressed. This study aimed to characterize the anatomopathology and etiologic agents involved in Nile tilapia fillet lesions. From May to September 2017, 70 fillets of Nile tilapia discarded as improper for human consumption at the inspection line were sampled, macroscopically characterized, and submitted to histopathology. In addition, Streptococcus agalactiae and Francisella noatunensis subsp. orientalis (Fno) were detected by PCR and qPCR, respectively. Out of all analyzed samples, 67% were positive either for Streptococcus agalactiae, Fno, or both. Three macroscopic patterns were identified, being two associated with bacterial pathogens. 85% of the Fno positive fillets had a black dot pattern and 90% of the S. agalactiae positive fillets had a cavity pattern on the surface of the fillet. Irregular white areas were not positive for any agent and were trauma related. The microscopic findings were pyogranulomatous and necrotizing myositis of varying degrees and distribution. The rate of discard in the lots affected by bacteria ranged from 0.2% to 2% of fillets. This is the first report of Fno as an important cause of fillet discharge in Nile tilapia slaughterhouses. The macroscopic patterns identified may be helpful in suggesting the etiology of these lesions and preventing economic loss.