Lactococcus garvieae and Yersinia raukeri co-infection in a rainbow trout (Oncorhynchus mykiss) farm: a case report

The current case study describes high mortality rate in a rainbow trout (Oncorhynchus mykiss) farm in Mazandaran province, north of Iran. The freshly dead fish were subjected to clinical examination to detect any observable symptoms. Biochemical and molecular investigations were conducted in order to determine the causative agents. The sensitivity of the isolated organisms to a range of antibiotics was assessed using the disk diffusion method. The main pathological findings consisted of ulcerations and haemorrhages on the skin, ventral part of the body, and muscles. Hyperemia in visceral organs, hyperemic spleen, and yellow ochre-colored liver with surface petechial hemorrhages were seen. The results of the biochemical tests and PCR assay confirmed the Lactococcus garvieae and Yersinia ruckeri co-infection. Co-infection has been documented in a diverse range of aquatic organisms. The initial introduction of a pathogen can modify the host's immune reaction to subsequent infections by either inhibiting or stimulating the immune system.