Study of Renibacterium salmoninarum infection in rainbow trout (Oncorhynchus mykiss) and the effect of temperature in an in-vivo cohabitation challenge

Renibacterium salmoninarum is the causative agent of bacterial kidney disease (BKD), an important infectious disease affecting wild and farmed salmonids worldwide. Besides enabling true vertical transmission, the intracellular lifestyle of the bacterium complicates both its detection and treatment and facilitates its persistence inside infected hosts. BKD therefore poses a health threat to farmed fish within aquaculture production systems. In rainbow trout, the disease has been poorly investigated under experimental conditions, and an experimental model to study the host-pathogen interaction is currently lacking. We demonstrate that cohabitation exposure effectively transmits R. salmoninarum in rainbow trout, mimicking natural horizontal transmission dynamics, with intraperitoneally injected shedder fish acting as transmission sources. Following infection, shedders exhibited a high bacterial burden, histopathological kidney lesions, and reduced survival. The in-vivo trial reproduced the subclinical stage of infection in cohabitants, characterized by moderate bacterial levels detected by qPCR, sporadic reisolation on SKDM agar during the 12 weeks trial, mild kidney lesions and no overt BKD signs. We further verify that R. salmoninarum infection development is temperature mediated. In cohabitants, lower temperatures (6 °C) were associated with a higher reisolation rate of viable bacteria at later stages of the trial, and in shedders, with a delayed onset of reduced survival. To complement the infection monitoring, targeted environmental DNA analysis was applied to monitor bacterial DNA in water. Altogether, findings emphasize the value of a multiple-method approach to obtain a comprehensive overview of R. salmoninarum infection.