Microscopic evidence of Piscirickettsia salmonis and IPNV infections. Phase contrast microscope appearance of cultures of CHSE-214 (top row) and RTS11 (bottom row) after the addition of either Piscirickettsia salmonis (middle column) or IPNV (right column). The cytopathic effect is seen as morphological changes (arrows) and as homotypic aggregation [highlighted by a box] in the case of RTS11 and P. salmonis. In CHSE-214 cultures, P. salmonis infection displayed the characteristic vacuolisation of cells. This CPE was visible in small groups of cells as early as three days after infection. Five days post-infection, vacuolisation was extended to approximately 40% of the population and nine days later around 80% displayed CPE. At this point some cells were detaching from the monolayer. In contrast to CHSE-214, RTS11 showed no evidence of vacuolization after infection, but homotypic aggregation of these monocyte-like cells became perceptible as infection advanced. Significant cell death and lysis took place at least 12 days post-infection. IPNV infection was evidenced by rounding up and blebbing of the plasma membrane in CHSE-214 cultures. As infection advanced, apoptotic bodies could be seen. Two days post-infection, over 50% of the population displayed CPE. For RTS11 cultures, strong homotypic aggregation became apparent 24 h post-infection followed by significant morphological changes such as plasma blebbing.