Outbreaks of WDS can occur in hatcheries but are mostly found in fish at the grow-out stage.

The disease typically starts in fish of medium body weight (100-200 g) and later spreads to the lower and higher weight classes.

Mortalities can be over 5% of the total population.

Usually, outbreaks occur when the water temperature drops below 14 °C (except in hatcheries where it can occur at higher temperatures) and persist until the temperature has risen to 18 °C.