Proliferative kidney disease (PKD)

Proliferative kidney disease is a devastating myxozoan disease of trout, some salmon and whitefish. It is caused by infection with a myxozoan parasite, which can only complete its life cycle in brown trout and Chinook salmon, but in other salmonids it triggers a runaway immune response in the fish's kidneys, leading to death.
The parasite's other natural host is freshwater bryozoans (moss animals), and this is one of the few known species to use both a fish and bryozoan host (and belongs to the myxozoan sub-group Malacosporea).