Myxozoa, or myxidium serotinum-myxobolus koi.
This parasite infects many types of genera and species.
If you
suspect an infection, check the gall bladder for cysts and spleen with crushed
saline mounts of gill tissue size around 0.2-1.
The infection is past
via faeces. Your fish are not the main host, the latter is not known.
It
is quite rare, most infections occur in small koi under 6". It can be a problem
with gills, although this infection is treatable, in most cases it does little
harm