Crayfish plague is caused by infection with the freshwater oomycete fungus, Aphanomyces astaci.

Disease signs at the farm level:

• high mortality at time of initial outbreak
• many dead or weak crayfish floating or lying in watercourses or ponds (mortalities may go unnoticed in the wild)
• crayfish in open water with an unsteady and raised gait ('walking on stilts')
• rapid tail escape response weakens
• crayfish fall over, unable to right themselves (more evident out of water)

Clinical signs of disease in an infected animal:

• fungal growth on soft, noncalcified parts of shell
• browning or blackening spots on the carapace, where fungal hyphae proliferate
• white necrotic musculature in tail instead of the pearl grey of healthy crayfish
• fine black lines on the soft shell underneath the tail
• melanised (black) shell in chronically infected individuals
• death, within weeks, of nonresistant European crayfish