This disease is caused by Aphanomyces astaci representing the family of Leptolegniaceae. It is deadly disease for every European crayfish species, while American species are disease vectors.
Aphanomyces astaci arrived to Europe (Italy) from the North America in 1860, probably with ballast waters of ships. It spread quickly in Europe decimating native crayfish populations. This protozoan infects the crust and its fungal spores grow in the inner parts of the body. In the North American crayfishes: the spiny-cheek crayfish and signal crayfish, fungal spores infect only the surface of the cuticle. The first symptoms of crayfish plaque are: progressing loss of limb co-ordination, staying on the land and shedding legs. There is no cure for crayfish plague.
Crayfish plague is primary and basic reason for crayfish extinction; however there are also other reasons determining this process. In the water reservoirs, in which crayfishes are extinct, the parasite „loses“its host and disappears.