Aphanomyces astaci; crayfish plague; invasive species; crayfish; Orconectes limosus

Crayfish plague, caused by the oomycet Aphanomyces astaci, is a very serious disease, which had destroyed many populations of native European crayfish in 19th and 20th century. Although the plague threat is still present in many countries, there are only few reports in literature about the occurrence of the disease in the Czech Republic. In recent years, mass mortalities of Astacus astacus with typical symptoms of crayfish plague were noticed in three streams - two in central Bohemia (1998-9: Pšovka, 1999: Loděnický brook) and one in Silesia (2004: Křivec). Three dead individuals from the last locality were tested for the presence of A. astaci by a PCR-based diagnostic method, all with the positive result. Although causes of mass mortalities from other two localities lack such a direct confirmation, the indirect evidence supports the same conclusion.Crayfish plague is usually transmitted by non-native crayfish species of North American origin, which are resistant to the disease.