The crayfish plague pathogen, Aphanomyces astaci (Saprolegniales, Oomycetes), is lethal for native European crayfish species and eliminated a number of their populations during the second half of the 19th and throughout the 20th centuries. The disease is spread by non-native North American crayfish species, known to be carriers of the pathogen, which are currently thriving in many European countries. This represents one of the major threats to the recovery of populations of native crayfish (the noble crayfish Astacus astacus and the stone crayfish Austropotamobius torrentium) in the Czech Republic. While only two suspected plague-caused crayfish mass mortalities were reported in the second half of the 20th century, five mass mortalities involving both native species were found to be caused by A. astaci between 2004 and 2007, and several additional ones, though unconfirmed, are likely.

We discuss possible reasons for this apparent increase in the number of crayfish plague outbreaks, and we report details of the four most recent mass mortalities in which the presence of A. astaci has been confirmed by molecular methods (PCR with species-specific primers and sequencing of the pathogen DNA) and microscopic analysis, and of additional suspicious cases of crayfish disappearance. As these plague outbreaks have been found only accidentally, it is likely that their actual frequency is higher. The Czech Republic is probably not exceptional in this respect, and plague monitoring within the whole Central Europe deserves more attention.