Crayfish plague pathogen detected in the Danube Delta - a potential threat to freshwater biodiversity in southeastern Europe

The crayfish plague, caused by the oomycete Aphanomyces astaci, is probably the most significant reason for declines in European freshwater crayfish species. One of its hosts, the North American spiny-cheek crayfish Orconectes limosus, extends its range in the river Danube and recently reached the territory of Romania. We used highly sensitive A. astaci-specific real-time PCR to test if the native narrow-clawed crayfish Astacus leptodactylus in the highly protected Danube Delta about 970 km downstream of the current invasion front of American crayfish is a carrier of the crayfish plague. Thirteen out of 40 analysed native A. leptodactylus tested positive for the crayfish plague pathogen, infected individuals were found at both sampled localities within the Danube Delta. Therefore A. astaci has a much wider range in this river than assumed. The pathogen seems to persist in local populations, as neither crayfish mass mortalities nor alien crayfish species have been reported from the region. Aphanomyces astaci may have reached the Delta by long-range passive dispersal of infected hosts or pathogen spores, or by gradually infecting populations of native crayfish in upstream regions of the Danube in a stepping-stone manner. Alternatively, the crayfish plague may have persisted in the Danube Delta as chronic infection from an old plague wave in the 19th century. In any case, the presence of this pathogen in the lower Danube may become a threat to conservation of European crayfish and to freshwater biodiversity in many regions of southeastern Europe, at present considered crayfish plague-free.