Chronic Aphanomyces astaci infection in wild white-clawed crayfish: risks and implications for breeding programs

Aphanomyces astaci is the oomycete responsible for crayfish plague, a disease with devastating effects on susceptible crayfish populations across Eurasia. Recently, low virulence strains of this oomycete, mostly belonging to genetic group A, have been reported to coexist with European crayfish species. Between 2011 and 2020, 19 populations of white-clawed crayfish, Austropotamobius pallipes complex, were monitored in North-eastern Italy to detect the presence of A. astaci using molecular assays. Chronic infection was documented in eight populations, two of which were found to be persistently thriving despite A. astaci infection, remaining positive for the pathogen for at least two years. For conservation purposes, broodstock were collected from wild populations of A. pallipes where A. astaci had not been detected and housed in a breeding facility. However, a few months after the last introduction of broodstock into the facility, a crayfish plague outbreak ensued, leading to an extended mortality period of seven months. The A. astaci strain responsible for the outbreak, identified as genotype and haplotype group A, was isolated and characterised by three independent molecular methods: RAPD-PCR, mtDNA and genotype-specific nuclear markers. Epidemiological evaluations suggested that the pathogen was introduced into the facility via broodstock carrying an undetected chronic infection. The observed mortality was probably triggered by a combination of biotic and abiotic stressors. These findings highlight the importance of an appropriate health monitoring of the broodstock collected for ex situ breeding purposes and are notably relevant to the conservation of the European crayfish species.