Aphanomyces astaci in wild crayfish populations in Slovenia: first report of persistent infection in a stone crayfish Austropotamobius torrentium population

All 5 crayfish species inhabiting Slovenian freshwaters, of which 3 are indigenous crayfish species (ICS: Astacus astacus, Austropotamobius pallipes, and A. torrentium) and 2 are non-indigenous (NICS: Pacifastacus leniusculus and Cherax quadricarinatus), were inspected for the presence of Aphanomyces astaci, the causative agent of crayfish plague. Wild crayfish populations showing no clinical signs of infection were inspected using A. astaci-specific real-time PCR. In addition, a conventional PCR assay was employed and confirmative sequencing was performed. Out of 88 analyzed crayfish, 15/27 (55.6%) specimens of A. torrentium from Borovniscica Brook and 4/35 (11.4%) of P. leniusculus from the Mura River tested positive, showing low to moderate levels of infection (agent levels A1-A4 and A1-A3, respectively). Results revealed the presence of A. astaci not only in the resistant NICS but also in ICS, since the infected population of A. torrentium presumably had no contact with the NICS carrier and appeared to sustain A. astaci infection in the 2 sampling years. Although the A. astaci genotype has not yet been identified, a connection between the latent infection in ICS and a Group A strain of A. astaci, co-evolving with A. torrentium since its first introduction to Slovenia, is suggested as the most plausible conclusion. This is the first reported population of the genus Austropotamobius with persistent infection, in addition to the already known populations of the genus Astacus. Findings of the presumed co-evolution of A. astaci and ICS hosts open new perspectives, necessitating additional studies on the presence of A. astaci genotypes in the persistently infected ICS populations.